Physics P3Topic 1 through to Topic 5

Radiation in treatment and medicineTopic One

Topic 1- Radiation in treatment and medicine

Diagnosis methods: Endoscopes, which use visible light CAT scans, which use x-rays Ultrasound, which use sound waves

Treatment methods: Ionising radiation, usually in the form of

gamma rays and x-rays Non-ionising radiation in the form of light

(lasers) and ultrasound

Topic 1 Continued Radiation is energy carried by waves or

particles from a source The intensity of radiation will decrease

with distance from a source according to the nature of the medium through which it is travelling

This is calculated through the equation: intensity=power of incident

radiation/area

Topic 1 ContinuedLight is focused on the retina by the actions of the cornea and the lens.

The average adult human eye has a near point of about 25cm and a far point of infinity.

Long sighted people can focus on distant objects but not near ones. It is caused by the eyeball being too short and therefore is fixed with a converging lens.

Short sighted people can focus on close objects but not far ones. It is caused by the eyeball being too long and therefore is fixed with a diverging lens.

Topic 1 Continued Contact lenses are also available to

correct vision- although they can cause infection and irritation to the eyes.

Laser correction is also an option where a laser is used to permanently alter the shape of the cornea to make it act as a converging/diverging lens dependant on the patients requirements

Topic 1 Continued

Here we have reflection.

This is refraction. This occurs because the light ray slows as it enters the glass due to the density. It therefore bends towards the normal.

This is total internal reflection, this occurs when the angle of incidence is greater than the critical angle.

To calculate the critical angle:Sin C=1/n(n being the refractive index)

Snell’s law explains refraction:

Sin i/sin r = nr/ni (with n being the refractive index)

Topic 1 Continued TIR in fibre optics: Light enters a fibre and reflects continually off the wall.

Some of the optical fibres carry light inside them which reflects off the inside of the body and is focussed by the lens.

This can be used in an endoscope in order to examine a patient, but also allow keyhole surgery and therefore a faster recovery time.

Ultrasound in treatment: High intensity ultrasound can break up kidney stones, and

absorption of energy can be used to treat muscle damage. Ultrasound in diagnosis: It’s able to locate hard deposits such as kidney stones,

accurately with a ‘real time’ image for doctors to see.

END OF TOPIC 1 QUESTIONS

How is light focussed on the retina? What is long sight, what is it’s cause

and how is it fixed? How does total internal reflection work? How is TIR used in fibre optics in an

endoscope? Name two uses of ultrasound in

medicine.

ANSWERS Light is focused on the retina by the action of both the

cornea and lens, as they refract the light. Long sight means you can’t focus on things close to you,

caused by a short eyeball and fixed by a converging lens. Total internal reflection occurs when the angle of

incidence is larger than the critical angle. It is used in fibre optics as it reflects all the way

throughout the fibre, it is used in an endoscope as some fibres carry light and then the reflected image is focussed by a lens.

Ultrasound can be used in diagnosis, through a scan or can be used in treatment in high intensity to break up hard deposits, e.g. Kidney stones

X-Rays and ECGsTopic Two

Topic 2- X-rays and ECGs The ability of X-rays to ionise materials depends on

their energy: The more energy means they are more ionising,

the energy is related to the frequency. Therefore, the more frequent, means more energy,

which in turn means they are more ionising.

The movement of charged particles is equivalent to an electric current as charged particles or electrons can move from the cathode to the anode to complete a circuit.

Topic 2 Continued X-ray machines consist of: Thermionic emission of electrons from an

electron gun (heated cathode) A large potential difference between the cathode

and the anode to accelerate the electrons A Vacuum to stop the electrons from colliding in

the tunnel It works to produce x-rays because most of the

kinetic energy from the collisions is transferred as thermal energy, yet some is converted into x-rays.

Topic 2 Continued Current = no of particles per sec X

charge on each particle Kinetic energy = 1/2mv2 or electronic

charge X accelerating potential difference

The inverse square law- when you double the distance from the source, the strength decreases to a quarter.

Topic 2 Continued The denser a material is, the more x-rays it

absorbs, hence becoming white on an x-ray. X-rays in CAT scans- several cross section scans

are taken to build up a 3D image on a computer X-rays in fluoroscopes- used to show a patients

organs working (a patient is placed between a source and a detector attached to a video camera)

Benefits- painless, non invasive and eliminates need for biopsy

Drawbacks- damages other tissues and not suitable for pregnant women/children

Topic 2 Continued An action potential is an electrical signal sent to a

muscle to tell it when to contract. These can be shown in the heart via an ECG which is a picture of the hearts action potentials.

For people who’s hearts do not work properly, a pacemaker can be fitted to correct the action potentials in order to make them spread across the heart.

Pulse oximetry works as it consists of two LEDs, one of red light and the other of infra red. Oxygenated blood surges after each heartbeat, and this absorbs more infra red light, therefore the pulse is worked out by how frequent the surges in absorption are. Then, it works out the percentage maximum of oxygen in the blood by comparing the absorbance between the two LEDs

Topic 2 Continued P= action

potential spreads across both atria.

QRS= action potential spreads up ventricle walls.

T= contraction spreads up from base of ventricles

Frequency of heartbeat- work out average time period per beat, then divide 1 by it.

Frequency(hertz)=1/time period

END OF TOPIC 2 QUESTIONS

How does an x-ray machine work? Why is the flow of electrons the same as

an electric current? How does a fluoroscope work? What is the inverse square law? In an ECG, what is QRS? How does a pulse oximeter show

oxygenated blood? Name a pro and con of x-rays.

ANSWERS An x-ray machine fires electrons from a cathode to a rotating

anode repeatedly in order to transfer some of the kinetic energy into x-ray energy.

The flow of electrons is the same as an electric current as they both carry charge and flow from the cathode to the anode to close the circuit.

A fluoroscope uses x-rays to show a real time view of a patients organs working.

The inverse square law says by doubling the distance from a source, the intensity decreases to a quarter.

QRS is the action potential spreading up the ventricle walls. A pulse oximeter can measure the oxygen content as oxygenated

blood absorbs more infrared light. PRO= painless, non-invasive, no need for biopsy, CON= damages

other tissues, not good for pregnant women/kids

Production, uses and risks of ionising radiation from radioactive sources Topic Three

Topic 3- Production, uses and risks of ionising radiation from radioactive sources

TYPE MASS CHARGE PROPERTIESAlpha 4 +2 Heavily ionising with low penetration (stopped after

10cm)Beta 1/1840 -1 Weakly ionising with low penetration (stopped by

aluminium)Positron 1/1840 +1 Weakly ionising with low penetration (stopped by

aluminium)Neutron 1 0 Not directly ionising with high penetration

Gamma 0 0 Not directly ionising with high penetration (stopped by thick lead)

In an atom, the number of protons equals the number of electrons.

B- decay is where a neutron becomes a proton plus an electron;B+ decay is where a proton becomes a neutron plus a positron

Topic 3 ContinuedDecay Atomic

numberMass number

a -2 -4B+ -1B- +1y 0 0

Nuclear equations:

Alpha: 4/2 HeElectron: 0/-1 ePositron: 0/+1 eGamma: 0/0 y

The N-Z curve for stable isotopes curves above the N=Z line, between the b+ and b- lines.

Nuclei with high values of z (above 82) usually undergo alpha decay.

An isotope above the curve has too many neutrons to be stable and will undergo b- decay. Whereas, an isotope below the curve has too many protons to be stable and therefore undergoes b+ decay.

Topic 3 Continued Protons and neutrons each contain three quarks. In a proton there are two up quarks and one down quark. In a neutron there are two down quarks and one up

quark.

In B- decay, a down quark changes into an up quark causing the neutron to become a proton.

In B+ decay, an up quark changes into a down quark causing the proton to become a neutron.

MASS

CHARGE

UP QUARK 1/3 +2/3DOWN QUARK 1/3 -1/3

Nuclei that have undergone radioactive decay often undergo nuclear rearrangement with a loss of energy as gamma radiation.

Topic 3 Continued Ionising radiation causes mutations in the

structure of DNA, it can also cause burns. Some precautions to prevent this are:

Increasing the distance from the source, Shielding, Containing the source, Minimising time spent exposed to radiation.

Internal radiation is usually a beta emitter, whereas external radiation is usually gamma rays or x-rays.

Topic 3 Continued Palliative care is used to improve

health, not cure the issue. E.G. Shrinking tumours.

Radioactive tracers are used with a PET scan to locate areas of abnormality. The isotopes used in the tracer have to be made nearby as the isotope has to have a short half life, so it doesn’t remain in the body and cause excess tissue damage.

END OF TOPIC 3 QUESTIONS

What is the charge of an alpha particle?

How ionising is beta radiation? Where on the stability curve would you

see b- decay, and why? What happens in b+ decay? Name a precaution for radiation? What is palliative care?

ANSWERS +2 Weakly ionising, but quite penetrative (stopped by

aluminium) B- decay would be above the curve as it has too

many neutrons. In b+ decay, the proton turns into a neutron plus a

positron- an up quark changes into a down quark. Limiting time spend with a source, distancing,

shielding. Palliative care is improving the condition of a

patients life, not curing their issue. E.g. Shrinking a tumour.

Motion of particlesTopic Four

Topic 4- Motion of particles By using particle accelerators, scientists can

attempt to make new particles to understand how the earth was created. Scientists also collaborate in big groups to have larger funds and bring together large amounts of expertise.

For motion in a circle, there must be a resultant force known as a centripetal force, that acts towards the centre of the circle.

Topic 4 Continued Particle accelerators called cyclotrons

cause charged particles to move in a circular or spiral path due to the magnetic field.

Certain stable elements can be bombarded with proton radiation to change them into radioactive isotopes.

Small cyclotrons are used in hospitals to produce isotopes with short half lives needed in PET scanners.

Topic 4 Continued In elastic collisions, both kinetic energy and momentum are

conserved. Whereas in inelastic collisions, only momentum is conserved.

Momentum of an object=massXvelocity

Gamma rays can be produced by the annihilation of an electron and a positron.

In positron electron annihilation, charge and momentum are conserved.

The masses of the annihilated electron and positron are converted into an equivalent amount of energy. Mass-energy is conserved.

E=mc2 – E is the energy of a system, m is the mass and c is the speed of light (3x10^8ms)

Topic 4 Continued Radio isotopes are used in PET

scanners to produce gamma rays. This is because the radio isotopes the emit positrons are injected into the blood in a tracer which accumulates in various tissues. The body contains naturally occurring electrons which then annihilate, creating gamma rays which are then picked up by the detectors around the patient.

END OF TOPIC 4 QUESTIONS

What is E=mc2 for? What is the centripetal force? Why are cyclotrons used in hospitals? What happens in an elastic collision? What happens in positron electron

annihilation? What is the speed of light? Why are radio isotopes used in PET

scans?

ANSWERS It shows how during annihilation, the masses are transferred into

energy. The centripetal force is the resultant force acting towards the

centre of a circle to create motion in a circle. Cyclotrons are used in hospitals to make radioactive isotopes for

use in scanners. In an elastic collision, kinetic energy and momentum are

conserved. In positron electron annihilation, charge and momentum are

conserved. Mass is converted into mass energy- which is conserved. Gamma rays are made.

Speed of light is (3x10^8)2 Radio isotopes that emit positrons are used as they collect in

tissues and they encounter electrons in the body and annihilate producing gamma rays, which are picked up by the PET scanner.

Kinetic theory and gasesTopic Five

Topic 5- Kinetic Theory and gases

Solids- Particles are held tightly together and cannot move, but can vibrate.

Liquids- Particles are still held together but are able to move over each other, but are uncompressible.

Gases- Particles are far apart and can move around quickly, also compressible.

The pressure of gases are caused by the particles hitting the walls of a container. The faster the particles move, the more collisions, therefore a higher pressure.

The higher the temperature, the more kinetic energy there is, so there is more collisions causing a higher pressure.

Topic 5 Continued At ‘absolute zero’ (-273c) it is theorised that

there is NO movement in any particles. Converting from Kelvin to Celsius, -273 Converting from Celsius to Kelvin, +273

The average kinetic energy of the particles in a gas is directly proportional to the Kelvin temperature of the gas.

In medicine, gases e.g. Oxygen, are stored at high pressure in order to store a higher volume.

Topic 5 Continued If the volume of a gas increases at a constant

temperature- pressure decreasesV1P1=V2P2 (v is volume m3) (p is pressure Pa) If the temperature of a gas is increased at a

constant pressure- the volume increasesV1=V2T1/T2 (t is temperature K) These two equations above can be combined:P1V1/T1 = P2V2/T2 (initial pressure X initial volume/initial temperature

= final pressure X final volume/final temperature)

END OF TOPIC 5 QUESTIONS

What happens at ‘absolute zero’? How do you convert from Celsius to

Kelvin? What happens if there is a gas at a

high temperature? If the volume of gas increases at a

constant temperature, what happens? If the temperature of a gas increases at

a constant pressure, what happens?

ANSWERS At absolute zero, it is theorised that no

particles can move. Add 273. The gas will move quicker, causing a

higher pressure. The pressure decreases. The volume increases.