Motion Graphs Distance-Time Graphs

Time

Dis

tanc

e

The gradient gives you the speed/velocity of the object Straight lines means the object is travelling at a constant speed. Curved lines means the object is accelerating or decelerating

Velocity-Time Graphs

Time (s)

Vel

ocit

y (m

/s)

The gradient gives you the acceleration of the object. The steeper the line the greater the acceleration. If the lines gradient is negative then deceleration is occurring. The area below the graph gives the distance travelled by the object

10

5

a = v – u t

d = s x t

Forces Different types of forces include: Air resistance, friction, weight, motive,

drag, upthrust, reaction

500N 300N

Resultant Force When the forces are added together the resultant force is given. When the resultant force is 0 the object is either stationary of moving at a constant speed. If it is not zero the object is either decelerating of accelerating.

When two objects exert a force on each other, the forces are opposite but equal.

Key Equations: Force = Mass x Acceleration Weight = Mass x Acceleration due to gravity

The mass An object will accelerate in the direction of the resultant force. A force on a large mass will accelerate it less than the same force on a smaller mass.

Forces and Braking

Thinking Distance This is the distance travelled by the car before the driver reacts and presses the brakes. Drugs, alcohol, speed of car etc. can affect the thinking distance. Braking distance This is the distance travelled once the cars brakes are applied.

Motion of the car The car has kinetic energy when it is moving. There is friction between the tyres and roads generates heat energy. Braking The faster the car is travelling, the greater the braking force needed to stop in a certain distance. When the car brakes, work is done against the kinetic energy and hence it decreases.

Tot

al s

topp

ing

dis

tanc

e =

thin

king

dis

tanc

e +

Bra

king

dis

tanc

e

Falling and Terminal Velocity Velocity-Time graph for a parachutist

When an object starts to fall, the resultant force is in the downwards direction due to the weight causing the object to accelerate. As the object accelerates the resistive force increases. Eventually the two become equal but opposite (there is no resultant force) so the object starts to travel at a constant velocity, this is known as the objects terminal velocity.

What would the weight of the parachutist be if he weighed 70kg? Take g=10m/s2

At terminal velocity, what would the value of the resistance forces

be? How is this force different to the parachutists weight

A force applied to an object can cause it to stretch or squash – it changes shape. An elastic object is one which can return to its original shape once the force applied is removed. Energy changes Work is done to change an objects shape. The energy transfer that occurs in this process results in elastic potential energy being stored in the stretched object.

Hooke’s Law: The extension of an elastic object is directly proportional to the force applied, provided its limit of proportionality is not exceeded. Force applied = spring constant x extension (N) (N/m) (m) Notes: 1. The spring constant is a measure of how stiff the

spring is. The stiffer the spring, the larger the constant.

2. Extension of a spring is the difference between the original length of the spring and the length of the spring when a force is applied

Stretching and Squashing

When a force causes an object to move work is done. Work done and energy transferred are often used interchangeably.

Work done = Force applied x Distance moved (J) (N) (m)

In symbol form:

W = F x d

The gravitational potential energy is the energy that an object has due to its position in a gravitational field.

Ep = m x g x h (J) (kg) (m/s2) (m)

As an object falls it loses gravitational potential energy and gains kinetic energy. If there is friction present work is done against the frictional forces. This causes heating.

The kinetic energy of an object is dependent on its mass and velocity.

Ek = ½ x m x v2 (J) (kg) (m/s)

Power is the rate at which work is done or energy is transferred.

Power = Energy Power = Work Done Time Time

Work, Energy, GPE and KE

Momentum and Impacts Momentum Momentum is a property all moving objects have. Momentum = Mass x Velocity (kg m/s) (kg) (m/s)

p = m x v

The Conservation of Momentum In a closed system, the momentum before an event must be equal to that of the momentum after the event.

mbefore x vbefore = mafter x vafter

Impacts Air bags and crumple zones increase the amount of time an impact takes place in. The longer the impact time, the more the impact force is reduced. Why is this the case? We know acceleration = velocity/time. If we increase the time the acceleration therefore decreases. We also know that force = mass x acceleration. As the acceleration has decreased this therefore means that the force decreases. Momentum As stated previously, momentum before a collision is equal to that of the momentum after the collision. If a car hits a stationary object then the car decelerates and the object accelerates (conservation of momentum). Therefore the car has lost momentum

Electric Circuits Static Electricity When two insulators are rubbed against each other, electrons transfer from one object to the other, making both electrically charged. The material that gains electrons becomes negatively charged and the material that loses electrons become positively charged. Like charges repel. Opposite charges attract.

Current In metals, there are free electrons that are not attached to the positive ions. When the electrons flow, a current is produced. Current is defined as the rate of flow of electric charge. Charge = Current x Time (C) (A) (s) Potential difference P.d is defined as the work done between two points per coulomb of charge that passes the points. P.d = Work done/Charge (V) (J) (C)

Resistance Electrons need to push themselves past vibrating ions. The faster these ions vibrate (they vibrate faster when the temperature increases leading to an increase in their kinetic energy), the harder it is for electrons to do this. This means there is a higher resistance.

Resistance = P.d / Current (Ω) (V) (A)

Current and potential difference graphs

Resistor/Wire Filament Lamp Diode Thermistor LDR

Ohm’s law states that, for a resistor, the current that flows

through it is directly proportional to the potential difference across

the resistor, providing it is at constant temperature.

A diode only allows current to flow in one

direction. LED’s are replacing

filament lamps as they can operate at

lower currents.

The resistance in a thermistor increases as the temperature

decreases. (High temp = red line low temp = blue line).

The resistance in a LDR

decreases as the light intensity increases. (Bright light = red line

low temp = blue line). Resistors get hot as current

flows through them.

Red

Blue

Red

Blue

Series and Parallel Circuits A series circuit is one in

which current only has one path to flow through.

There is the same current through each component. A1=A2=A3

The total resistance of the circuit is the sum of the resistance of each component. The potential difference is shared by each components. E.g. A 12V power supply could supply 8V to the resistor and 4V to the bulb.

A parallel circuit is one in which there is more than one

path/junction, current can flow through.

When the current reaches a junction is ‘splits’. However the total current is the sum of the current through the components. A1=A2+A3=A4 The potential difference across each branch is the same. It would then be shared by the components in that branch. V1=V2=V3

Mains Electricity

peak voltage

time period

The frequency is given by the number of cycles per second frequency = 1/time for one

cycle

figure 1

Cables Some appliances only have two

wires, they are missing the earth wire. This is because they have a plastic casing. The cables are said to be

two-core cables. This is also known as double insulation.

Metal cased appliances tend

to be earthed.

Cells and batteries supply current that always passes in the same direction. This is known as a direct current

(d.c). An alternating current is one

that is constantly changing direction. Mains electricity has a frequency of 50Hz and

is about 230V

The fuse A fuse contains a thin

wire which heats up and melts if too much current is passed

through it. The fuse is always placed in series

with the live wire.

Circuit breakers A circuit breaker is an electromagnet switch that opens when there is a fault, that stops the current in the live wire flowing. They are quicker to reset than fuses and

work faster. An RCCB works even faster than an ordinary circuit breaker, as it is more sensitive. It

‘trips’ when the current in the live wire is different to that of the neutral wire.

Mains Electricity The rate at which energy is transferred by an

appliance is called the power.

Power (W) = Energy (J) Time (s)

Power, potential different and current are related by

the following equation:

Power (W) = Current (A) x Potential difference (V)

Higher tier only Energy, transferred, potential difference and charge

are related by the equation:

Energy (J) = Potential difference (V) x Charge (C)

The equations from this chapter:

Q = I x t

P = I x V

V = W / Q

V = I x R

P = E / t

P = I x V

E = V x Q (HT only)

Note: These two

are the same

equation!

Prefixes Remember to always check the units of the numbers given. If there is a prefix, you will need to convert

the number into standard form first: M = 1x106 k = 1x103 m = 1x10-3 μ = 1x10-6

Radioactivity The nuclear model Rutherford’s scattering experiment determine todays nuclear model of the atom, which replaced the plum pudding model. Rutherford fired positive alpha particles towards gold foil. He found that most of the alpha particles passed straight through, thus concluding that the atom is mostly empty space. He found that some alpha particles were deflected, which showed that the entre of the atom was positively charged. Very few alpha particles bounced straight back, leading Rutherford to conclude that the nucleus contained positive protons and neutrons which had electrons which orbit it.

Atoms In an atom there are the same number of protons as there are electrons. The number is denoted by the atomic number. Chlorine has 17 of each. If an atom loses of gains an electron it becomes an ion. The number of protons and neutrons in an atom is the mass number. Chlorines mass number is 35.

Radioactivity An unstable atom becomes more stable by releasing radiation. There are 3 types of radiation: Alpha: An alpha particle is two protons and two neutrons. It is a helium nucleus. Beta: A beta particle is a fast moving electron emitted by a nucleus which has too many neutrons. Gamma: Gamma radiation is an electro-magnetic wave, therefore has no charge.

Particle Relative mass

Relative charge

proton 1 +1

neutron 1 0

electron 0.0005 −1

Deflection in a magnetic or electric field

Alpha, Beta & Gamma Nuclear reactions Alpha emission: Beta emission:

Penetrating and Ionisation We can test absorbed materials by placing the material between the source and a Geiger-Muller counter. As you add the absorber material the count rate will drop to 0 i.e. all the radiation has been absorbed. You can find the penetrating power in air by moving the source further from the counter. You find that alpha only travel a few centimetres, due to it being highly ionising, beta travels about 1m and gammas range is unlimited.

The charged particles deflect in a magnetic or electric field. The beta particle deflects more than the alpha particle as it is lighter.

Evaluating the uses - Medical Tracers Doctors may use radioactive chemicals called tracers for medical imaging. Radiation detectors placed outside the body detect the radiation emitted and, with the aid of computers, build up an image of the inside of the body. When a radioactive chemical is used in this way it is not normally harmful, because: • it has a short half-life and so

decays before it can do much damage

• it is not poisonous Emitters of beta radiation or gamma radiation are used because these types of radiation readily pass out of the body, and they are less likely to causes ionisation of the cells compared to alpha radiation.

Uses of radioactivity Here are some examples of how radiation is used: • in smoke detectors • for sterilising medical instruments • for killing cancer cells • for dating rocks and materials such as

archaeological finds • in chemical tracers to help with medical diagnosis • for measuring the thickness of materials in, for

example, a paper factory

Radioactivity Half-life There are two definitions for the half-life of a radioactive substance, either is just as valid as the other: 1. The time it takes for the count rate to half 2. The time it takes for half of the radioactive

nuclei to decay

Fusion Nuclear fusion is the joining together of smaller nuclei to form larger nuclei. This is the process which energy is released in stars. When the smaller nuclei fuse together there is a drop in mass compared to their separate masses. The difference in mass is released as energy. The future We are trying to create fusion generators on Earth but it is proving difficult to generate the conditions needed (high temperature and pressure). Background radiation and radiation dose Background radiation stems from nuclear fallout, rocks, medical, cosmic rays. Air travel and occupation such as medicine increases the exposure.

Fission and Fusion Fission In a nuclear reactor, fission is sued to generate energy and hence electricity. The two most common fissionable substances used in nuclear reactors are urnaium-235 and plutonium-239. When either absorbs a neutron, the fissionable substance, uranium in the example below, splits into two smaller nuclei. When it does so, further neutrons are also released which can then be absorbed by other uranium nuclear, this creating a chain reaction. Control rods in a nuclear reactor absorb neutrons to control the rate of fission.

Energy is also released when the uranium nuclei splits which is what is harnessed to produce electricity in a nuclear power station.

A moderating material slows down neutrons so that they can be absorbed. Fast moving electron have little effect.

H H (with neutron)

He

Stars and Element Formation

Stars form when dust and gas is pulled together by gravitational attraction. Once enough matter is pulled together a protostar forms which continues

collecting dust and gas, becoming hotter and hotter. Eventually a star is born and fusion can

begin in a main sequence star.

A main sequence star is a stable star where the radiation

pressure, caused by fusion, balances the gravitational

force of the star.

Hydrogen nuclei form to become helium nuclei in a main sequence star. This process

continues up until, and including, iron. After this too much energy is required for

heavier elements to be formed.

The heavier elements form when a star collapses and then explodes as a supernova, where

there is enough energy available for heavier elements

to form.