· web viewa fixed mass of ideal gas at a low temperature is trapped in a container at...

A2 Extended Writing Practice

367 minutes

331 marks

Q1. (a) A spring, which hangs from a fixed support, extends by 40 mm when a mass of 0.25 kg is suspended from it.

(i) Calculate the spring constant of the spring.

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(ii) An additional mass of 0.44 kg is then placed on the spring and the system is set into vertical oscillation. Show that the oscillation frequency is 1.5 Hz.

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(b) With both masses still in place, the spring is now suspended from a horizontal support rod that can be made to oscillate vertically, as shown in the diagram below, with amplitude 30 mm at several different frequencies.

The response of the masses suspended from the spring to the vertical oscillations of the support rod varies with frequency. Describe and explain, as fully as you can, the motion of the masses when the support rod oscillates at a frequency of (i) 0.2 Hz, (ii) 1.5 Hz and (iii) 10 Hz.

The quality of your written answer will be assessed in this question.

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(Total 10 marks)

Q2. Deep space probes often carry modules which may be ejected from them by an explosion. A space probe of total mass 500 kg is travelling in a straight line through free space at 160 m s–

1 when it ejects a capsule of mass 150 kg explosively, releasing energy. Immediately after the explosion the probe, now of mass 350 kg, continues to travel in the original straight line but travels at 240 m s–1, as shown in the figure below.

(a) Discuss how the principles of conservation of momentum and conservation of energy apply in this instance.

The quality of your written communication will be assessed in this question.

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(b) (i) Calculate the magnitude of the velocity of the capsule immediately after the

explosion and state its direction of movement.

magnitude of velocity = ....................................... m s–1

direction of movement ............................................................(3)

(ii) Determine the total amount of energy given to the probe and capsule by the explosion.

answer = ....................................... J(4)

(Total 13 marks)

Q3. A student was required to design an experiment to measure the acceleration of a heavy cylinder as it rolled down an inclined slope of constant gradient. He suggested an arrangement that would make use of a capacitor-resistor discharge circuit to measure the time taken for the cylinder to travel between two points on the slope. The principle of this arrangement is shown in the figure below.

S1 and S2 are two switches that would be opened in turn by plungers as the cylinder passed over them. Once opened, the switches would remain open. The cylinder would be released from rest as it opened S1. The pd across the capacitator would be measured by the voltmeter.

(a) Describe the procedure the student should follow, including the measurements he should make, when using this arrangement. Explain how he should use the measurements taken to calculate the acceleration of the cylinder down the slope.


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(b) When the student set up his experiment using the arrangement shown in the figure above, he used a 22 μF capacitor, C, and a 200 kΩ resistor, R. In one of his results, the initial pd was 12.0 V and the final pd was 5.8 V. The distance between the plungers was 2.5 m.

(i) From the student’s result, calculate the time taken for the cylinder to reach the second plunger.

answer = ................................... s(3)

(ii) What value does this result give for the acceleration of the cylinder down the slope, assuming the acceleration is constant?

answer = ............................m s–2

(2)(Total 11 marks)

Q4. (a) Long cables are used to send electrical power from a supply point to a factory some distance away, as shown in Figure 1. An input power of 500 kW at 25 kV is supplied to the cables.

Figure 1

(i) Calculate the current in the cables.

answer = .................................A(1)

(ii) The total resistance of the cables is 30Ω. Calculate the power supplied to the factoryby the cables.

answer = ...............................kW(2)

(iii) Calculate the efficiency with which power is transmitted by the cables from the input at the supply point to the factory.

answer = ................................%(1)

(b) In Great Britain, the electrical generators at power stations provide an output at 25 kV. Most homes, offices and shops are supplied with electricity at 230 V. Power is transmitted from the power stations to the consumers by the grid system, the main principles of which are shown in Figure 2. In this network, T1, T2, T3, etc, are transformers.

Figure 2

(i) Explain how a step-down transformer differs in construction from a step-up transformer.

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(ii) Explain why the secondary windings of a step-down transformer should be made from thicker copper wire than the primary windings.

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(c) Discuss the principles involved in high voltage transmission systems, explaining why a.c. is used in preference to d.c. and how the energy losses are minimised.


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(Total 13 marks)

Q5. (a) A transformer operating on a 230 V mains supply provides a 12 V output. There are 1150 turns on the primary coil.

(i) Calculate the number of turns on the secondary coil.

answer = ........................... turns(1)

(ii) A number of identical lamps rated at 12 V, 24 W are connected in parallel across the secondary coil. The primary circuit of the transformer includes a 630 mA fuse.Calculate the maximum number of lamps that can be supplied by the transformer if its efficiency is 85%.

answer = .......................... lamps(2)

(iii) The transformer circuit includes a fuse. Explain why this is necessary.

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(iv) Why is the fuse placed in the primary circuit rather than in the secondary circuit?

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(b) The figure below shows an experimental arrangement that can be used to demonstrate magnetic levitation. The iron rod is fixed vertically inside a large coil of wire. When the alternating current supply to the coil is switched on, the aluminium ring moves up the rod

until it reaches a stable position ‘floating’ above the coil.

(i) By reference to the laws of electromagnetic induction explain

• why a current will be induced in the ring,

• why the ring experiences a force that moves it upwards,

• why the ring reaches a stable position.

The quality of your written communication will be assessed in your answer.

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(ii) What would happen to the ring if the alternating current in the coil was increased without changing the frequency? Explain your answer.

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(Total 13 marks)

Q6. (a) Define the gravitational potential at a point in a gravitational field.

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(b) The figure below, which is not drawn to scale, shows the region between the Earth (E) and the Moon (M).

(i) The gravitational potential at the Earth’s surface is –62.6 MJ kg–1. Point X shown in the figure above is on the line of centres between the Earth and the Moon. At X the resultant gravitational field is zero, and the gravitational potential is –1.3 MJ kg–1.

Calculate the minimum amount of energy that would be required to move a Moon probe of mass 1.2 × 104 kg from the surface of the Earth to point X. Express your answer to an appropriate number of significant figures.

answer = .................................. J(3)

(ii) Explain why, once the probe is beyond X, no further energy would have to be supplied in order for it to reach the surface of the Moon.

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(iii) In the vicinity of the Earth’s orbit the gravitational potential due to the Sun’s mass is –885 MJ kg–1. With reference to the variation in gravitational potential with distance, explain why the gravitational potential due to the Sun’s mass need not be considered when carrying out the calculation in part (b)(i).

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(c) The amount of energy required to move a manned spacecraft from the Earth to the Moon is much greater than that required to return it to the Earth. By reference to the forces involved, to gravitational field strength and gravitational potential, and to the point X, explain why this is so.


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(Total 14 marks)

Q7.The diagram below shows the orbits of two Earth satellites, a communications satellite in a geosynchronous orbit and a monitoring satellite in a low orbit that passes over the poles.

(a) The time period, T, of any satellite in a circular orbit around a planet is proportional to r3/2, where r is the radius of its orbit measured from the centre of the planet. For a satellite in a low orbit that passes over the poles of the Earth, T is 105 minutes when r is 7370 km.

(i) Calculate the height above the surface of the Earth, in km, of a satellite in a geosynchronous circular orbit.Give your answer to an appropriate number of significant figures.

height above surface .............................. km(4)

(ii) Calculate the centripetal force acting on the polar orbiting satellite if its mass is650 kg.

centripetal force ................................ N(2)

(b) These geosynchronous and polar satellites have different applications because of their different orbits in relation to the rotation of the Earth.

Compare the principal features of the geosynchronous and polar orbits and explain the consequences for possible uses of satellites in these orbits.

In your answer you should explain why:

• a low polar orbit is suitable for a satellite used to monitor conditions on the Earth.

• a geosynchronous circular orbit above the Equator is especially suitable for a satellite used in communications.

The quality of your written communication will be assessed in your answer.(6)

(Total 12 marks)

Q8.A radioactive source used in a school laboratory is thought to emit α particles and γ radiation. Describe an experiment that may be used to verify the types of radiation emitted by the source. The experiment described should allow you to determine how the intensity of radiation varies with distance in air or with the thickness of suitable absorbers.

Your answer should include:

• the apparatus you would use and any safety precautions you would take

• the measurements you would make

• how the measurements would be used to reach a final decision about the emitted radiation.

The quality of your written communication will be assessed in your answer.(Total 6 marks)

Q9.Many astronomical observations rely on a Charge Coupled Device (CCD) to obtain an image. Describe the structure and operation of the CCD and discuss the advantages of using a CCD for astronomical observations.

The quality of your written communication will be assessed in this question.(Total 6 marks)

Q10.(a) Sound waves are incident on the ear canal of a normal human ear. Describe the physical processes involved in the transmission of the energy from the air through to the inner ear.

Include an outline of how the variations in air pressure in the ear canal are amplified to produce greater pressure variations in the inner ear.


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(b) Define intensity of sound.

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(c) A human ear has a threshold of hearing of 54 dB at a given frequency. Calculate the intensity of sound incident on the ear at this frequency.

Give your answer to an appropriate number of significant figures.

Io = 1.0 × 10−12 W m−2

intensity of sound ................................... W m−2

(3)(Total 11 marks)

Q11.(a) Explain why the compression stroke of a diesel engine is considered to be an adiabatic change.

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(b) Figure 1 shows the cylinder of a diesel engine. The pressure of the air at the start of the compression stroke is 1.0 × 105 Pa and the volume above the piston is 4.5 × 10−4 m3.

Figure 1

Figure 2

Figure 2 shows the same cylinder at the instant just before the fuel is injected. The pressure above the piston is now 6.2 × 106 Pa. The compression is adiabatic with no leakage of air past

the piston or valves.adiabatic index for air = 1.4

(i) Calculate the volume above the piston at the instant just before the fuel is injected.

Give your answer to an appropriate number of significant figures.

volume ......................................... m3

(3)

(ii) The temperature of the air in the cylinder at the start of the compression stroke is 297 K. Calculate the temperature of the air at the instant just before the fuel is injected.

temperature ........................................... K(2)

(iii) Explain why, in a diesel engine, the fuel starts to be injected into the cylinder slightly before the piston reaches its highest point in the cylinder.

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(c) Figure 3 shows the indicator (p – V) diagram for a real diesel engine compared tothe p – V diagram for a theoretical diesel cycle of the same maximum and minimum volumes and fuel injection cut-off.

Figure 3

Compare the real engine cycle with the theoretical cycle. In your account you should:

• discuss the important differences between the cycles

• explain why the overall efficiency of the real engine is less than that predicted by an analysis of the theoretical cycle.

The quality of your written communication will be assessed in your answer.(6)

(Total 14 marks)

Q12.In his investigation of radio waves, Hertz created stationary waves by using a large flat metal sheet to reflect radio waves as shown in the diagram below.

(a) Explain why stationary waves are formed in this arrangement and describe how the wavelength of the radio waves can be determined by moving a suitable detector along XY.


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(b) Hertz knew the frequency of the radio waves from the electrical characteristics of the transmitter. He found the wavelength from the investigation described in part (a) and was then able to calculate the speed of the radio waves. Explain the significance of the result of this calculation.

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(Total 8 marks)

Q13. (a) On the figure below sketch a graph to show how the radius, R, of a nucleus varies with its nucleon number, A.

(1)

(b) (i) The radius of a gold-197 nucleus is 6.87 × 10–15 m.Show that the density of this nucleus is about 2.4 × 1017 kg m–3.

(2)

(ii) Using the data from part (b)(i) calculate the radius of an aluminium-27

nucleus, .

answer = ...................................... m(2)

(c) Nuclear radii have been investigated using α particles in Rutherford scattering experiments and by using electrons in diffraction experiments.Make comparisons between these two methods of estimating the radius of a nucleus.Detail of any apparatus used is not required.For each method your answer should contain:

• the principles on which each experiment is based including a reference to an appropriate equation

• an explanation of what may limit the accuracy of each method

• a discussion of the advantages and disadvantages of each method.


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(Total 11 marks)

Q14. TRAPPIST is a robotic telescope designed to detect exoplanets, which are planets outside our solar system.

(a) The charge coupled device (CCD) attached to TRAPPIST has a quantum efficiency of 96% for light of wavelength 750 nm.Explain what is meant by the quantum efficiency of a CCD.

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(b) (i) The optical arrangement of the telescope includes an objective mirror of diameter 0.60 m.Calculate the minimum angular separation of two objects which can be resolved by the telescope for light of wavelength 750 nm.

answer = .................................... rad(1)

(ii) One of the nearest exoplanets orbits the star Epsilon Eridani, which is 10.5 light years from Earth. The exoplanet has an elliptical orbit, whose orbital radius varies from 1 AU to 5 AU.Calculate the maximum angular separation of the star and the planet when viewed from a distance of 10.5 light years.

answer = .................................... rad(3)

(iii) TRAPPIST detects the presence of exoplanets by measuring the reduction in light intensity that occurs as the planet passes in front of the star.Explain why it is unlikely that the telescope could be used to observe such planets directly.

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(c) Different types of telescope are used to detect the various parts of the electromagnetic spectrum. Discuss with reference to three different parts of the electromagnetic spectrum, the factors which should be taken into account when deciding the siting and size of telescopes.The quality of your written communication will be assessed in your answer.

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(Total 12 marks)

Q15. (a) Explain how and why ultrasound is used to obtain an image of an unborn foetus.You might consider the following points in your answer

• the method of obtaining the image

• practical considerations for the scan

• safety issues.


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(b) Explain why the pulses of ultrasound used in medical imaging must be of short duration.

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(Total 8 marks)

Q16. Figure 1 shows a model steam engine used in a school to demonstrate energy transfers.The steam engine drives a dynamo which requires a constant torque.By means of valves, high pressure steam is applied to one side of the piston on the outward stroke (as shown) and to the other side of the piston on the inward stroke. The motion of the piston is converted to rotary motion by a connecting rod and crank. A flywheel (not shown) is fitted to the crankshaft.

Figure 1

Figure 2 shows how the torque on the crankshaft due to the engine varies with the crankshaft angle θ for one rotation of the crankshaft. The broken line shows the constant dynamo torque required from the output.

Figure 2

(a) State and explain how you could use Figure 2 to determine the work done by the engine in one revolution of the crankshaft.

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(b) The dynamo has a low moment of inertia.

• Explain why the engine torque varies over a cycle.

• Explain why, in terms of kinetic energy or angular momentum, it is necessary to fit a flywheel to the crankshaft of the engine.

• Discuss how the motion of the crankshaft is influenced by the value of the moment of inertia of the flywheel.


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(Total 7 marks)

Q17. (a) Light has a dual wave-particle nature. State and outline a piece of evidence for the wave nature of light and a piece of evidence for its particle nature. For each piece of evidence, outline a characteristic feature that has been observed or measured and give a short explanation of its relevance to your answer. Details of experiments are not required.


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(b) An electron is travelling at a speed of 0.890 c where c is the speed of light in free space.

(i) Show that the electron has a de Broglie wavelength of 1.24 × 10–12 m.

(2)

(ii) Calculate the energy of a photon of wavelength 1.24 × 10–12 m.

answer = ...................................... J(1)

(iii) Calculate the kinetic energy of an electron with a de Broglie wavelength of1.24 × 10–

12 m.Give your answer to an appropriate number of significant figures.

answer = ...................................... J(2)

(Total 11 marks)

Q18. A fixed mass of ideal gas at a low temperature is trapped in a container at constantpressure. The gas is then heated and the volume of the container changes so that the pressure stays at 1.00 × 105 Pa. When the gas reaches a temperature of 0 °C the volume is 2.20 × 10–3m3.

(a) Draw a graph on the axes below to show how the volume of the gas varies withtemperature in °C.

(2)

(b) Calculate the number of moles of gas present in the container.

answer = .................................moles(2)

(c) Calculate the average kinetic energy of a molecule when this gas is at atemperature of 50.0 °C. Give your answer to an appropriate number ofsignificant figures.

answer = .........................................J(3)

(d) Calculate the total internal energy of the gas at a temperature of 50.0 °C.

answer = .........................................J

(1)

(e) By considering the motion of the molecules explain how a gas exerts a pressure and why the volume of the container must change if the pressure is to remain constant as the temperature increases.


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(Total 14 marks)

Q19. (a) On the axes below draw the Hertzsprung-Russell (H-R) diagram, labelling the main sequence stars, giant stars and white dwarf stars. Complete the vertical axis by labelling a suitable absolute magnitude scale.

(3)

(b) Deneb is the brightest star in the constellation Cygnus.

(i) The black-body radiation curve for Deneb shows a peak at a wavelengthof 3.4 × 10–7 m. Calculate the black-body temperature of Deneb. Give youranswer to an appropriate number of significant figures.

answer = ........................................K(3)

(ii) The power output of Deneb is 70000 times greater than the Sun. Calculate theradius of Deneb.

surface temperature of the Sun = 5700K

answer = .......................................m(3)

(c) The spectrum of Deneb contains Hydrogen Balmer absorption lines. Describe how Hydrogen Balmer absorption lines are produced in the spectrum of a star.


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(Total 15 marks)

Q20. (a) An ECG trace is to be obtained for a patient. State and explain the procedure and some design features of the equipment needed to ensure a good trace is obtained.


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(b) The figure below shows an ECG trace for a healthy person.

(i) Add a suitable scale and unit to the potential axis.(2)

(ii) Add a suitable scale to the time axis.(1)

(iii) State the electrical events which give rise to the points:

P ........................................................................................................

R ........................................................................................................

T ….....................................................................................................(3)

(Total 12 marks)

Q21. (a) Newton suggested a theory that light is composed of corpuscles. He used his theory to explain the refraction of a light ray travelling from air to glass, as shown in Figure 1. Huygens explained the refraction of light using his own theory that light consists of waves.

Figure 1

(i) State one reason why Huygens’ theory of light was rejected for many years after itwas first proposed, in favour of Newton’s corpuscular theory of light.

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(ii) Explain why the eventual measurement of the speed of light in water led to thedefinite conclusion that light consists of waves and not corpuscles.

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(b) Young demonstrated that a pattern of alternate bright and dark fringes was observedwhen light from a narrow single slit passed through double slits, as shown in Figure 2.

Figure 2

Newton’s corpuscular theory predicted incorrectly that just two bright fringes would be

formed in this pattern. Use Huygens’ theory of light to explain why more than two bright fringes are formed in this pattern.


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(Total 9 marks)

Q22. (a) In a thermal nuclear reactor, one fission reaction typically releases 2 or 3 neutrons. Describe and explain how a constant rate of fission is maintained in a reactor by considering what events or sequence of events may happen to the released neutrons.


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(b) Uranium is an α emitter. Explain why spent fuel rods present a greater radiation hazard than unused uranium fuel rods.

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(Total 10 marks)

Q23. (a) In 1997 a type 1a supernova was observed which contributed to the controversial conclusion that the expansion of the Universe is accelerating.

Explain why observations of supernovae led to the conclusion that the Universe is expanding at an accelerating rate and discuss why this conclusion is controversial.


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(b) Measurements of the shift in the 21 cm H1 line in the spectrum of galaxy M84 suggests that it is receding at a velocity of 900 km s–1.

(i) Calculate the value of the red shift, z, for this galaxy.

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z = ..........................................(1)

(ii) Calculate the distance to this galaxy.

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distance = ...................................... Mpc(2)

(Total 9 marks)

Q24. (a) On the axes below, sketch the action potential of a nerve cell. Indicate units and scales on both axes.

(3)

(b) Explain in terms of ion movement, starting at resting potential, how bioelectrical signals are produced in muscle fibres.The quality of your written answer will be assessed in this question.

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(Total 10 marks)

Q25. (a) Figure 3 shows the indicator diagram for a theoretical or ideal four-stroke petrol engine (Otto) cycle.

Figure 1

Use Figure 1 to describe the process that occurs during each of the parts A to B, B to C,C to D and D to A of the cycle. Describe whether heating or cooling is taking place, the type of process and whether work is being done on or by the air.


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(b) Show, on Figure 2, how the indicator diagram might be expected to appear if measurements of pressure and volume were made on a real four-stroke petrol engine of the same volume under operating conditions. The ideal cycle is shown in dashed lines as a guide.

Figure 2

(2)(Total 8 marks)

Q26. Figure 1 shows the probe of a scanning tunnelling microscope (STM) above a metal surface.

Figure 1

(a) Explain why electrons can cross the gap between the tip of the probe and the surface, provided

• the gap is sufficiently narrow

• a potential difference is applied between the tip and the surface.


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(b) The probe is moved horizontally in a straight line across the surface. As it moves, the current due to the transfer of electrons between the surface and the probe decreases then returns to its initial value at the end of the line, as shown in Figure 2.

Figure 2

Explain why the current changes in this way.

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(2)(Total 8 marks)

Q27. (a) Sketch, using the axes provided, a graph of neutron number, N, against proton number, Z, for stable nuclei over the range Z = 0 to Z = 80. Show suitable numerical values on the N axis.

(2)

(b) On the graph indicate, for each of the following, a possible position of a nuclide that might decay by

(i) α emission, labelling the position with W,

(ii) β– emission, labelling the position with X,

(iii) β+ emission, labelling the position with Y.(3)

(c) Used fuel rods from a nuclear reactor emit β– particles from radioactive isotopes that were not present before the fuel rod was inserted in the reactor. Explain why β– emitting isotopes are produced when the fuel roads are in the reactor.

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(d) A nuclear power station is a reliable source of electricity that does not produce greenhouse gases but it does produce radioactive waste. Discuss the relative importance of these features in deciding whether or not new nuclear power stations are needed.


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(Total 14 marks)

Q28. Modern astronomy relies on the analysis of radiation from many different parts of the electromagnetic spectrum. Compare the main features of telescopes used to detect radio waves with those of optical reflecting telescopes. Explain the differences in their resolving and collecting powers.

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Q29. (a) The discovery of photoelectricity and subsequent investigations led to the wave theory of light being replaced by the photon theory. State one feature of photoelectricity that could not be explained using the wave theory of light and describe how it is explained using photon theory.The quality of your written answer will be assessed in this question.

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(b) A certain metal has a work function of 2.2 eV.

(i) Explain what is meant by this statement.

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(ii) The surface of the metal is illuminated with light of wavelength 520 nm.Calculate the maximum kinetic energy of electrons emitted from the surface.

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(Total 11 marks)

Q30. Charge coupled devices (CCDs) are commonly used in astronomy because of their highquantum efficiency.

(a) Describe the structure and operation of a CCD.

You may be awarded additional marks to those shown in brackets for the quality of written communication in your answer.

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(b) Explain what is meant by quantum efficiency, and state a typical value of the quantum efficiency of a CCD.

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Q31. The figure below shows the design of an X-ray image intensifier.

The main components are labelled A to D. Name each component and state its purpose in the process of image intensification.

You may be awarded additional marks to those shown in brackets for the quality of written communication in your answer.

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Q32. Describe the main features of the Big Bang theory and the evidence that supports it.You may be awarded marks for the quality of written communication in your answer.

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Q33. The diagram below shows the probe tip of a scanning tunnelling microscope (STM) above a metal surface. The probe tip is at a constant negative potential relative to the metal surface.

(a) Explain why electrons can cross the gap between the probe tip and the surface, provided the gap is sufficiently narrow.

You may be awarded marks for the quality of written communication in your answer.

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(b) Describe one way in which an STM is used to investigate a surface.

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(3)(Total 7 marks)

M1. (a) (i) mg = ke (1)

k = = 61(.3) N m–1 (1)

(ii) T = (1) (= 0.667 s)

(1) (= 1.5(0) Hz)4

(b) The marking scheme for this part of the question includes an overallassessment for the Quality of Written Communication (QWC). Thereare no discrete marks for the assessment of QWC but the candidates’QWC in this answer will be one of the criteria used to assign a leveland award the marks for this part of the question.

Level Descriptoran answer will be expected to meet most of the criteria in the

level descriptor

Mark range

Good 3 – answer supported by appropriate range of relevant points

– good use of information or ideas about physics, going beyond those given in the question

– argument well structured with minimal repetition or irrelevant points

– accurate and clear expression of ideas with only minor errors of spelling, punctuation and grammar

5-6

Modest 2 – answer partially supported by relevant points

– good use of information or ideas about physics given in the question but limited beyond this

– the argument shows some attempt at structure

– the ideas are expressed with reasonable clarity but with a few errors of spelling, punctuation and grammar

3-4

Limited 1 – valid points but not clearly linked to an argument structure 1-2

– limited use of information or ideas about physics

– unstructured

– errors in spelling, punctuation and grammar or lack of fluency

0 – incorrect, inappropriate or no response 0

examples of the sort of information or idea that might be used tosupport an argument

• forced vibrations (at 0.2 Hz) (1)

• amplitude fairly large (≈ 30 mm) (1)

• in phase with driver (1)

• resonance (at 1.5 Hz) (1)

• amplitude very large (> 30 mm) (1)

• oscillations may appear violent (1)

• phase difference at 90º (1)

• forced vibrations (at 10 Hz) (1)

• small amplitude (1)

• out of phase with driver or phase lag of π on driver (1)[10]

M2. (a) The candidate’s writing should be legible and the spelling,punctuation and grammar should be sufficiently accurate for themeaning to be clear.

The candidate’s answer will be assessed holistically. The answer will beassigned to one of three levels according to the following criteria.

High Level (Good to excellent): 5 or 6 marks

The information conveyed by the answer is clearly organised, logical andcoherent, using appropriate specialist vocabulary correctly. The form andstyle of writing is appropriate to answer the question.

The candidate states that momentum is conserved, supported by reasoningto explain why the conditions required for momentum conservation aresatisfied in this case.

The candidate also gives a statement that total energy is conserved, giving

detailed consideration of the energy conversions which take place,described in the correct sequence, when there is an explosion on a bodythat is already moving.

Intermediate Level (Modest to adequate): 3 or 4 marks

The information conveyed by the answer may be less well organised andnot fully coherent. There is less use of specialist vocabulary, or specialistvocabulary may be used incorrectly. The form and style of writing is lessappropriate.

The candidate states that momentum is conserved, but the reasoning ismuch more limited.

and/or

There is a statement that (total) energy is conserved, with basicunderstanding that some energy is released by the explosion.

Low Level (Poor to limited): 1 or 2 marks

The information conveyed by the answer is poorly organised and may notbe relevant or coherent. There is little correct use of specialist vocabulary.The form and style of writing may be only partly appropriate.

The candidate indicates that either momentum or energy is conserved, orthat both are conserved. There are very limited attempts to explain eitherof them.

The explanation expected in a competent answer should include acoherent selection of the following points concerning the physicalprinciples involved and their consequences in this case.

Momentum

• momentum is conserved because there are no external forcesacting on the overall system (probe plus capsule) – or because it’sfree space

• they are moving in free space and are therefore so far from largemasses that gravitational forces are negligible

• during the explosion, there are equal and opposite forces actingbetween the probe and the capsule

• these are internal forces that act within the overall system

• because momentum has to be conserved, and it is a vector, thecapsule must move along the original line of movement after theexplosion

Energy

• total energy is always conserved in any physical process becauseenergy can be neither created nor destroyed

• however, energy may be converted from one form to another

• the probe is already moving and has kinetic energy

• in the explosion, some chemical energy is converted into kineticenergy (and some energy is lost in heating the surroundings)

• the system of probe and capsule has more kinetic energy than theprobe had originally, because some kinetic energy is released bythe explosion

max 6

(b) (i) conservation of momentum gives (500 × 160)= 150 v + (350 × 240) (1)from which v = (−)26(.7) (m s−1) (1)

direction: opposite horizontal direction to larger fragment[or to the left, or backwards] (1)

3

(ii) initial Ek = ½ × 500 × 1602 (1) (= 6.40 × 106 J)

final Ek = (½ × 350 × 2402) + (½ × 150 × 26.72) (1) (= 1.01 × 107 J)

energy released by explosion = final Ek − initial Ek (1)

= 3.7 × 106 (J) (1)4

[13]

M3. (a) The candidate’s writing should be legible and the spelling, punctuationand grammar should be sufficiently accurate for the meaning to beclear.

The candidate’s answer will be assessed holistically. The answer will beassigned to one of the three levels according to the following criteria.

High Level (good to excellent) 5 or 6 marks


The candidate provides a comprehensive and logical description of thesequence of releasing the ball and taking measurements of initial and finalvoltages. They should identify the correct distance measurement and showa good appreciation of how to use these measurements to calculate thetime and acceleration from them. Time should be found from capacitordischarge, using known C and R values. Repeated readings would beexpected in any answer worthy of full marks, but five marks may beawarded where repetition is omitted.

Intermediate Level (modest to adequate) 3 or 4 marks


The candidate provides a comprehensive and logical description of thesequence of releasing the ball and taking measurements of the initial andfinal voltages. They are likely to show some appreciation of the use of suvatequations to calculate the acceleration, although they may not recognise theneed to measure a distance.

Low Level (poor to limited) 1 or 2 marks

The information conveyed by the answer is poorly organised and may notbe relevant or coherent. There is little correct use of specialist vocabulary.The form and style of writing may only be partly appropriate.

The candidate is likely to have recognised that initial and final voltagesshould be measured, but may not appreciate the need for any othermeasurement. They may present few details of how to calculate theacceleration from the voltage measurements.

The explanation expected in a competent answer should include acoherent selection of the following points.

Measurements

• initial pd across C (V0) from voltmeter (before releasing roller)

• distance s along slope between plungers

• final pd across C (V1) from voltmeter

• measurements repeated to provide a more reliable result

Analysis

• time t is found from V1 = V0e-t/RC, giving t = RC ln (V0/V1)

• from s = ut + ½ at2 with u = 0, acceleration a = 2s/t2

• repeat and find average a from several results

(b) (i) RC = 22 × 10–6 × 200 × 103 [or = 4.4 (s)] (1) (4.40)

5.8 = 12.0 e–t/4.40 (1)

gives t = 4.40 ln (12.0/5.8) = 3.2 (3.20) (s) (1)3

(ii) (1)

= 0.49 (0.488) (m s–2) (1)2

[11]

M4. (a) (i) current I = 20 (A) 11

(ii) wasted power (I2 R) = 202 × 30 = 1.20 × 104 (W) (12.0 kW)

power output from cables = 500 – 12 = 488 (kW)

or voltage drop along cables = IR = 20 × 30 = 600 (V)

output voltage = 25000 – 600 = 24400 (V)

power output = IV = 20 × 24400 = 4.88 × 105 (W)

(iii) efficiency = = 98 (97.6) (%) 1

(b) (i) primary coil must have more turns than secondary 1

(ii) to reduce heating (I2R) loss [or energy/power/copper loss]

(because) IS > IP

and R is reduced (by use of thicker wire) max 2

(c) The candidate’s writing should be legible and the spelling,punctuation and grammar should be sufficiently accurate forthe meaning to be clear.

The candidate’s answer will be assessed holistically. The answerwill be assigned to one of three levels according to the following criteria.


The information conveyed by the answer is clearly organised, logicaland coherent, using appropriate specialist vocabulary correctly. The formand style of writing is appropriate to answer the question.

The candidate provides a comprehensive and logical description of themain principles of the grid system. They should identify I2R heating asthe main cause of energy loss, and know that this can be reduced by using

transformers to raise voltage and therefore decrease current (for the samepower), and that transformers require ac. They may not have referred tosafety and insulation issues that ultimately require the voltage to be reducedagain or to energy losses from transformers.



The candidate provides a description of the main features of the grid systemwhich recognises that heating losses can be reduced by use of transformersto decrease the current. They should know that transformers require ac.They may not fully explain the reasoning for the use of a higher voltage andthey are unlikely to refer to safety and insulation issues that require thevoltage to be reduced again.



The candidate recognises that the use of higher voltage will reducetransmission losses and that transformers need ac. They give a muchweaker account (if any) of the underlying principles.

Incorrect, inappropriate of no response: 0 marks

No answer or answer refers to unrelated, incorrect or inappropriatephysics.

The explanation expected in a competent answer shouldinclude a coherent selection of the following points concerningthe physical principles involved and their consequencesin this case.

voltages are changed using transformers, which work with acbut not with dc

ac generation and transmission is therefore essential

current in cables causes joule heating ( or I2R loss)

resistance of cables should be as low as possible

losses are reduced if current in cables can be reduced

current can be reduced (for same power I V) if voltage is increased

the higher the voltage, the smaller the proportion of theinput power that is wasted

high voltage introduces insulation problems and raises safety issues

voltage must be reduced as the supply reaches its consumers

this is done in stages as the supply is moved from overheadcables to underground wires

transformers cause energy losses because they are not perfectly efficient

features are incorporated in the design of transformers to reducelosses from themmax 6

[13]

M5. (a) (i) use of = gives NS = = 60 (turns) 1

(ii) max output power = 0.85 × 0.630 × 230 (= 123 W)

max number of lamps = 5 (no mark for non-integer answer)

[or efficiency = gives 0.85 = (and max IS = 10.3 (A))

max number of lamps = 5 ]2

(iii) fuse prevents transformer from overheating [or prevents transformer from supplying excessive currents]

1

(iv) (all of) transformer is disconnected from supply when fuse fails [or fuse in secondary circuit would leave primary circuit live]

1

(b) (i) The candidate’s writing should be legible and the spelling, punctuationand grammar should be sufficiently accurate for the meaning to be clear.


High level (good to excellent) 5 or 6 marks

The information conveyed by the answer is clearly organised, logical and coherent, using appropriate specialist vocabulary correctly. The form and style of writing is appropriate to answer the question.

The candidate states that the ac in the coil produces a constantly changing magnetic field that passes through the ring, causing an emf to be induced according to Faraday’s law.

The candidate recognises that the induced emf will cause a current to flow in the ring, that the current is likely be large because the coil acts as a single conductor with low resistance, and that this current also produces a magnetic field.

The candidate appreciates that Lenz’s law indicates that the direction of the induced current is such as to produce a magnetic field that will oppose the existing field, and that the two fields will interact.

The candidate refers to the force that acts on a current-carrying conductor when it is in a magnetic field and that this force lifts the ring upwards (into an area where the magnetic field is weaker) until the upwards magnetic force is equal to the downwards weight of the ring.

Intermediate level (modest to adequate) 3 or 4 marks

The information conveyed by the answer may be less well organised and not fully coherent. There is less use of specialist vocabulary, or specialist vocabulary may be used incorrectly. The form and style of writing is less appropriate.

The candidate is familiar with either or both Faraday’s and Lenz’s laws but only applies one of them to explain what happens in this demonstration. There are correct references to the two forces that act on the ring, and a reasonable explanation of why the ring reaches a stable position.

Low level (poor to limited) 1 or 2 marks

The information conveyed by the answer is poorly organised and may not be relevant or coherent. There is little correct use of specialist vocabulary. The form and style of writing may be only partly appropriate.

The candidate refers much more superficially to either Faraday’s or Lenz’s law (or to both of them) but shows some understanding of why the forces acting on the ring cause it to reach equilibrium.

The explanation expected in a competent answer should include a coherent selection of the following points concerning the physical principles involved and their consequences in this case.

Faraday’s law

• An emf is induced whenever there is a change in the magnetic flux passing through a conductor.

• The magnitude of the emf is proportional to the rate of change of magnetic flux linkage.

• The induced emf will cause a current to flow in any complete circuit, such as a single conducting ring.

• Because the ring is made from aluminium, which is a good conductor, a large initial current will be induced in it.

Lenz’s law

• The induced current flows in such a direction as to oppose the increase in magnetic flux when the current is switched on in the coil.

• The current produces a magnetic field in the opposite direction to that produced by the coil.

• These two (alternating) fields interact like the fields between two facing like magnetic poles, giving repulsion.

Forces

• The ring is a current-carrying conductor in a magnetic field, andconsequently it experiences a force.

• This magnetic force acts upwards, in the opposite direction to theweight of the ring.

• As the ring rises, the magnetic field to which it is exposed becomesweaker as it moves away from the coil.

• This reduces the induced current, reducing also the magnetic forceon the ring.

• The ring reaches a stable height when the magnetic force hasdecreased to the point where it is equal to the weight of the ring.

6

(ii) ring would ‘float’ higher [or be expelled upwards]

because (initial) current or emf (induced) in ring is greater

or ring moves into weaker field until magnetic force balances weight [or (initially) magnetic force exceeds weight]

2[13]

M6. (a) work done (or energy required) per unit mass

in moving a mass from infinity to the point 2

(b) (i) ΔV (= – 1.3 – (–62.6)) = 61.3 (MJ kg–1)

energy required (= mΔV) = 1.2 × 104 × 61.3 × 106

= 7.4 × 1011 (J) to 2SF only 3

(ii) beyond X, gravitational potential decreases as Moon is approached [or gravitational field (or force) of Moon will now attract the probe]

1

(iii) distance from Earth to Sun » distance from Earth to Moon

change in Vsun (or in gsun) over Earth to Moon distance is negligible

value of Vsun (or gsun) is not (significantly) changed by relative positions of E+M

2

(c) The candidate’s writing should be legible and the spelling, punctuation and grammar should be sufficiently accurate for the meaning to be clear.

The candidate’s answer will be assessed holistically. The answer will be assigned to one of three levels according to the following criteria.

High Level (Good to excellent): 5 or 6 marksThe information conveyed by the answer is clearly organised, logical and coherent, using appropriate specialist vocabulary correctly. The form and style of writing is appropriate to answer the question.The candidate discusses the forces of attraction due to the Earth and due to the Moon, appreciates that they act in opposite directions, and that the former is generally much greater than the latter.The candidate discusses the resultant gravitational field between E and M, understands that there is a ‘neutral’ point at which the resultant field strength is zero and that this point is much closer to M than E. It is recognised that this point has to be passed for the journey to be completed in either direction.There is a discussion of gravitational potential, in which it is pointed out that the resultant potential rises to a maximum at the neutral point. There is a reference to the much greater amount of work that has to be done on the spacecraft to reach this point from E than from M.

Intermediate Level (Modest to adequate): 3 or 4 marksThe information conveyed by the answer may be less well organised and not fully coherent. There is less use of specialist vocabulary, or specialist vocabulary may be used incorrectly. The form and style of writing is less appropriate.

The candidate discusses the forces of attraction due to the Earth and the Moon, and appreciates either that they act in opposite directions, or that the former is much greater than the latter. There is a relevant discussion of field strength or potential. The significance of the neutral point may not be appreciated. The candidate is likely to make some reference to the work that has to be done on the spacecraft.

Low Level (Poor to limited): 1 or 2 marksThe information conveyed by the answer is poorly organised and may not be relevant or coherent. There is little correct use of specialist vocabulary. The form and style of writing may be only partly appropriate.The candidate has some understanding of the forces that act during the journey but makes very limited references to the significance of the variation of the gravitational field. Discussions of gravitational potential and/or work done are likely to be superficial and may be absent.


Gravitational forces

• The spacecraft experiences gravitational attractions to both the Earth and the Moon during its journey.

• These forces pull in opposite directions on the spacecraft.

• Because E is much more massive than M, for most of the outward journey the force towards E is greater than that towards M.

• Only in the later stages of the outward journey is the resultant force directed towards M.

• On the return journey the resultant force is predominantly towards E.

Gravitational field strength

• During the outward journey E’s gravitational field becomes weaker and M’s becomes stronger.

• The resultant field is the vector sum of those due to E and M separately.

• A point (X) is reached at which these two component fields are equal and opposite, giving zero resultant.

• X is much closer to M than E.

• Once X has been passed, the spacecraft will be attracted to M by M’s gravitational field.

• On the return journey the spacecraft will ‘fall’ to E once it is beyond X.

Gravitational potential

• The gravitational potential due E increases (i.e. becomes less negative) as the spacecraft moves away from E.

• The resultant gravitational potential is the (scalar) sum of those due to E and M separately.

• At X the gravitational potential reaches a maximum value before decreasing as M is approached.

• In order to reach M on the outward journey, the spacecraft has to be given at least enough energy to reach X, and vice-versa for the return.

• Much more work is needed to move the spacecraft from E to X than from M to X, since a larger force has to be overcome over a larger distance.

6[14]

M7.(a) (i)

from which = 5.73

and rE (= 5.73 × 7370) = 42 200 (km)

height above surface = 42 200 − 6370 = 35 800 or 35 900 (km) answer to 3SF only

Full solution derived from Newton’s law of gravitation is acceptable for all 4 marks.

[or Newton ’s law approach for 1st two marks:

∴rE3 = (=7.54 × 1022)

from which rE = 42 200 (km) ]For 3 rd mark, final answer must be expressed in km.3SF mark is independent.

4

(ii) centripetal force (= m ɷ2r) = = 4800 (4760) (N) If both T and r values for the geosynchronous satellite are substituted, award 0 marks for (ii).

[or centripetal force and v =

gives v = 7350 (m s−1) and centripetal force =

= 4800 (4760) (N) ]

[or centripetal force

= 4800 (4770) (N) ]If only one correct T or r value for the polar satellite is substituted, mark (ii) to max 1.

2

(b) The candidate’s writing should be legible and the spelling, punctuation and grammar should be sufficiently accurate for the meaning to be clear.


High Level (Good to excellent): 5 or 6 marksThe information conveyed by the answer is clearly organised, logical and coherent, using appropriate specialist vocabulary correctly. The form and style of writing is appropriate to answer the question.

Four aspects must be considered in a high level answer:-Features of polar orbit.Features of geosynchronous orbit.Why polar orbit is suitable for monitoring.Why geosynchronous orbit is suitable for communication.

The candidate gives a comprehensive comparison of the principal features of the satellite orbits and explains the consequences for the uses of the two types of satellites. There are clear statements showing good understanding of why the polar satellite is suitable for monitoring, and of why the geosynchronous satellite is useful for communications.


The candidate’s comparison of the principal features of the orbits is less complete and the consequences for the uses of satellites in them are less well understood. The candidate has an acceptable appreciation of why the polar satellite is suitable for monitoring, and of why the geosynchronous satellite is useful for communications.

Low Level (Poor to limited): 1 or 2 marksThe information conveyed by the answer is poorly organised and may not be relevant or coherent. There is little correct use of specialist vocabulary. The form and style of writing may be only partly appropriate.

The candidate has a much weaker knowledge of the principal features of the orbits and very limited knowledge of consequences for the uses of satellites in them. Understanding of why the polar satellite is suitable for monitoring, and why the geosynchronous satellite is suitable for communications, is limited or absent.

The explanation expected in a competent answer should include a coherent selection of the following points.

Low polar orbit

• Orbital period is a few hours• Earth rotates relative to the orbit• Many orbits with different radii and periods are possible• Orbit height is less than geosynchronous satellite• Speed is greater than that of geosynchronous satellite• Satellite scans the whole surface of the Earth• Applications: surveillance of conditions / installations on Earth, mapping, weather

observations, environmental monitoring• Gives access to every point on Earth’s surface every day• Can collect data from regions inaccessible to man

• Contact with transmitting / receiving aerial is intermittent• Aerial is likely to need a tracking facility• Lower signal strength required than that for geosynchronous satellite

Geosynchronous orbit above Equator

• Orbital period matches Earth’ rotational period exactly• Satellite maintains same position relative to Earth• Only one particular orbit radius is possible• Travels west to east above Equator (in same direction as Earth’s rotation)• Orbit height is greater than polar orbit satellite• Speed is less than that of polar orbiting satellite• Scans a restricted (and fixed) area of the Earth’s surface only• Applications: telecommunications generally, cable and satellite TV, radio, digital

information, etc.• Satellite is in continuous contact with transmitting / receiving aerial• Aerial can be in a fixed position• Higher signal strength required than that for polar satellite

max 6[12]

M8.The mark scheme for this part of the question includes an overall assessment for the Quality of Written Communication (QWC).

QWC

Descriptor

Mark range

High Level (Good to excellent)

The candidate refers to all the necessary apparatus and records the count-rate at various distances (or thicknesses of absorber). The background is accounted for and a safety precaution is taken. The presence of an α source is deduced from the rapid fall in the count rate at 2 – 5 cm in air. The presence of a ɣ source is deduced from the existence of a count-rate above background beyond 30 -50 cm in air (or a range in any absorber greater than that of beta particles, e.g. 3 – 6 mm in Al) or from the intensity in air falling as an inverse square of distance or from an exponential fall with the thickness of a material e.g. lead. The information should be well organised using appropriate specialist vocabulary. There should only be one or two spelling or grammatical errors for this mark.

If more than one source is used or a different experiment than the question set is answered limit the mark to 4

5-6

Intermediate Level (Modest to adequate)

The candidate refers to all the necessary apparatus and records the count-rate at different distances (or thicknesses of absorber). A safety precaution is stated. The presence of an α source is deduced from the rapid fall in the count rate at 2 – 5 cm in air and the ɣ source is deduced from the existence of a count-rate beyond 30 -50 cm in air (or appropriate range in any absorber, e.g. 3 -6 mm in Al). Some safety aspect is described. One other aspect of the experiment is given such as the background. The grammar and spelling may have a few shortcomings but the ideas must be clear.

To get an idea of where to place candidate look for 6 items:

1.Background which must be used in some way either for a comparison or subtracted appropriately2.Recording some data with a named instrument

3-4

Low Level (Poor to limited)

The candidate describes recording some results at different distances (or thicknesses of absorber) and gives some indication of how the presence of α or ɣ may be deduced from their range. Some attempt is made to cover another aspect of the experiment, which might be safety or background. There may be many grammatical and spelling errors and the information may be poorly organised.

3.Safety reference appropriate to a school setting – not lead lined gown for example4.Record data with more than one absorber or distances5.α source determined from results taken6.ɣ source determined

1-2

The description expected in a competent answer should include a coherent selection of the following points.

apparatus: source, lead screen, ruler, ɣ ray and α particle detector such as a Geiger Muller tube, rate-meter or counter and stopwatch, named absorber of varying thicknesses may be used.

safety: examples include, do not have source out of storage longer than necessary, use long tongs, use a lead screen between source and experimenter.

measurements: with no source present switch on the counter for a fixed period measured by the stopwatch and record the number of counts or record the rate-meter reading

with the source present measure and record the distance between the source and detector (or thickness of absorber)

then switch on the counter for a fixed period measured by the stopwatch and record the number of counts or record the rate-meter reading

repeat the readings for different distances (or thicknesses of absorber).from results takenthis is a harder mark to achieveit may involve establishing an inverse square fall in intensity in air or an exponential fall using thicknesses of leadif a continuous distribution is not used an absorber or distance in air that would just eliminate ɣ (30-50cm air / 3-6mm Al) must be used with and without the source being present or compared to background

use of measurements:

for each count find the rate by dividing by the time if a rate-meter was not used

subtract the background count-rate from each measured count-rate to obtain the corrected count-rate

longer recording times may be used at longer distances (or thickness of absorber).

plot a graph of (corrected) count-rate against distance (or thickness of absorber) or refer to tabulated values

plot a graph of (corrected) count-rate against reciprocal of distance squared or equivalent linear graph to show inverse square relationship in air

analysis:

the presence of an α source is shown by a rapid fall in the (corrected) count-rate when the source detector distance is between 2 – 5 cm in air

the presence of a ɣ source is shown if the corrected count-rate is still present when the source detector distance is greater than 30 cm in air (or at a range beyond that of beta particles in any other absorber, e.g. 3 mm in Al)

the presence of a ɣ source is best shown by the graph of (corrected) count-rate against reciprocal of distance squared being a straight line through the origin

6[6]

M9.The marking scheme for this part of the question includes an overall assessment for the Quality of Written Communication (QWC). There are no discrete marks for the assessment of written communication but the quality of written communication will be one of the criteria used to assign the answer to one of three levels.

The candidate’s writing should be legible and the spelling, punctuation and grammar should be sufficiently accurate for the meaning to be clear.

There are three areas:Structure: silicon chip into pixels


Function: photon incident, electron excited, electron trapped in potential well, one electron per photon, no of electrons (and therefore charge) proportional to number of incident photons, after sufficient exposure charge on each pixel measured and image producedAdvantage: most will say the QE>70%

High Level (Good to excellent): 5 or 6 marksThe information conveyed by the answer is clearly organised, logical and coherent, using appropriate specialist vocabulary correctly. The form and style of writing is appropriate to answer the question.

A 6 mark answer need not be “perfect” but should be substantially complete, correct and free from major errors. One of the above points may be missing. Eg charge integration

The candidate provides a comprehensive and logical description of the structure of the CCD. The answer includes a clear description of how the light causes a release of charge and why the charge is stored. The answer also includes an explanation of what is meant by quantum efficiency and a correct value for the q.e. of a CCD.Confusion with the photoelectric effect would reduce a 6 mark answer to 5.

5 marks may have 2 missing eg silicon chip and charge integration


4 probably has more than 2 missing or no correct advantage

The candidate provides a comprehensive and logical description of the CCD. The answer demonstrates some understanding of how the light is used to generate charge. The answer also includes some reference the efficiency of the CCD or other advantage

Low Level (Poor to limited): 1 or 2 marks.The information conveyed by the answer is poorly organised and may not be relevant or coherent. There is little correct use of specialist vocabulary. The form and style of writing may be only partly appropriate.

The candidate demonstrates an understanding that an image is formed on the CCD and that this image is transferred to a computer.

Zero: Incorrect, inappropriate or no response.

Points that can be used to support the explanation:

• The CCD is a silicon chip• The chip is divided into picture elements• Each picture element is associated with a potential well in the silicon• Incident photons are focused on the CCD• The photons cause the release of electrons within the semiconductor• The number of electrons liberated is proportional to the intensity of the light.• Electrons are trapped in the potential wells• An electron pattern is built up which is identical to the image formed on the CCD• When exposure is complete the charge is processed to form an image.

Advantages:High quantum efficiency > 70%Light integration – using long exposure times to capture faint images.Device can be directly linked to computer for capture and analysis.

6[6]

M10.(a) The candidate’s writing should be legible and the spelling, punctuation and grammar should be sufficiently accurate for the meaning to be clear.The candidate’s answer will be assessed holistically. The answer will be assigned to one of three levels according to the following criteria.

Good to ExcellentThe information conveyed by the answer is clearly organised, logical and coherent, using appropriate specialist vocabulary correctly. The form and style of writing is appropriate to answer the question.

The candidate explains the principles of transfer of vibrations, from mechanical vibration of the ear drum, through mechanical oscillations of the malleus, incus and stapes acting as a lever system, producing mechanical vibration of the oval window and then pressure waves in the fluid in the cochlea. They use the correct names of the relevant parts of the earThey then explain the increase in pressure with sensible use of numbers, reduction in area of about 20 and increase in force of about 1.5, resulting in pressure increase of about 30.

5-6

Modest to AdequateThe information conveyed by the answer may be less well organised and not fully coherent. There is less use of specialist vocabulary, or specialist vocabulary may be used incorrectly. The form and style of writing is less appropriate.

The candidate explains some of the principles of transfer of vibrations and mentions some of the names of the relevant parts of the ear. They talk about the increase in pressure, but may fail to add relevant numbers

3-4

Poor to LimitedThe information conveyed by the answer is poorly organised and may not be relevant or coherent. There is little correct use of specialist vocabulary. The form and style of writing may be only partly appropriate.

The candidate explains a principle of transfer of vibrations or explains the increase in pressure and mentions at least one of the names of the relevant parts of the ear

2-1

Incorrect, Inappropriate or No ResponseNo answer at all or answer refers to unrelated, incorrect or inappropriate physics.

0

The explanation expected could include the following:Outer ear acts as a funnel gathering waves into the ear canalpressure waves incident on eardrumeardrum vibrates, mechanical vibrationsmechanical vibrations passed through a system of three bones acting as leversmalleus, incus and stapesthe last bone sets the oval window into mechanical vibrationthis produces pressure waves in the liquidin the cochlea.three bone lever system increase force by about 1.5 (1.3 to 1.7) times cross sectional area of the oval window about 20 (15 to 25) times less than the cross sectional area of the eardrum larger force / smaller area gives pressure about 30 times greater, (any answer for pressure, to agree with other values quoted)

(b) Intensity is the power per unit (cross-sectional) area (in path of the wave) At normal incidence

2

(c) rearrange equation to give I = correct answer 2.5(12) × 10−7 W m−2 correct to 2 sig figs

3[11]

M11.(a) (Adiabatic change requires) no heat transfer / energy transfer / heat to escape / heat loss (to surroundings)

Do not accept heat or energy ‘change’ .

(Compression stroke occurs in short time / very quickly) so no time for heat transfer

(Therefore change can be considered to be adiabatic)2

(b) (i) P1V1γ = P 2V2

γ

Significant figure mark is an independent mark

1.0 × 10 5 × (4.5 × 10−4)1.4 = 6.2 × 106 × V21.4

V2 = 2.4 × 10−5 m3 2 sig fig 3

(ii) use of CE from b i

If 2.36 × 10−5 m3used for V2, T2 = 966 K

OR use of n = p1 V1 / R T1 and T2 = p2V2 / nR

Leading to T2 = 982 K 2

(iii) So that the fuel has partially evaporated / started to burn when piston is at top of stroke (so max pressure obtained when piston is at top of stroke / top dead centre).

Accept ‘diesel’ instead of ‘fuel’

OR If injected at top dead centre. by the time fuel has started to burn, piston would be on its way down cylinder, (so max possible pressure not obtained).

1

(c) Good – Excellent

The information conveyed by the answer is clearly organized, logical and coherent, using appropriate specialist vocabulary correctly. The form and style of writing is appropriate to answer the question.

The candidate gives a comprehensive account of the differences between the two cycles,with reasons. There are clear statements relating to the need for induction and exhaust processes / strokes in a real engine only, that adiabatic processes are not possible in the real engine, that constant pressure and constant volume processes are impossible, and / or that the corners of the real engine diagram are rounded.

They will refer to the lower efficiency of the real engine, linking this to the smaller area loop, or the fact that the pumping loop has to be subtracted from the main loop and/or that heat transfers occur during compression and expansion and that in the real engine friction has to be overcome / power has to be expended in driving ancillaries.

5-6

Modest – Adequate

The information conveyed in the answer may be less well organized and not fully coherent. There is less use of specialist vocabulary or specialist vocabulary may be used or spelled incorrectly. The form and style of writing is less appropriate.

The candidate’s comparisons are less complete but good understanding is shown of some of the major differences between the diagrams, with some reasons given.

They should be able to give at least one valid reason for the lower efficiency of the real engine cycle.

3-4

Poor – Limited

The information conveyed by the answer is poorly organized and may not be relevant or coherent. There is little correct use of specialist vocabulary.

The candidate is more likely to describe the differences rather than explain them. They are likely to make reference to the rounded corners, and the induction / exhaust strokes in the ‘real’ diagram, but not be able to say why these do not exist in the theoretical diagram. They may not be able to give a valid reason for the lower efficiency of the real engine cycle, or may give vague reasons in terms of ‘heat losses’ or ‘friction’ without further detail.

The descriptions and explanations expected in a good answer should include several of the following physics ideas

• Real engine needs ‘pumping loop’ at near atmos. pressure for induction and exhaust

• Work needed for induction and exhaust – so efficiency lower than theoretical

• Area of pumping loop has to be subtracted from main loop, hence reducing net work and hence efficiency of real engine

• Theoretical cycle needs no pumping loop / same air continuously taken through repeated cycles

• Corners rounded on real engine diagram [because valves are needed and take finite time to open/close]

• Cooling cannot occur at constant volume in real engine [because piston would have to stop]

• Heating does not occur at constant pressure [because impossible to control rate of burning of fuel during injection]

• Compression and expansion do not take place infinitely quickly heat is lost; therefore not adiabatic processes, lowering efficiency

• Area of loop is smaller for real engine, less work done per cycle so lower efficiency

• Friction between moving surfaces has to be overcome / energy expended in driving oil and water pumps, opening and closing valves etc.

• Always some exhaust gases present in cylinder.

• Theoretical cycle does not make reference to any mechanism

• Calorific value of fuel is never fully realised2-1

[14]

M12.(a) Quality of written communication:

Good – Excellent

The candidate provides a comprehensive, coherent and logical explanation which recognises what a stationary wave is and that the conditions for the formation of a stationary wave are present. They should know that nodes and antinodes are formed at alternate positions along XY which are equally spaced with nodes every half wavelength. They should know how the detector is used to locate the position of each node or antinode and how the wavelength is determined from the distance between two such positions. They may know that the nodes can be located more accurately than the antinodes and that their chosen two positions should be as far apart as possible.

Their answer should be well-presented in terms of spelling, punctuation and grammar.For top band,explanation = at least b and edescription = at least f, g,h

(5-6 marks)

Modest – Adequate

The candidate provides a logical explanation which recognises what a stationary wave is and what some of the conditions for the formation of a stationary wave are. They may know that nodes and antinodes are formed at alternate positions along XY with nodes every half- wavelength. They may know how the detector is used to locate the position of each node or antinode and how the wavelength is determined from the distance between two such positions. They may know that the nodes can be located more accurately than the antinodes and that their chosen two positions should be as far apart as possible.Their answer should be well-presented in terms of spelling, punctuation and grammar.

For middle band ,explanation = at least any two of a-edescription = at least any two of f-i

(3-4 marks)

Poor to Limited

The candidate may recognise that the reflector reflects radio waves which then form a stationary wave pattern with the incident waves. They may be unaware what the conditions for the formation of a stationary wave are and their understanding of nodes and antinodes may be poor. They may have some awareness that the stationary wave causes the detector signal to vary with position along XY and that the wavelength can be determined from this variation although they might not be able to link the wavelength to the changes of detector position correctly.

Their answer may lack coherence and may contain a significant number of errors in terms of spelling and punctuation.

For lowest band,Any 2 points ,must be 1 of each for 2 marks

The explanations expected in a good answer should include most of the following physics ideas

Explanation of stationary wave formation;-a. radio waves from the transmitter are reflected back towards the transmitter b. reflected and incident waves pass through eachother

c. both waves have same frequency (and speed) andamplitude d. superposition (of reflected and incident waves)occurs to form a stationary wave (as above) e. equally spaced nodes and antinodes formed along XY Description of measurement of wavelength;-f. Detector signal is zero ( or least) along XY atnodes g. distance between adjacent nodes is ½λ h. move detector along XY to measure distancebetween adjacent nodes and double to give thewavelength i. measure distance over n nodes and divide by n-1to give distance between adjacent nodes

6(1-2 marks)

(b) Speed of radio waves (obtained by Hertz ) is the same as the speed of light

Speed of electromagnetic waves (calculated or predicted by Maxwell) is the same as the speed of light ( or of radio waves) so radio waves are electromagnetic waves

2(Total 8 marks)

M13. (a) graph starting (steeply) near/at the origin and decreasing in gradient 1

(b) (i) (use of density = mass/volume)

mark for top line and mark for bottom line

(allow use of 1.66 x 10-27)

Lose mass line mark if reference is made to mass of electrons

= 2.4(2) × 1017 kg m-3

2

(ii)

= 3.54 × 10-15 m

or

m

m

or

volume = mass/density =

= 3.54 × 10-15 m 2





The candidate makes 5 to 6 points concerning the principles of the method, the limitations to the accuracy and the advantages and disadvantages of a particular method







The explanation expected in a competent answer should include a coherent selection of the following points concerning the physical principles involved and their consequences.

principles

• α scattering involves coulomb or electrostatic repulsion

• electron diffraction treats the electron as a wave having a de Broglie wavelength

• some reference to an equation, for example λ = h/mv ; eV = mv2/2 ; Qq/4πεor = Eα ; sinϴ = 0.61λ/R

• reference to first minimum for electron diffraction

accuracy

• α’s only measure the least distance of approach, not the radius

• α’s have a finite size which must be taken into account

• electrons need to have high speed/kinetic energy

• to have a small wavelength or wavelength comparable to nuclear diameter, the wavelength determines the resolution

• the wavelength needs to be of the same order as the nuclear diameter for significant diffraction

• requirement to have a small collision region in order to measure the scattering angle accurately

• importance in obtaining monoenergetic beams

• cannot detect alpha particles with exactly 180° scattering

• need for a thin sample to prevent multiple scattering

advantages and disadvantages

• α-particle measurements are disturbed by the nuclear recoil

• Mark for α-particle measurements are disturbed by the SNF when coming close to the nucleus or electrons are not subject to the strong nuclear force.

• A second mark can be given for reference to SNF if they add electrons are leptons or alpha particles are hadrons.

• α’s are scattered only by the protons and not all the nucleons that make up the nucleus

• visibility – the first minimum of the electron diffraction is often difficult to determine as it superposes on other scattering events

6[11]

M14. (a) the percentage of photons hitting the CCD which are detected and/or producea signal

1

(b) (i) use of ϴ = λ/D

to give ϴ = 750 × 10–9/0.60

= 1.25 × 10–6 (rad) 1

(ii) use of s = rϴ

to give ϴ = 5 × 1.5 × 1011 /10.5 × 9.46 × 1015

= 7.55 × 10–6 (rad)3

(iii) either

answer (b)(i) is theoretical limit – and in reality resolving power will be muchpoorer than this (due to atmosphere etc)

or

planets will be far too dim to see (next to star) 1





The candidate provides a comprehensive and logical explanation which considers the detection of three named parts of the electromagnetic spectrum.The candidate describes how the optimum siting of a telescope is determined by the effect the atmosphere and, for full marks, other factors, such as light pollution, have on three named parts of the electromagnetic spectrum.They also demonstrate an understanding of how resolving power, and for full marks, the collecting power, of a telescope is affected by the size of the aperture, and relate the resolving power to the wavelengths of the three different parts of the electromagnetic spectrum.



The candidate provides a comprehensive and logical explanation which names two or three parts of the electromagnetic spectrum and discusses both siting and size for at least two for 4 marks, or for one of them for 3 marks.The candidate may recognise that some telescopes need to be in orbit due to the absorption of some parts of the em spectrum by the atmosphere. They may also discuss the positioning of telescopes eg they should be high up to reduce absorption of IR. Their

answer may refer to only resolving or collecting power when discussing the size of telescopes and there may be little attempt or, for three marks, no attempt to relate resolving power to wavelength.



The candidate recognises some telescopes are in orbit, and for two marks, they may describe a part of the electromagnetic spectrum being detected.They may confuse which parts of the electromagnetic spectrum are absorbed by the atmosphere and which pass through. They may make, for two marks a vague reference to the size of telescopes, and for one mark they may make no reference at all.


Siting

• Apart from visible and some parts of the radio wave section, all the other parts of the em spectrum are significantly absorbed by the atmosphere.

• To reduce the effects of absorption, IR telescopes are often placed in dry areas and/or very high up.

• UV is significantly absorbed by the ozone layer, so UV telescopes are generally put into orbit.

• X-ray telescopes are also put into orbit to avoid atmospheric absorption.

• To avoid atmospheric distortion, visible telescopes are often placed high up.

• To avoid interference from terrestrial sources, radio telescopes may be situated away from centres of population.

• To avoid light pollution, visible telescopes are often placed a long way from centres of population.

Size

• Telescopes are often built as large as possible in order to increase the collecting power, which is proportional to the diameter2.

• The diameter of the objective of telescopes is also often as large as possible in order to improve the resolving power, as minimum angle resolved is proportional to 1/diameter.

6[12]

M15. (a) The candidate’s writing should be legible and the spelling, punctuation and grammar should be sufficiently accurate for the meaning to be clear.




The answer will discuss the multi-array of transducers in a linear formation and the use of gel between the skin and the probe will be explained. There will be mention of the transducers acting as receivers and why ultra sound echoes occur. There will be some discussion of the processing of the received signal to produce an image. The fact that this is non-ionising and thus has no known side effects will be included.



The answer will contain at least one property of the probe and either the use of gel or the transducer acting as a receiver should be discussed. The processing of the signal will be sketchy, but the reason that ultrasound is safe is likely to be mentioned.



There will be a few of the guidance points mentioned, but there will be little cohesion in the writing.


Method of obtaining the image

Ultra sound reflected at interface between two different acoustic impedances

Each transducer emits pulse in turn and receives the echoes from the interfaces directly in line with it

Each echo displayed as a bright spot on screen

The brightness is determined by the intensity of the echo

The y position is determined by the time taken from transmission to the time of the echo

The x position is determined by the position of the transducer

Images are produced at about 25 per second and thus appear as a real time moving image

Practical considerations

Probe has line of transducers (approx 100)

High frequency ac pulse applied to each transducer in turn

Each transducer has piezoelectric crystal to generate ultra sound

Use of gel between probe and skin to eliminate air

Transducer acts as receiver

Safety

No harmful side effects known – does not use ionising radiation.

Always allow details of other correct probes.6

(b) The transducer to be damped/stop oscillating before the echo returns to allow the transducer to act as a receiver.

(This time is very short) as distances travelled are short

Emitted pulse must cease before echo arrives so that there is no overlapping at the transducer/ no interference

3[9]

M16. (a) Either W = area under (engine torque) graph from 0 to 2π rad

OR W = area under graph because W = Tϴ

OR W = dynamo torque × 2π

OR W = area under dotted line / dynamo torque because W = Tϴ 1

(b) The candidate’s writing should be legible and the spelling, punctuation and grammar should be sufficiently accurate for the meaning to be clear.




The candidate is aware that at two points in the cycle the engine torque is zero and can give a reason, perhaps mentioning moments or variation in steam pressure

The candidate identifies the flywheel as a store of rotational kinetic energy and can relate the energy changes in one cycle to the varying torque and clearly relates the fluctuation in speed to the value of the M of I of the flywheel.

Alternatively, the candidate states that the flywheel tends to maintain angular momentum and so takes the crank over the dead centres. The changing torque has the effect of changing the angular momentum (I∆ω) but if I is large, ∆ω is small.The candidate may go on to discuss effect of I being very large (e.g long acceleration time from start).


The information conveyed by the answer may be less well organised and not fully coherent. There is less use of specialist vocabulary, or specialist vocabulary may be used incorrectly. The form and style of writing is less appropriate.The candidate correctly identifies that a flywheel will make for smoother motion and may show an understanding that a flywheel acts as an energy reservoir, but may not be able to link the motion of the flywheel to the engine torque graph. Candidates answering in terms of angular momentum appreciate that the flywheel’s angular momentum will take it over the dead centres. The candidate identifies that an increase in moment of inertia gives smoother running/less variation in speed per cycle. Reasons for the variation in torque may not refer to moments but candidate may state that torque is zero when crank and con rod are in line.


The information conveyed by the answer is poorly organised and may not be relevant or coherent. There is little correct use of specialist vocabulary. The form and style of writing may be only partly appropriate.The candidate may be able to give a reason why the motion is not smooth and can identify that a flywheel will make for smoother running. There may be some reference to the flywheel storing energy. They may confuse power or angular momentum with energy.


• without flywheel motion will be jerky/unsmooth/cause vibrationsOR flywheel makes motion smoother/less fluctuation in speed

• flywheel needed to take crank over dead centres (wtte)

• because torque is zero at dead centres

• torque varies because pressure on piston varies

• because force is in line with c’shaft/ no moment of force about c’shaft

• flywheel stores rotational kinetic energy when engine torque > dynamo torque

• flywheel gives up energy when engine torque < dynamo torque

• flywheel’s ang. momentum takes it over dead centres

• the greater I, the less the fluctuation in speed over one cycle

• over one cycle, work done by engine = work needed by dynamo

• so average engine torque = average dynamo (load) torque

• torque = rate of change of ang. momentum – high I gives less change in ω.6

[7]

M17. (a) The candidate’s writing should be legible and the spelling, punctuation and grammar should be sufficiently accurate for the meaning to be clear.




The candidate provides a comprehensive and coherent answer that includes a stated property of light such as interference or diffraction that can only be explained in terms of the wave nature of light and a stated property such as photoelectricity that can only be explained in terms of the particle nature of light. In each case, a relevant specificobservational feature should be referred to and should be accompanied by a coherent explanation of the observation. Both explanations should be relevant and logical.

For full marks, the candidate may show some appreciation as to why the specific feature of either the named wave property cannot be explained using the particle nature of light or the named particle property cannot be explained using the wave nature of light.



The candidate provides a logical and coherent explanation that includes a stated property of light such as interference or diffraction that can only be explained in terms of the wave nature of light and a stated property such as photoelectricity that can only be explained in terms of the particle nature of light.

For 4 marks, the candidate should be able to refer to a relevant specific observational feature of each property, at least one of which should be followed by an adequate explanation of the observation. Candidates who fail to refer to a relevant specific observational feature for one of the properties may be able to score 3 marks by providing an adequate explanation of the observational feature referred to.



The candidate provides some relevant information relating to two relevant stated properties for 1 mark. Their answer may lack coherence and may well introduce irrelevant or incorrect physics ideas in their explanation.


Wave-like nature property

• property is either interference or diffraction

• observational feature is either the bright and dark fringes of a double slit interference pattern or of the single slit diffraction pattern (or the spectra of a diffraction grating)

• explanation of bright or dark fringes (or explanation of diffraction grating spectra) in terms of path or phase difference

• particle/corpuscular theory predicts two bright fringes for double slits or a single bright fringe for single slit or no diffraction for a diffraction grating

Particle-like nature

• property is photoelectricity

• observational feature is the existence of the threshold frequency for the incident light or instant emission of electrons from the metal surface

• explanation of above using the photon theory including reference tophoton energy hf, the work function of the metal and ‘1 photon beingabsorbed by 1 electron’

• wave theory predicts emission at all light frequencies or delayedemission for (very) low intensity

6

(b) (i) m (= mo (1 - v 2 / c 2) –0.5 = 9.11 × 10–31 (1 - 0.8902)–0.5)

(= 1.998 × 10–30 kg) = 2.0(00) × 10–30 kg

(= 1.2(4) × 10–12m)2

(ii) = 1.6(0)× 10–13 J 1

(iii) EK= (m - mo) c2

= (1.998 × 10–30 – 9.11 × 10–31) × (3.0 × 108)2

= 9.78 × 10–14 J 3 sf only 2

[11]

M18. (a) graph passes through given point 2.2 × 10–3 m3 at 0 °C straightline with positive gradient

(straight) line to aim or pass through –273 °C at zero volume 2

(b) (use of n = P V/R T)

1.00 × 105 × 2.20 × 10–3/8.31 × 273

n = 0.0970 (moles) 2

(c) (use of mean kinetic energy = 3/2 K T)

= 3/2 × 1.38 × 10–23 × 323

6.69 × 10–21 (J) 3 sfs 3

(d) total internal energy = 6.69 × 10–21 × 0.0970 × 6.02 × 1023 = 390 (J) 1

(e) The candidate’s writing should be legible and the spelling,punctuation and grammar should be sufficiently accurate for themeaning to be clear.

The candidate’s answer will be assessed holistically. The answerwill be assigned to one of three levels according to the following criteria.


The information conveyed by the answer is clearly organised,logical and coherent, using appropriate specialist vocabularycorrectly. The form and style of writing is appropriate to answerthe question.

The candidate provides a comprehensive and coherent sequenceof ideas linking the motion of molecules to the pressure they exerton a container. At least three of the first four points listed below must begiven in a logical order. The description should also show awarenessof how a balance is maintained between the increase in speed andshortening of the time interval between collisions with the wall to maintaina constant pressure.To be in this band, reference must be made to force being the rate ofchange of momentum or how, in detail, the volume compensates for theincrease in temperature.


The information conveyed by the answer may be less well organisedand not fully coherent. There is less use of specialist vocabulary, orspecialist vocabulary may be used incorrectly. The form and style ofwriting is less appropriate.

The candidate provides a comprehensive list of ideas linking themotion of molecules to the pressure they exert on a container. At leastthree of the first four points listed below are given. The candidate alsoknows than the mean square speed of molecules is proportional totemperature. Using this knowledge, an attempt is made to explain howthe pressure is constant.


The information conveyed by the answer is poorly organised and maynot be relevant or coherent. There is little correct use of specialist vocabulary.The form and style of writing may be only partly appropriate.

The candidate attempts the question and refers to at least two of thepoints listed below.



Statements expected in a competent answer should include some ofthe following marking points.

molecules are in rapid random motion/many molecules are involved

molecules change their momentum or accelerate on collision withthe walls

reference to Newton’s 2nd law either F = ma or F = rate of change ofmomentum

reference to Newton’s 3rd law between molecule and wall

relate pressure to force P = F/A

mean square speed of molecules is proportional to temperature

as temperature increases so does change of momentum or change invelocity

compensated for by longer time between collisions as the temperatureincreases

as the volume increases the surface area increases which reduces thepressuremax 6

[14]

M19. (a)

main sequence curvature correct

giants and (white) dwarfs correct

absolute magnitude scale correct (from 15 to –10) 3

(b) (i) use of λmax T = 0.0029

gives T = 0.0029/3.4 × 10–7

= 8.5 × 103 (K) 3

(ii) use of PD/PS = σADTD4/(σASTS

4)

gives AD/AS = PDTS4/(PS TD

4)

= 70000(5700/8500)4

= 1.42 × 104

so rD/rs = √(1.48 × 104) = 119

gives rD = 119 × 6.96 × 108 = 8.28 × 1010 (m) 3

(c) The candidate’s writing should be legible and the spelling,punctuation and grammar should be sufficiently accurate forthe meaning to be clear.

The candidate’s answer will be assessed holistically. Theanswer will be assigned to one of three levels according tothe following criteria.



The candidate states that the atmosphere of the star containshydrogen with electrons in the n = 2 state and includes a cleardescription of the absorption process in the atmosphere of thestar, with reference to energy jumps corresponding to specificfrequencies of light. They describe at least one reason for thegap in the spectrum in terms of the de-excitation process.


The information conveyed by the answer may be less wellorganised and not fully coherent. There is less use of specialistvocabulary, or specialist vocabulary may be used incorrectly.The form and style of writing is less appropriate.

The candidate may not state that electrons start in the n = 2 state.Only one of the processes, excitation or de-excitation, issatisfactorily described. There should be some link betweenenergy and frequency but they may not make a clear referenceto E = hf.


The information conveyed by the answer is poorly organisedand may not be relevant or coherent. There is little correctuse of specialist vocabulary. The form and style of writing maybe only partly appropriate.The candidate recognises that changes in electron energy levels areinvolved. They may confuse absorption for emission and theirexplanation of why the frequency of the light is important may be vague. There may also be confusion between absorption due tothe star’s atmosphere and the Earth’s atmosphere.



Statements expected in a competent answer should includesome of the following

marking points.

• the atmosphere of the star has hydrogen atoms withelectrons in the n = 2 state

• light from the star passes through the atmosphere of the star

• electrons (in the n = 2) are excited into higher energy states

• they can only absorb certain amounts of energy

• these certain energies are related to specific frequencies (E=hf)

• the electrons then de-excite

• the electrons may de-excite through different energy level changes

• when the electrons de-excite the light is radiated in all directions

• this means that the intensity of the light at particular frequenciesis reduced, resulting in absorption lines

max 6[15]

M20. (a) The candidate’s writing should be legible and the spelling,punctuation and grammar should be sufficiently accuratefor the meaning to be clear.

The candidate’s answer will be assessed holistically. The answerwill be assigned to one of three levels according to the followingcriteria.



The candidate accurately describes measures to ensure goodcontact between the electrodes and the skin including the useof conducting gel. The candidate will mention the need for morethan one electrode and the need for the patient to remain relaxedand still. They will need at least one property of the amplifier.


The information conveyed by the answer may be less wellorganised and not fully coherent. There is less use of specialistvocabulary, or specialist vocabulary may be used incorrectly.The form and style of writing is less appropriate.

The candidate will include most measures to ensure good contactbetween electrodes and the skin. They might give a property of theamplifier or mention the need for the patient to remain relaxed and still.


The information conveyed by the answer is poorly organised andmay not be relevant or coherent. There is little correct use ofspecialist vocabulary. The form and style of writing may be onlypartly appropriate.

The candidate will mention electrodes connected to the skinand might make another sensible comment on the arrangement.

Statements expected in a competent answer should includesome of the following marking points.

To reduce contact resistance

• sandpaper skin to remove hairs and some dead skin

• apply conducing gel

• securely attach more than one electrode

To remove unwanted signals

• electrodes should be non-reactive

• patient to remain relaxed and still

• shielded leads/reducing interference from ac sources

Properties of amplifier

• amplifier has large input impedance/high gain/low noisemax 6

(b) (i) 0 marked where line meets axis with maximum value of 1

unit mark mV 2

(ii) uniform scale starts at 0 and has value 0.7 (0.9 to 0.5)at end of T wave

1

(iii) P depolarisation of atria

R depolarisation of ventricles (and repolarisation of atria)

T repolarisation of ventricles 3

[12]

M21. (a) (i) Newton’s other theories were successful (or Newton wasmore eminent so Newton’s view was accepted)

alternatives, Huygens’ theory was based on longitudinalwaves which cannot explain polarisation or

Huygens’ theory could not explain sharp shadows1

(ii) either

Newton predicted that light travels faster in glass thanin air, Huygens predicted the opposite

or

there was no evidence (for many years) that light travelsslower or faster in glass than in air

the speed of light in water (or glass) was (eventually) found to beless than the speed of light in air

diffraction/interference observations not conclusive max 2

(b) The candidate’s writing should be legible and the spelling,punctuation and grammar should be sufficiently accurate for themeaning to be clear.

The candidate’s answer will be assessed holistically. The answerwill be assigned to one of three levels according to the followingcriteria.



The candidate provides a comprehensive, coherent and logicalexplanation which recognises that the pattern is due to interferenceof light which is a wave property. They should know that at a brightfringe, the waves from the two slits are in phase and thereforereinforce each other and this can happen at positions where thepath difference is zero or a whole number of wavelengths.They may not refer to the need for the waves to be coherent.Their answer should be well-presented in terms of spelling,punctuation and grammar.


The information conveyed by the answer may be less well organisedand not fully coherent. There is less use of specialist vocabulary,or specialist vocabulary may be used incorrectly. The form and styleof writing is less appropriate.

The candidate provides a logical explanation which recognises thatinterference of light is a wave property. They should know either abright fringe is where the waves from the two slits are in phase or adark fringe is where they are out of phase by 180° and be awarethere are different positions where these conditions apply. Theymay know the general condition for the path difference for a brightfringe or a dark fringe although they may not recognise that thiscondition explains why there are more than two bright fringes.Their answer should be adequately or well-presented interms of spelling, punctuation and grammar.


The information conveyed by the answer is poorly organised andmay not be relevant or coherent. There is little correct use ofspecialist vocabulary. The form and style of writing may be onlypartly appropriate.

The candidate recognises that interference of light is a waveproperty and that the waves from the two slits reinforce at abright fringe or cancel at a dark fringe. They may confuse pathdifference and phase difference and their explanation of why there are more than two bright fringes may be vague or absent.Their answer may lack coherence and may contain a significantnumber of errors in terms of spelling and punctuation.



Statements expected in a competent answer should includesome of the following marking points.

the pattern is due to interference of light from the two slits

interference is a wave property

light from the two slits is in phase at a bright fringe and thereforereinforces

the path difference (from the central bright fringe to the two slits)is zero

either bright fringes are formed away from the centre wherever thepath difference is a whole number of wavelengths or dark fringesare formed away from the centre wherever the path difference isa whole number of wavelengths + a half wavelength

the path difference for the mth bright fringe from the centre is mwavelengths where m is any whole number

since m is any whole number, more than two bright fringes areobservedmax 6

[9]





The candidate can explain the role of the moderator and control rods inmaintaining a critical condition inside the reactor. The explanation is givenin a clear sequence of events and the critical condition is defined in terms ofneutrons. To obtain the top mark some other detail must be included. Suchas, one of the alternative scattering or absorbing possibilities or appropriatereference to critical mass or detailed description of the feedback to adjustthe position of the control rods etc.



The candidate has a clear idea of two of the following:the role of the moderator, the role of the control rods or can explain thecritical condition.



The candidate explains that a released neutron is absorbed by uranium tocause a further fission. Alternatively the candidate may explain one of thefollowing:the role of the moderator, the role of the control rods or can explain thecritical condition.

The explanation expected could include the following events thatcould happen to a released neutron.

a neutron is slowed by the moderator

taking about 50 collisions to reach thermal speeds

then absorbed by uranium-235 to cause a fission event

one neutron released goes on to cause a further fission is the criticalcondition

a neutron may leave the reactor core without further interaction

a neutron could be absorbed by uranium-238

a neutron could be absorbed by a control rod

a neutron could be scattered by uranium-238

a neutron could be scattered by uranium-235max 7

(b) it is easy to stay out of range or easy to contain an α source or ß/γ havegreater range/are more difficult to screen (1)

most (fission fragments) are (more) radioactive/unstable (1)

and are initially most likely to be beta emitters/(which also) emit γradiation/are neutron rich/heavy (1)

ionising radiation damages body tissue/is harmful (1)max 3

[10]





The candidate states that the distance an object is away can be determinedif its absolute magnitude is known and its apparent magnitude is measured.The candidate also gives a statement that the absolute magnitudes of somesupernovae is known and that evidence shows that the Universe isexpanding at a faster rate than when the supernovae were produced.



The candidate states that the distance to some supernovae can bedetermined, but the reasoning is much more limited.There is a statement that there is evidence that suggests that the expansionof the Universe is accelerating and that there is a controversy, but they maynot recognise that Hubble’s Law shows that the supernovae should bebrighter (ie closer).



The candidate recognises that there is a controversy about the expansion ofthe Universe. They may confuse the two methods of determining distanceand their explanation of why there is evidence for an accelerating Universemay be vague.


• the absolute magnitude of (some) supernovae is known, this allowssupernovae to be used as standard candles

• using the inverse square law (or from values of absolutemagnitudes) allows the distance to be calculated

• supernovae are very bright – so they can be seen in very distantgalaxies

• it has taken billions of years for the light from the most distantgalaxies to reach Earth; these supernovae were therefore producedwhen the Universe was young

• measurement of red shift (to measure velocity) and use of Hubble’sLaw shows that these supernovae are fainter than expected

• this indicates that the Universe is expanding faster now than whenthe supernovae exploded as the light has had to travel further toreach us than expected by a constant rate of expansion

max 6

(b) (i) use of z = v/c

to give z = 900 × 103/3 × 108

= 3(.00) × 10–3 (1)

1

(ii) use of v = Hd

to give d = v/H

= 900/65 (1)

= 13.8 (1)2

[9]

M24. (a) general shape, must be both positive and negative values (1)

action potential axis scale and unit – allow –70 to +30,or –90 to +20 mV(these values will be consistent in part a and part b)minimum –90 maximum +45 (1)

time scale and unit 0 to 6 ms – this will depend on curve drawn, pulselasting no less than 1 ms and no more than 6 ms (1)

3

(b) The candidate’s writing should be legible and the spelling,punctuation and grammar should be sufficiently accurate for themeaning to be clear.




The candidate provides a correct and detailed description of the movementof ions into and out of the fibre. They include the terms depolarisation andrepolarisation with reference to change in potential. Final mention is madeto the slower process to restore the equilibrium concentrations.



The description of ion movement and the terms depolarisation andrepolarisation might not be clearly named, but the candidate refers to thechange in potential, although actual values might not be included. Theremay be mention of a final movement restoring equilibrium.



Some reference to ion movement and a resulting change in potential. Oneof the terms depolarisation or repolarisation might be included.


Points which can be used to support the explanation:

• at resting potential, high concentration of K+ ions inside and Na+outside

• when stimulated, membrane becomes permeable to Na+ ionsentering the core increasing membrane potential

• good answer will say depolarisation from –70 mV to 0 mV andreverse polarisation from 0 mV to +30 mV; but allow depolarisationfrom –70 mV to +30 mV

• membrane becomes impermeable to Na+ ions and permeable to K+ions leaving the core

• reducing membrane potential to –70 mV, repolarisation

• after this, a much slower process returns the axon to itsinitial state with Na+ ions outside and K+ ions inside

max 7[10]





The candidate correctly identifies the two adiabatic and two constantvolume processes described in their correct sequence and gives detailedconsideration of where heating, cooling and work transfers take place. The

candidate states that work is done on the gas only in A → B and by the gasonly in C → D, and/or states that the area of the loop is net work done.There is also some reference to temperatures and pressures increasingor decreasing



The candidate correctly identifies where some of the heat and worktransfers take place but the answer is much more limited. The adiabaticprocesses may not be named as such, but there is a statement that work isdone in these processes or that there is no heat transfer. One processmight be missed out altogether or be incorrect. There may be reference totemperatures, volumes and pressures increasing or decreasing.



The candidate may refer (incorrectly) to the four processes as making upthe four strokes of the engine real engine and some credit may be given ifthere is a statement that A → B is the compression stroke and C → D is thepower stroke.The answer may be written mainly in terms of pressures, volumes andtemperatures increasing or decreasing with little reference to the type ofprocess or heat or work transfers.The candidate may relate the answer to a real engine rather than thetheoretical cycle eg ‘the spark occurs at B’


A to B work done on air adiabatic compression no heat transfer (from air) temperature rises

B to C no work done heating at constant volume temperature rises (and also pressure)

C to D work done by air accept power stroke adiabatic expansion no heat transfer (to air) work done at expense of kinetic energy of molecules

D to A cooling (or heat rejected) at constant volume with no work done temperature falls

accept answers which cover above points and include reference to piston, cylinder, valves

max 6

(b)

induction and exhaust ‘pumping’ loop shown (1)

loop of smaller area than ideal loop with ‘rounded’ corners (1)2

[8]





The candidate gives a comprehensive explanation based on the knowledgethat electrons have a wave-like nature so there is a finite probability ofcrossing the gap. They should recognise how the transfer of electronsacross the gap is affected by the gap width and why a pd is necessary andwhy it should be constant. Their explanation should have the key ideaslinked effectively in an appropriate sequence.



The candidate includes the main idea that electrons can transfer or ‘tunnel’across the gap because they have a ‘wave-like’ nature. They should showawareness that the transfer of electrons is in one direction only because apd is applied and that the transfer is affected by the gap width. Theirexplanation should not include contradictory or incorrect physics ideas (egthe use of electrostatic attraction).



The candidate knows that electrons have a wave-like nature and this isrelevant in this context. They may show some awareness of the effect ofthe gap width. They may not appreciate why the wave nature of theelectron is relevant here. They may well introduce irrelevant or incorrectphysics ideas in their explanation.


• electrons have a wave like nature

• there is a finite probability that electrons can cross the gap

• electrons can tunnel across the gap

• pd is necessary so electrons cross in one direction only (no nettransfer of electrons for zero pd)

• the narrower the gap, the greater the number of electrons (persecond) that cross the gap

• electrons transfer from – to +

• constant pd provides one less variable (to affect the current)(de Broglie) wavelength is of the order of the gap width

max 6

(b) as the probe moves along, the gap width increases (as the currentdecreases) then decreases (as the current increases) (1)

the current decreases (or increases) because the tunnelling effect (orprobability of crossing the gap) decreases (or increases) (1)

2[8]

M27. (a) graph passes through N = 100 to 130 when Z = 80 (1)

and N = 20 when Z = 20 (1)2

(b) (i) W at Z > 60 just below line (1)

(ii) X just above line (1)

(iii) Y just below line (1)3

(c) fission nuclei (or fragments) are neutron-rich and thereforeunstable (or radioactive) (1)

neutron-proton ratio is much higher than for a stable nucleus(of the same charge (or mass)) (1)

β– particle emitted when a neutron changes to a proton(in a neutron-rich nucleus) (1)

3

(d) The marking scheme for this part of the question includes an overallassessment for the Quality of Written Communication (QWC). Thereare no discrete marks for the assessment of written communicationbut the quality of written communication will be one of the criteriaused to assign the answer to one of three levels.

Level Descriptoran answer will be expected to meet most of the criteria in the

level descriptor

Mark range

Good 3 – answer supported by appropriate range of relevant points

– good use of information or ideas about physics, going beyond those given in the question

– argument well structured with minimal repetition or irrelevant points

– accurate and clear expression of ideas with only minor errors of spelling, punctuation and grammar

5-6

Modest 2 – answer partially supported by relevant points

– good use of information or ideas about physics given in the question but limited beyond this

– the argument shows some attempt at structure

– the ideas are expressed with reasonable clarity but with a few errors of spelling, punctuation and grammar

3-4

Limited 1 – valid points but not clearly linked to an argument structure 1-2

– limited use of information or ideas about physics

– unstructured

– errors in spelling, punctuation and grammar or lack of fluency

0 – incorrect, inappropriate or no response 0

examples of the sort of information or ideas that might be used tosupport an argument:

• reduction of greenhouse gas emissions is (thought to be) necessaryto stop global warming (1)

• long term storage of radioactive waste is essential because theradiation from it damages (or kills) living cells (1)

• radioactive isotopes with very long half lives are in the used fuelrods (1)

• nuclear power is reliable because it does not use oil or gas fromother countries (1)

• radioactive waste needs to be stored in secure and safe conditionsfor many years (1)

conclusioneithernuclear power is needed; reduction of greenhouse gases is a greaterproblem than the storage of radioactive waste because

1 global warming would cause the ice caps to melt/sea levels torise (1)2 safe storage of radioactive waste can be done (1)

ornuclear power is not needed; storage of radioactive waste is agreater problem than reduction of greenhouse gases because

1 radioactive waste has to be stored for thousands of years (1)2 greenhouse gases can be reduced using renewable energysources (1)

[14]

M28. 3 marks for any of the following 3 features

• compared with optical reflecting telescopes, radio telescopes:

• are much longer

• have a much lower resolving power

• are not as affected by the atmosphere and so their positioning is lesscritical

• have only one reflecting surface rather than two

• have a similar structure in that a concave reflecting surface reflectsthe em radiation to a detector at the focal point

3

explanations of resolving power

radio telescopes have a lower resolving power:

because the ratio of wavelength to telescope diameter is larger (1)

because radio wavelengths are very much larger than optical wavelengths(even though the diameters of radio telescopes are larger) (1)

explanations of collecting power:

collecting power depends on the area of the objective which is much largerfor radio telescopes (depends on the square of the diameter) (1)

3[6]

M29. (a) one feature (1 mark for one of the following)

• there is a threshold (minimum) frequency (of light) forphotoelectric emission from a given metal

• photoelectric emission is instant

explanation

• light consists of photons (or wavepackets) (1)

• energy of a photon = hf where f is the light frequency (1)

• work function of metal is the minimum amount of energyit needs to escape (1)

• 1 electron absorbs 1 photon and gains energy hf (1)

• electron can escape if energy gained hf > (1)6

(b) (i) an electron requires 2.2 eV of energy to escape from themetal surface (1)

(ii) photon frequency, f (= c/λ = ) = 5.77 × 10–19 J (1)

photon frequency (= hf) = 6.63 × 10–34 × 5.77 × 1014 = 3.83 × 10–19 J (1)

EK max (= hf – ) = 3.83 × 10–19 – (2.2 × 1.6 × 10–19) (1)= 3.1 × 10–20 J (1)

5[11]

M30. (a) silicon chip divided into picture elements (pixels) (1)incident photons release electrons (1)number liberated proportional to intensity (1)image identical to electron pattern (1)when exposure complete, charge processed to give image (1)

4

(b) ratio of the number of photons falling on a device that producea signal to the total number of photons falling on the device (1)≥70%(for CCD) (1)

2[6]

M31. A scintillator crystal(s)/fluorescent screen (1)convert X-ray photons into light (1)

B photocathode (1)light energy releases electrons (1)number of electrons released proportional to X-ray intensity (1)

C anodes (1)increase energy of the electrons (1)focus the electrons to form an image (1)

D fluorescent screen (1)converts electron energy into light photons (1)

[8]

M32. main features : expanding Universe (from single point) (1) suggest about 15 billion years ago (1) ‘explosion’ - creation of space/matter/time (1)

evidence : red shift of distant galaxies (1) in keeping with Hubble’s law (1) Hubble’s law can be used to age Universe (1)

max 4QWC 2

[4]

M33. (a) electrons can behave as waves[or electrons can tunnel across gap] (1)waves can cross narrow gaps[or non-zero probability of crossing gap] (1)electron waves would be attenuated too much by large gap[or probability of transfer negligible if gap too wide][or the narrower the gap, the greater the probability] (1)electron transfer is from – to + (1)

4QWC 2

(b) constant height mode:

tip height constant (1)

current varies as gap width changes (1)

image built up as tip moves across surface

[or as tip moves across, a decrease (or increase)

of current means the gap widens (or narrows)] (1)3

[or constant current mode:

tip height altered (1)

to keep current constant (1)

image built up as above or as tip moves across,

the tip

height rises (or falls) if the surface rises or

falls (1)][7]

E2. It was evident from their attempts at part (a) that during their courses many candidates had

considered the application of conservation of momentum to events involving an explosion. It was less clear that they had ever considered an explosion that takes place in a moving object, or considered how conservation of energy applies in an explosion. Consequently, part (a) of the question proved to be difficult, not least because it was unfamiliar territory for so many. Part (b), which was formulaic and involved much less original thinking, brought much more success for the majority.

In part (a) only a very small proportion of the candidates were able to produce answers that were well organised, coherent, detailed and contained correct physics to merit a ‘high level’ mark of five or six. More answers fell into the ‘intermediate level’ (three or four marks) and even more into the ‘low level’ (one or two marks). A major failing in most answers was to overlook the question’s requirement to address the two conservation laws ‘in this instance’. For a high level answer, it was necessary to consider an explosion on a moving space vehicle travelling in a straight line in deep space. All of the italicised section is significant. The system has momentum before exploding (unlike a straightforward recoil example); this momentum has to be conserved because there are no external forces in deep space. Hence the probe speeds up and the capsule must be ejected along the original line of movement (although it may not be possible to tell that this is ‘backwards’ until the calculation has been done). Forces between probe and capsule during the explosion are equal and opposite, but they are internal forces for the system. When considering momentum, it was common for candidates to conclude that ‘momentum must be conserved because momentum is always conserved’.

In the explosion, chemical energy is converted into kinetic energy; this increases the total kinetic energy of the system, which is shared between probe and capsule. Examiners saw many very weak answers that showed total confusion – such as momentum being converted into energy, mass being converted into energy, or energy not being conserved. A serious omission in many answers was that of the word ‘kinetic’ before ‘energy’, whilst many answers referred to the event as an ‘inelastic collision’. There was seldom any reference to conservation of the total energy of the system taken as a whole.

Most candidates recovered from their poor attempts at part (a) to gain all three marks for the calculation in part (b) (i). There were also many awards of full marks in part (b) (ii), where the main mistake was to calculate only the kinetic energy of the system (probe + capsule) after the explosion, and to regard this as the answer to the question. Apparently, the candidates who did this had not realised that the system had an initial kinetic energy.

E3. Part (a), which was about a student’s proposed experimental arrangement for measuring the acceleration of a cylinder down a slope, was also a test of candidates’ quality of written communication. Answers were rather better than those on the quality of written communication questions in the 2010 unit 4 examinations. Perhaps this is because candidates now have a clearer idea about what is expected in this type of question and are making greater efforts to address these requirements, or perhaps this question may have been more accessible because it involved an experimental technique. The main errors saw candidates not answering the specific aspects posed in the question; what procedure should be followed, what measurements should be taken, and how would these measurements be used to calculate the acceleration? Less able candidates in particular, ended up writing much of their answer about how the circuit would work. Common misconceptions over measurements included the need to measure the mass of the cylinder, or the angle of the slope. Some candidates thought the time should be measured with a stopwatch rather than by the intended discharge circuit, whilst others thought

there would be value in repeating the measurements for a different angle of slope. The best answers were usually brief and to-the-point, showing excellent understanding of what this experiment would involve.

The need to measure the distance from S1 to S2 was sometimes overlooked. The omission of any reference to the calculation of acceleration was a serious error and tended to limit the mark that examiners could award. Among the answers making an attempt to show how a could be determined, a frequent error was to suggest that this could be done by dividing the average speed by time rather than by dividing the final speed (ie twice the average speed) by time. The more successful answers were those using s = ut + ½ at2, with u taken to be zero.

Many candidates achieved full marks in parts (b) (i) and (ii), showing confidence in solving an exponential equation to determine the time and then applying an appropriate uniform acceleration equation to calculate a. Some of the less convincing attempts to solve the exponential equation omitted the essential minus sign in t = –RC ln (V/V0). A small proportion of the candidates used log10 rather than loge, therefore ending up with the incorrect answer. Many attempts at part (b) (ii) received no credit because the acceleration was found by dividing the average speed by the time of descent – the error already noted in part (a).

E4. Most candidates could score only half marks in part (a). There was very little difficulty in (i), but the most common answers in (ii) and (iii) were 12 kW and 2.4% respectively. These incorrect answers were the result of just calculating the power dissipated in the cables, instead of the power transmitted to the factory by them. It seemed that only the most perceptive candidates appreciated that the power transmitted would be 500 – 12 = 488 kW and therefore the efficiency of transmission 97.6%.

In part (b)(i) answers which did not address constructional differences (ie referring to turns ratios) went unrewarded. The expected answer was that the secondary of a step-down transformer has fewer turns than the primary (whereas a step-up transformer has more secondary turns than primary turns).

It was essential to make these comparisons for the same kind of transformer: answers such as ‘a step up has fewer secondary turns but a step-down has fewer primary turns’ were not considered satisfactory.

Part (b)(ii) was generally answered well, with many candidates realising that thicker wire would have a lower resistance and hence reduce the power lost by heating of the secondary. It was quite rare to find an answer in which it was stated that this is important in a step-down transformer because the secondary carries a higher current than the primary. Frequent misapprehensions were that a thicker wire would have greater resistance (and thereby reduce the current), and that thicker wire would have a lower resistivity.

Part (c) was used to assess candidates’ quality of written communication. In this type of question, a large proportion of candidates struggle when trying to present an organised and coherent piece of writing that answers the main issues raised by the question. Consequently, the marks awarded were generally low. Relatively few answers could be placed in the high level category of response.

Perhaps this is to be expected when, as here, the communication question is attempted at the end of a fairly demanding examination. The main temptation appeared to be to devote most of

the answer to energy losses in a transformer, which had been the topic tested by a communications question in a principles of power transmission. Clear statements that transformers will only work continuously with an ac supply and that power loss through heating of the cables can be reduced if the current is reduced (whilst maintaining the same power) by increasing the voltage, were uncommon. Good answers were expected to enhance this by referring to P = I V and P = I2R and outlining the need for voltage reduction on safety grounds at the consumer’s end, for low resistance cables etc. It appeared that in many centres, this topic has only been given a very superficial treatment. It was reassuring for examiners to occasionally come across a good answer, in which the principles involved in power transmission were discussed in a logical and organised manner.

E5. The transformer turns ratio equation was familiar territory for most in part (a) (i), but correct application of the efficiency formula proved to be a greater challenge in part (a) (ii). Many correct answers were seen, and almost all students knew that the number of lamps has to be an integer. Most difficulties arose from mixing up data for the secondary coil with that for the primary (for example, multiplying the primary current by the secondary voltage).

Parts (a) (iii) and (iv) proved to be an exacting test of whether students could think through to the real reasons or had enough practical experience of transformers to know these reasons.Many attempts at part (iii) were general answers about the reason for fitting a fuse in any circuit rather than specifically in a transformer’s circuit. Very few students stated that transformer coils can overheat and become damaged when they handle excessive currents and that they therefore need to be protected.

Similarly, it was only a small minority of answers to part (iv) that were properly valid; that stopping the primary current would isolate the whole transformer from the mains or that a failed fuse in the secondary circuit would leave the primary live.

Most students find that electromagnetic induction is one of the most demanding topics in the specification. In these circumstances perhaps it should not be surprising that many of the attempts to answer part (b) (i) were very disappointing. Even when pointed at a logical and sequential structured answer by three bullet points, many students could not construct a coherent, ordered response. In assessing the Quality of Written Communication, one aspect that must be taken into consideration is the appropriate use of technical terminology. This was often absent from the responses seen. The term induction has a very special meaning in physics; magnetic induction involves magnetising a material by applying a magnetic field, electrostatic induction involves charge separation by applying an electric field, electromagnetic induction involves producing an emf by applying a changing magnetic field. In all cases, direct contact is unnecessary. Many answers contained the word ‘induced’ used much more carelessly than its technical meaning in physics; a current was stated to be induced in the coil because it was connected to the ac supply, for example. This current was then said to induce a magnetic field. Many students seemed obsessed by effects in the iron rod, rather than in the aluminium ring. The aluminium ring was confused with the coil for example ‘the coil is pushed upwards by the magnetic field’. It was evident that a large proportion of students were familiar with statements of the laws of electromagnetic induction but could not apply them meaningfully to explain what happens in this demonstration. The field produced was regularly referred to as an electric field. The repulsion of the ring was sometimes attributed to Coulomb’s law and the repulsion between charges. Fleming’s left hand rule was confused with his right hand rule, or with the right hand grasp rule.

Broadly, an outline plan of an appropriate answer to this question would be along the following lines. The ac current in the coil produces an alternating magnetic field, which is concentrated in the iron rod and passes through the ring. This changing magnetic field induces an emf in the ring. Because the ring is aluminium it is a good conductor and the emf causes a large current in it. A current-carrying conductor in a magnetic field experiences a force so this current produces a magnetic field whose direction opposes the applied field. Interaction between these fields gives a net upwards repulsion of the ring. As the ring moves upwards the magnetic field becomes weaker and the force on the ring decreases. The ring’s position becomes stable when the upwards magnetic force balances its weight. Answers written in this fashion were rare but not difficult to identify and they were rewarded well.

Answers to part (b) (ii) suffered from the same lack of general understanding as the previous part. It was often realised that the ring would move to a higher position, or be expelled upwards from the rod. Reasons were less well presented. Reference to a larger induced current (or emf) in the ring was considered a prerequisite for an acceptable explanation.

E6. In the definition of gravitational potential (part (a)) the common failings were the omission ofper unit mass and a wrong direction of displacement – from a point in the field to infinity instead of in the opposite direction. The calculation in part (b)(i) was usually well rewarded. The number of significant figures to be quoted in the final answer should have been limited to the smallest number of significant figures provided in the data, which was 2 in this case. Not all students realised that, in order to reach the Moon, the minimum increase in gravitational potential required was the difference between the values at X and at the Earth’s surface, some thinking that the probe had to be given an increase of 62.6 MJ kg–1 (which would be enough to remove it to infinity).

Part (b)(ii) had a high proportion of correct answers, usually given in terms of field strength or gravitational force rather than gravitational potential. A common error here was to state that the Earth’s field had no effect beyond X. Very few answers to part (b)(iii) scored both marks and most received no credit at all. The simplistic, incorrect answer – that V is proportional to 1/rimplied that the potential would be negligible at large r – was given by many. The question had informed students that near the Earth the value of gravitational potential due to the Sun is –885 MJ kg–1, which is certainly not negligible! The important facts in this part were that the Earth-Moon distance is much less than the Earth-Sun distance, so the change in V due to the Sun is negligible over the distance moved by the probe.

To merit a mark of 5 or 6 (a high level answer) in part (c) the students were expected to make detailed correct references to all of the four factors mentioned in the question: forces, field strengths, potentials and the significance of point X. Few were able to do this, and so the majority of answers fell to the intermediate or low level. The understanding of gravitational potential was particularly weak, compounded by misinterpretation of the negative sign in a scalar quantity. It was commonly stated that the gravitational potential of the Earth is greaterthan the gravitational potential of the Moon; in fact the values are –62.6 and –3.9 MJ kg–1respectively, so it appears that a large proportion of A level students did not understand that 0 is greater than a negative number. Most answers concentrated on the Earth’s gravitational field being larger than the Moon’s because of their different masses. Many answers reiterated the question, without explaining the reasons satisfactorily, and in a coherent, sequenced, well organised way. Students should have been able to explain why X is closer to the Moon than to Earth, that work has to be done on the probe only as far as X, and that the larger distance from the Earth to X,

and the average larger force required to get there, imply that more work has to be done on the probe when travelling to the Moon than when returning to Earth. Loose use of terminology was a frequent detractor when making these answers: “the Earth’s larger potential pulls the probe back to Earth”, etc. References to escape speed, which is not very relevant in the context of this question, were fairly common.

E7.Use of the relation T ε r3/2 provided a swift method to the required answer in part (a)(i), but the mathematics involved appeared to be beyond the skills of many of the candidates. Answers

which derived an expression for r 3 from first principles, or which quoted r = (GMT2/4π2)1/3directly, were equally acceptable. The major error in most answers was a failure to subtract the radius of the Earth from the value of r when finding the height of the satellite. On this occasion the number of significant figures required in the final answer was three, because all of the data had been provided to at least 3SF. Following the expectation of recent papers, a large proportion of answers only included 2SF.

Candidates had been concentrating on the geosynchronous satellite in part (a)(i) and a proportion of them failed to read the question sufficiently carefully in part (a)(ii), thereby failing to spot that it was about the polar orbiting satellite. Their responses therefore quoted T and r values for the geosynchronous orbit, not answering the question set. For those who used the Tand r values given in the question, this part usually provided two straightforward marks using F= mω2r, or F = mv2 /r, or F = GMm/r2.

The features of the orbits and the applications of the two types of satellite were quite well known. This gave most candidates a better opportunity to score a rather better mark on a communications question in part (b) than has often been the case in previous Unit 4 examinations. Nevertheless, only a few answers giving a comprehensive and coherent treatment, expected for 5 or 6 marks, were produced. The majority gave some relevant (and sometimes unrelated) facts and many were written sufficiently well to merit an intermediate level mark. In the case of the polar orbit, only a minority of answers made any reference to the fact that the Earth rotates under the orbiting satellite, and that it is this feature which allows the satellite to provide complete coverage of the Earth’s surface. Other features of the polar satellite that were often overlooked were that orbits with different radii are possible, that data can be collected from inaccessible regions, and that contact is intermittent. Features of the geosynchronous orbit not often mentioned included the fact that the radius of the orbit is unique, that the direction of travel is west to east and that the signal strength required is higher than that for a low orbit. A few candidates stated that the orbital period of the geosynchronous satellite is one year.

E8.On the whole most candidates knew what approach to take and attempted to explain a suitable experiment. Weak candidates had issues over the language used to answer this question. Often they would state that the intensity of radiation needs to be calculated rather than a count rate needs to be recorded. Also they often stated facts instead of describing an experiment. For example, alpha particles can be stopped by a sheet of paper is a poor substitute for explaining what data to take and how to interpret the data to arrive at the conclusion that alpha rays are emitted from the source. Slightly better candidates started to discuss the background radiation but they did not always carry on to explain how this would be used in the analysis. Safety in the experiment was usually given but a majority of candidates tended to overstate the precautions necessary. It was common to see references to remote handling, lead gowns, and keeping

metres away from the source. Only the better candidates could adequately determine that gamma rays were given out by the source. These either talked about count-rate falling with the inverse square of distance or they discussed an absorber, which would have eliminated any beta radiation but still allows some radiation to pass through. The only way to know radiation passes through is to compare the count-rate with the background radiation. It was this last point that many candidates missed. Overall candidates seemed to lack planning. They often missed important considerations and bolted them on at the end. The standard of English still leaves a lot to be desired. The writing in several cases was virtually illegible and keywords were often misspelled. Fortunately there were candidates in contrast to this description who performed the writing task exceedingly well.

E9.This question assessed the student’s quality of written communication. Many answers included all the relevant points detailed in the specification and correctly described the quantum efficiency. Some showed a poorer understanding by referring to the photoelectric effect as the process by which the electrons are excited or by unclear descriptions of the pixels and potential wells.

E10.Part (a) was the QWC question and proved accessible to nearly all the candidates who were able to write something of relevance and score at least 1 mark. There were a noticeable number of candidates who were able to order their answers in a logical way and score either 5 or 6 marks. However, there were many answers spoiled by poor Physics such as the ossicles transferring the pressure wave through the ear, or the ossicles acting as a pressure multiplier. Some candidates glossed over the question asked and spent much more of their time talking about the way that the cochlea transformed the pressure variations into electrical impulses which was beyond the scope of the question.

In part (b), many candidates were able to look at the unit and suggest that Intensity was power per unit area, but only a few added at normal incidence to the path of the wave.

Part (c) was answered well by many candidates, but there were still a noticeable number who failed to look carefully at the information given and realise that the final answer should only be quoted to 2 significant figures.

E11.In part (a) most candidates were aware that in an adiabatic compression no heat transfer occurs, strictly from the gas, but as long as they had the idea of zero heat transfer the mark was given. The second mark was harder to get: candidates had to state or imply clearly that there was no time for heat transfer – it was not enough simply to say the compression stroke occurred quickly.

Candidates are well practised in calculations involving pVγ = c and pV / T= c and parts (b) (i) and (b) (ii) were tackled well by the majority of candidates, though some failed to achieve the significant figure mark in part (b) (i). Part (b) (iii) proved more difficult – all too many candidates thought that all the fuel had to be injected into the cylinder before the piston reached the top of its stroke, or before it was ignited. Some thought there would not be enough space in the cylinder for the fuel when the piston was at the top of its stroke.

There were some very confident answers to part (c), with a pleasing proportion of candidates showing a good understanding of both theoretical and real diesel cycles as represented on a p – V diagram. Candidates did not find it difficult to describe the most important differences, but many failed to give reasons, some described the differences but gave the wrong reasons for the differences. A very common misunderstanding was to think that friction accounted for a difference in the area enclosed by the loops in the diagrams, not realizing that an indicator diagram is taken before frictional losses are accounted for. The better answers to the efficiency part of the question mentioned where friction occurred, usually by giving an example (e.g. at the bearings), and also mentioned the losses due to oil viscosity and / or the work needed to circulate cooling water. Lower scoring answers simply mentioned ‘friction’, or missed out this

part of the question altogether. Another fairly common misconception was to think that the theoretical engine cycle must be 100% efficient. Examiners were wary of vague statements about ‘heat losses’ as there has to be heat loss (i.e. cooling) in the theoretical engine cycle as well as the real.

E12.(a) Most candidates made some progress in their description of how to measure the wavelength although some were unclear about the use of the detector. Quality of written communication was tested in this question and it is pleasing to see that many candidates were able to write clearly and logically on this topic. However, candidates were often unable to recognise the key physics principles involved in the formation of stationary waves in this situation although they usually recognised the significance of nodes and antinodes.

Misunderstandings were often seen in their explanations such as nodes at maxima rather than minima or superposition occurring at antinodes only. Lengthy explanations that were about double slits interference rather than stationary waves were not uncommon.

(b) Many candidates knew that the speed of radio waves was found to be the same or very close to the speed of light but only a minority appreciated why it was concluded that radio waves and light are electromagnetic waves.

E13. Only the less able students tried to draw graphs of completely the wrong shape by showing peaks etc. in part (a). A significant minority however failed to get the mark because they drew the graph with a horizontal asymptote. Part (b)(i) also scored well. Only the bottom 25% had difficulty over the use of the density equation or the volume of a sphere. Not many students got caught out by powers of 10 in the calculation but this could have been because of the ‘show that’ nature of the question. Part (b)(i) proved to be much more difficult and only the top third of the students scored the 2 marks. Some unsuccessful attempts showed the equation for the radius in terms of the atomic mass number but they did not know where to obtain ro from the information supplied. Part (c) was a good discriminator and the mean mark was between 3 and 4 out of 6. Two thirds of the students supplied information about alpha particles being scattered electrostatically. Many hinted at the idea that the least distance of approach is connected to a measure of the radius of the nucleus. This group of students also referred to electrons behaving as waves to explain diffraction. The bottom third of students scored poorly because they did not add much information to what they would have covered at GCSE. It was common to see an explanation of the scattering distribution of alpha particles and give nothing else. In this way they almost completely ignored the wording of the question. Students had obviously been taught this section of the specification in a vast number of different ways. To give students the greatest benefit, no individual marking point was required for any particular score. Any of the selection of points listed in the marking scheme were noted and taken into consideration along with the quality of communication. As a consequence, for example, some students scored full marks even though they did not refer to any equations. Most students lost marks by not including enough of the points listed. They did not include many statements that were wrong apart from one notable exception. A majority of students who gave the equation to find the least distance of approach for an alpha particle related the initial kinetic energy of the alpha particle with the Coulomb force expression rather than the potential energy expression.

E14. The definition of quantum efficiency required in part (a) proved to be quite demanding.

Although several alternative answers were given credit, the mark was lost by students who failed to refer to the photons incident on the CCD in their answer. There were also a significant number of answers suggesting that the students had not come across the term before.The calculation in part (b)(i) proved to be very straight forward for the majority of students. There were some careless mistakes with students inverting the values when substituting them into the equation, or incorrectly converting nanometres to metres.The calculation in (b)(ii) proved to be more demanding. Some students had difficulties converting the two distances into the same unit, despite the conversion factors being provided in the data booklet. Some students were also confused by the two values of radius quoted, and tried to include them both in their answer by adding, subtracting or averaging them. The inclusion of the wavelength (750 nm) in answers was interpreted as a physics error and no credit was given. Another relatively common problem was due to students calculating their answers in degrees and failing to change the unit in the answer line, or convert their answer to radian.Whilst there were many very pleasing answers to (b)(iii), many students failed to get the mark because they assumed that the problem was due to the angular resolution of the telescope, despite having correctly shown that the angular separation of the planet and star was greater than the angular resolution of the telescope. Many correct answers suggested that the planet would be too dim, but there were also many other approaches that gained credit.Question (c) incorporated the quality of written communication assessment. Most students were able to write about the telescopes they have studied. Unfortunately a significant number simply wrote down everything they knew about telescopes without restricting themselves to three parts of the electromagnetic spectrum, or to siting and size. Irrelevant material included descriptions of chromatic aberration for example. There was also evidence for confusion about what parts of the spectrum do get through the atmosphere, students commonly suggesting that x–ray and gamma–ray telescopes can be ground based, or that radio telescopes need to be in orbit. There was also confusion between infra red and ultra violet radiation.

E15. This question was related to ultrasound scans. Part (a) was the part where written communication was examined and related to prenatal scans. This was felt to be a topic that the students would be able to write on in a logical manner, but most answers were lacking in detail and coherence, with poor spelling and sentence construction. The use of a multi-array probe in a B-scan was very poorly described and very few answers could be graded in the high level as good to excellent. Many students still refer to the gel used in ultrasound as ‘Conducting’ confusing this with the gel used in an ecg examination, even suggesting that the patients stomach should be rubbed with sandpaper to remove unwanted hairs. Many students suggested that the ultrasound was diffracted or scattered at boundaries. The reasons for the partial reflections were not often given. The use of the gel to eliminate air was often not discussed. Students often referred to the scan being safer than using X-rays, but failed to state that this was because ultrasound is non-ionising. Part (b) provided many students with a single mark. Very few students related the length of pulse to the short distances travelled and thus the short time available between the start of the transmitted and reflected pulses. Several students gave incorrect answers suggesting that the short pulses were used to allow the reflected pulse to be received before the next pulse was transmitted.

E16. Part (a) asked students to state and explain how to use a graph of torque against angular velocity to find work done in one revolution. Many students failed to gain a mark here, mainly because they wrote only “find the area under the graph” without giving any explanation. All that was wanted in the way of explanation was to link the area to W = Tθ, or to show that they knew the area required was the area under either graph between 0 and 2π rad.Although there were some well-written answers to part (b), covering several of the points in the mark scheme, these were very few and far between. It was clear that the majority of students were totally unprepared for this. The question highlighted three areas for discussion, and it was expected that students would be able to show some understanding, perhaps not of all three but certainly of one or two. The specification asks for an understanding of the use of flywheels in machines, and an important use of a flywheel is in smoothing out the otherwise irregular motion in reciprocating machinery.All too often answers just described in words the T against ω graph of Figure 3 or said that because the acceleration varies, and T = I α, the torque varies, without mentioning forces or moments. Many tried to use conservation of energy and/or angular momentum, not taking into account the fact that the engine drove the flywheel so external torques and work input were involved. Many wrote that the larger the moment of inertia the smaller would be the engine speed and vice versa. A significant number drew on the previous year’s QWC question on the design of a flywheel for maximum energy storage, and a lot of what they wrote was not relevant here.

E17. In part (a) it was pleasing to see some well-written accounts that covered most if not all the relevant facts. Many students failed to support a reasonable or good account of one of the two properties with a similar account of the other property. Many students who were able to supply a reasonable ‘wave’ explanation of the double slits experiment often gave a limited account of photoelectricity as a particle property with little more than a statement of the meaning of the threshold frequency. Explanations often lacked depth as many students failed to link the threshold frequency to the work function and the photon energy equation. However, a significant number of students did provide a brief outline of why interference fringes could not be accounted for using corpuscular theory or why the threshold frequency could not be explained using wave theory.In part (b)(i), many students did not realise the relativistic mass needed to be calculated even though the speed of the electron was given in terms of the speed of light. In (ii) and (iii), whereas most students were able to calculate the photon energy in (ii), only the best students were able to calculate the kinetic energy in (iii). Frequent errors included the use of ½ mv2, some with the correct mass of the electron and some with its rest mass. A significant number of students did calculate the total energy correctly but then failed to subtract the rest energy.

E18. The graph in part (a) was done well by most, but the less able candidates were not careful in reading the temperature scale and did not place the x-axis intercept at absolute zero. In some cases they had drawn a curve that had no intercept on the x-axis.

Parts (c) (d) and (e) were tacked well by more able candidates. The less able could only manage to do part (b) but then started either to substitute the wrong data, eg temperature in °C, or quote

incorrect equations in the parts that followed. It was appreciated that not enough space was given to answer.

Part (e) allowed almost all candidates to score some marks, but the scores tended to be grouped in the following way. Less able candidates scored a couple of marks by discussing movement of molecules but did not go any further because of their poor use of physics in using phrases such as, ‘the molecules have more energy and so hit each other harder giving more pressure’.

Some candidates started to use Newton’s second law more effectively and referred to pressure in a more scientific manner.

The more able candidates could explain how increasing the volume allowed the pressure to remain constant as the temperature increased in terms of molecular motion.

E19. Part (a) produced a range of answers with many candidates obtaining full marks, but others dropping marks through carelessness or lack of detail. It was common to see the absolute magnitude scale upside down, or marked as relative luminosity. Candidates are expected to know that the scale goes from +15 to –10. Similar errors to previous years were seen when drawing the main sequence – some candidates drew it as a line or as a simple straight band. Others had the ends curving the wrong way. None of these got credit. The position of the giant stars was generally correct, but the positioning of the white dwarfs strayed too far to the right in some cases.

The calculation in (b)(i) proved to be relatively straightforward, although some scripts showed confusion with the ‘m’ in the unit of the Wien constant being interpreted as milli rather than metre. There was also some evidence that many candidates ignored the requirement for the answer to be expressed to the correct number of significant figures – two in this case.

There was the opportunity for many careless errors in question (b)(ii). One of the more common was the use of the incorrect formula for the surface area of a sphere, despite this being on the equation sheet. This was not a problem for those candidates who chose to answer using a ratio argument but it was more common to see answers which calculated the power of the Sun and then used this value to calculate the area, and then radius, of Deneb. Other careless answers used the wrong value for the radius of the Sun, or used 7000 rather than 70000 for the ratio of the powers.

Part (c) included the assessment of written communication. Unsurprisingly, perhaps, the content proved to be more accessible than last year’s question on dark energy. However, there was still evidence of much confusion. Some candidates suggested that it was the hydrogen itself that was being absorbed, or that hydrogen Balmer was a type of hydrogen. The best answers made it clear that light of all wavelengths passed through the atmosphere of the star, described the absorption process with reference to E = hf, and explained why the gaps were present despite the energy being re-emitted.

E20. In part (a), much of what was written did not answer the question clearly. The impression often given was that this topic had been learnt parrot-fashion from a revision guide, or dictated notes, because candidates who clearly did not understand what was being measured could often quote ‘high gain, large input impedance, low noise’ impeccably, without relating it to an amplifier, often just to the ‘sensors’, or just the ECG machine. Many candidates, presumably confused by the use of a gel, were drawn into describing some features of an ultrasound scan. Coupling gel was often used to reduce impedance, matching that of the skin, and sometimes an A scan was received on the oscilloscope.

Generally, answers were so mixed up with actual features of an ECG scan that it was difficult to be sure what was being described. Features that seemed to have registered well were the shaving of the chest and the absolute requirement not to use the right leg. Sometimes it was clear from the answer that the candidate thought that the electrodes were being used to supply a voltage to the patient – which is why the leads needed to be screened so as not to shock or electrocute them. ‘Low noise’ was a giveaway to the poor understanding of other candidates when it was clear that it was being related to sound. The ECG machine had to be quiet, the room had not to be noisy either otherwise it would ‘spoil the trace’.

In (b)(i) many candidates gained the unit mark, mV, but the scale mark was often lost by either placing the zero in the wrong place or by confusing it with that for the nerve action potential.

In (b)(ii) the scale was generally well known, but sometimes it was drawn looking non-uniform because candidates were trying to relate peaks on the graph to remembered values.

In (b)(iii) there were many answers which gained full marks. Those that lost marks did so because they did not give the electrical events, but described the resulting movements of the heart.

E21. Many candidates did not score the mark in (a)(i) because they gave general statements which could apply to a comparison of any two individuals rather than referring to Newton’s greater scientific reputation.

In part (a)(ii), most candidates knew that light travels slower in water than in air but many lost a mark because they only gave the correct prediction of one of the two theories.

Although considerable variation was seen in the depth of knowledge and understanding of candidates in part (b), many candidates were able to express their ideas adequately. Relatively few candidates were hampered by very poor quality of written communication. Most candidates gained some credit for knowing the light from the two slits produced an interference pattern and were able to give a simple explanation of why bright and dark fringes were formed. Many candidates knew that bright fringes were formed where the light waves were in phase but a significant number of candidates did not indicate the exact phase difference for the formation of a dark fringe and merely stated that the waves needed to be out of phase. Few candidates were able to state the correct path difference for a bright fringe or for a dark fringe, often referring to ‘phase’ difference in wavelengths or stating the path difference for a bright fringe as one wavelength instead of a whole number of wavelengths. The key explanation of why there are more than two bright fringes was often absent or too vague to gain any credit. The more able candidates wrote clearly that bright fringes are formed where the path difference is a whole number of wavelengths and that because the light is diffracted at each slit, there will be several positions where the path difference condition for a bright fringe is fulfilled.

E22. This question was a good discriminator. Most candidates, in part (a), knew how the core of the reactor functions. Some candidates too readily used the wording of the question as their answer. Others did not refer to neutrons even though this was asked for in the question. One example of a phrase given by candidates that did not quite answer the question but sounded reasonable was, ‘the power levels were kept constant by keeping a constant rate of fission using control rods’. This offers much of what was in the question itself and it does not refer to neutrons. The quality of the writing was generally good.

Again question (b) was a good discriminator. The majority of candidates were aware that fission products are normally unstable because they tend to be neutron rich or that they release beta and gamma rays. Less able candidates thought used fuel meant that they had undergone alpha emission.

E23. Part (a) was used to assess Quality of Written Communication. It produced answers across the whole mark range and was one of the highest discriminators on the paper. The specification approaches the ideas behind the accelerating Universe as a controversy. There are two conflicting pieces of evidence about how far away distant galaxies appear to be. Type 1a supernovae can be used as standard candles – they have a distinctive light curve with a well defined absolute magnitude peak. Using this value (–19.3) and the measured apparent magnitude peak gives one value of distance. The other measurement comes from red-shift and Hubble’s Law. Measurements of the light from distant galaxy show a shift towards the red end of the spectrum. The Doppler equation allows the recessional velocity and, using v = Hd, the distance can be calculated. These two values are not compatible. Essentially, high red shift supernovae are fainter than they should be. This is interpreted as evidence for an accelerating Universe. The controversy is that there is no known mechanism driving this acceleration – which is why cosmologists are using ideas such as dark energy to try to account for it.

Some candidates were able to describe the two methods of measurement, their incompatibility and the controversial nature of theories needed to explain the accelerating Universe, using correct spelling, punctuation and grammar and the correct scientific terms. Candidates who did this obtained full credit. It was common to see answers which only described one measurement or simply described what supernovae were. Very poor answers suggested that measurements of distance and velocity were made of the same supernova over a period of time and that this resulted in acceleration. Similar poor answers implied that Hubble’s Law is an indication of acceleration, as more distant objects are moving faster.

The inclusion of these ideas on the specification give students an opportunity to look into aspects of cosmology which are at the forefront of our understanding of how the Universe works. Questions on this, however, are likely to be limited to the ideas discussed in this report.

Part (b) (i) was one of the most accessible on the paper. The common difficulty for those who did not obtain full marks was due to a difference in units for the velocity of the galaxy and the velocity of light.

Part (b) (ii) was also very accessible. Again the most common error was with the unit of velocity.

Having changed it to m s–1 for part (i), several candidates did not use the correct unit for Hubble’s Law. This usually resulted in candidates not gaining one of the available marks.

E24. Part (a) was about the action potential of a nerve, but a significant number of candidates sketched the ecg trace, and a small number sketched the action potential of the heart. If the correct shape was drawn, then most candidates were able to score the marks for labelling the axes with the correct scale and unit.

In part (b) there were quite a few candidates who used about three quarters of the space available talking about the sodium potassium pump getting the muscle into equilibrium and then tried to squeeze the action potential into two lines at the end. There were some good descriptions of the ion movements, but the majority of candidates answers fell within the middle strand as shown in the mark scheme. The main faults were not using specific physics terms such as depolarisation, reverse polarisation and repolarisation, or not relating the ion movement with specified changes of potential.

E25. In part (a), candidates were asked to write an extended descriptive answer on the ideal petrol engine (Otto) cycle, and they were told that the quality of their written answers would be assessed. There were some extremely well written, concise and correct answers which even included application of the first law of thermodynamics to each process, but these were very few and far between. The best answers were those that concentrated on the processes and not on the workings of an engine. In an ideal cycle a fixed mass of air is taken through the four processes irrespective of any kind of mechanism, or any particular method of providing the heating. Common errors were thinking that the compression and/or expansion were isothermal, that work is done during a constant volume process, that the ‘spark occurs at point C’, and that the four processes represented the four strokes of the engine. Most candidates seemed to appreciate that they were expected to express their ideas in sentences rather than a series of terse notes.

In part (b) there were some well-drawn diagrams showing a good knowledge of what a real indicator diagram looks like, but many candidates concentrated only on drawing a loop with rounded corners, not realising that in the real engine the area of the loop would be less than the area of the ideal cycle. Many candidates completely missed out a line or narrow loop for the induction and exhaust strokes, and some drew a cycle for an engine of considerably different maximum and minimum volumes than the ideal.

E26. In (a) most candidates found it difficult to score high marks. Only about a quarter of candidates reached the high or intermediate levels and about a quarter of the candidates scored zero marks. The main reason was that the candidates did not say or realise that the essential point of this answer was that the electrons were behaving as waves. Very few realised the significance of the pd many saying that a high pd is required to pull the electrons across the gap.

In part (b), the majority of candidates could score one mark by saying that the current decreases as the gap between the probe and surface widens, but did not go on and link it to probability or tunnelling.

E30. The structure and operation of a CCD in part (a) has been asked before and this year many candidates obtained full marks by simply restating the mark scheme from previous papers. Several incorrect answers implied that the pixels emit photons or suggested that the process of analysis is continuous. It was clear that many candidates had no idea what was happening, suggesting that some centres had not taught this topic. It would be worth searching the Internet for animations of how a CCD works to help explain the important processes involved.

In part (b) the best answers quoted the general form of the definition for quantum efficiency, although answers, which were based on the CCD, were allowed. Most candidates knew that the value is greater than 70% and a wide range of values greater than this were accepted.

E31. Some candidates were able to answer this question clearly and concisely, earning all eight marks. Quite a few lost two marks by turning electrons into photons, rather than the electron’s energy into photons, and vice versa. The remainder, which was the majority, found the question difficult and guessed at many of the answers.

E32. Unfortunately this was incorrectly labelled in the paper but this printing error did not cause candidates any problems. Almost all candidates managed to write something about the Big Bang. Those who gained no credit included candidates who described the processes in a supernova. The expansion of the Universe also caused problems with some students. It was commonly suggested that everything was expanding and that Hubble’s red shift was measured for stars, with no mention of galaxies. Background radiation was often given as evidence for the Big Bang, but this was not given credit unless it was made clear what form this radiation takes.

The inclusion of this subject in the Specification gives candidates the opportunity to read about one of the most important theories in modern cosmology. Unfortunately too many answers were vague and ambiguous and suggested little more knowledge or understanding than that of the general public.

E33. Although most candidates knew in part (a) that electrons crossed the gap in the scanning tunnelling microscope because of the wave nature of the electron, they usually referred to tunnelling and probability rather than attenuation of the wave. Many candidates knew that the probability of an electron crossing the gap was greater the narrower the gap. The best

candidates were able to explain satisfactorily why a potential difference was necessary although many candidates considered the potential difference caused insulation breakdown between the tip and the surface.

Very few good answers were seen in part (b). Most candidates knew that the tunnelling current varied according to the gap width but few gave a clear answer in terms of a constant current or the tip being at a constant height. Many candidates switched from constant current to constant height, often as a result of confusion about feedback giving a constant gap width. Some candidates failed to mention that the current or the tip height was monitored as the tip was scanned across the surface.

· web viewa fixed mass of ideal gas at a low temperature is trapped in a container at...

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