Physics Revision Notes

Physics

Representing motion

The slope on a distance-time graph represents the speed of an object.

The velocity of an object is its speed in a particular direction. The slope on a velocity-time graph represents the velocity of an object. The distance travelled is equal to the area under a velocity-time graph

Speed, distance and time You should recall from your Key Stage 3 studies how to calculate the speed of an object from the distance travelled and the time taken.  The equationWhen an object moves in a straight line at a steady speed, you can calculate its speed if you know how far it travels and how long it takes. This equation shows the relationship between speed, distance travelled and time taken:                                                                          

For example, a car travels 300m in 20s.  Its speed is 300 ÷ 20 = 15m/s.   Distance-time graphs You should be able to draw and explain distance-time graphs for objects moving at steady speeds or standing still.  Background informationThe vertical axis of a distance-time graph is the distance travelled from the start, and the horizontal axis is the time from the start.  Features of the graphsWhen an object is stationary, the line on the graph is horizontal.  When an object is moving at a steady speed, the line on the graph is straight, but sloped.  The diagram shows some typical lines on a distance-time graph.  

Note that the steeper the line, the greater the speed of the object. The blue line is steeper than the red line

because it represents an object moving faster than the one represented by the red line.  

The red lines on the graph represent a typical journey where an object returns to the start again. Notice that the line representing the return journey slopes downwards.  

Velocity-time graphsYou should be able to explain velocity-time graphs for objects moving with a constant velocity or a constant acceleration.  Background informationThe velocity of an object is its speed in a particular direction. This means that two cars travelling at the same speed, but in opposite directions, have different velocities.  The vertical axis of a velocity-time graph is the velocity of the object and the horizontal axis is the time from the start.  Features of the graphsWhen an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a constant acceleration, the line on the graph is straight, but sloped. The diagram shows some typical lines on a velocity-time graph.  

The steeper the line, the greater the acceleration of the object. The blue line is steeper than the red line because it represents an object with a greater acceleration than the one represented by the red line.  Notice that a line sloping downwards (with a negative gradient) represents an object with a constant deceleration (slowing down).  

Acceleration You should be able to calculate the acceleration of an object from its change in velocity and the time taken.  The equationWhen an object moves in a straight line with a constant acceleration, you can calculate its acceleration if you know how much its velocity changes and how long this takes. This equation shows the relationship between acceleration, change in velocity and time taken:                                                                                               For example, a car accelerates in 5s from 25m/s to 35m/s.  Its velocity changes by 35 – 25 = 10 m/s.  So, its acceleration is 10 ÷ 5 = 2m/s2.  

Distance-time graphs (Higher Tier) You should be able to calculate gradients on distance-time graphs.  BackgroundTo calculate the gradient of the line on a graph, divide the change in the vertical axis by the change in the horizontal axis.  

Distance-time graphsThe gradient of a line on a distance-time graph represents the speed of the object. Study this distance-time graph.                                                                   What is the speed represented by the blue line?  The object travels 10m in 2s. Its speed is 10 ÷ 2 = 5m/s.  

Velocity-time graphs (Higher Tier) You should be able to calculate gradients of velocity-time graphs and the areas under the graphs.  

The gradientThe gradient of a line on a velocity-time graph represents the acceleration of the object. Study this velocity-time graph.  

What is the acceleration represented by the sloping line?The object increases its velocity from 0m/s to 8m/s in 4s. Its acceleration is 8 ÷ 4 = 2m/s2.  

The areaThe area under the line in a velocity-time graph represents the distance

travelled. To find the the distance travelled in the graph above we need to find the area of the light blue triangle and the dark blue square  

Step 1 - Area of light blue triangleThe width of the triangle is 4 seconds and the height is 8 metres per second.

To find area you use the equation:area of triangle = 1/2 x base x height

so the area of the light blue triangle is 1/2 x 8 x 4 = 16m.  Step 2 - Area of dark blue rectangle

The width of the rectangle is 6 seconds and the height is 8 metres per second so the area is 8 x 6 = 48m.  

Step 3 - Area under the whole graphThe area of the light blue triangle plus the area of the dark blue rectangle is:

16 + 48 = 64m.This is the total area under the distance time graph and this area represents

the distance covered.  To summarise:  

the gradient of a velocity time graph represents the acceleration the area under a velocity time graph represents the distance covered

Check your understanding of this by having a go at the following activity.  

Force, mass and acceleration

If the resultant force acting on an object is zero, a stationary object remains stationary. A moving object keeps moving at the same speed and in the same direction.

If the resultant force acting on an object is not zero, a stationary object begins to accelerate in the direction of the force. A moving object speeds up, slows down or changes direction.

Acceleration depends on the force applied to an object and the mass of the object.

Resultant force You should be able to use the idea of the resultant force on an object to determine its movement.  The resultant force  An object may have several different forces acting on it. They can have different strengths and directions. But they can be added together to give the resultant force. This is a single force that has the same effect on the object as all the individual forces acting together.  When the resultant force is zero  When all the forces are balanced, the resultant force is zero. In this case:  A stationary object remains stationary. A moving object keeps on moving at the same speed in the same direction. For example, in the diagram of the weightlifter, the resultant force on the bar is zero, so the bar does not move. Its weight acting downwards is balanced by the upward force provided by the weightlifter.  

The longer the arrow, the bigger the force. In this diagram, the arrows are the same length, so we know they are the same size.  

When the resultant force is not zero  When all the forces are not balanced, the resultant

force is not zero. In this case:  A stationary object begins to move in the direction of

the resultant force. A moving object speeds up, slows down or changes direction depending on the direction of the resultant

force. In this diagram of the weightlifter, the resultant force on the bar is not zero. The upwards force is bigger

than the downwards force. The resultant force acts in the upwards direction, so the bar moves upwards.  

In this next diagram of the weightlifter, the resultant force on the bar is also not zero. This time, the upwards force is smaller than the downwards force. The resultant force acts in the downwards direction, so the bar moves downwards.  

Forces and accelerationYou should know that objects accelerate when the resultant force is not zero, and understand the factors that affect the size of the

acceleration.  Size of the force  

An object will accelerate in the direction of the resultant force. The bigger the force, the greater the acceleration.  

Doubling the size of the (resultant) force doubles the acceleration.  

The mass  A force on a small mass will accelerate it more than the same

force on a larger mass. It takes more force to accelerate a larger mass.  

Doubling the mass halves the acceleration.  

Forces and acceleration calculationsYou should know the equation that shows the relationship between

resultant force, mass and acceleration, and be able to use it.  The equation  

resultant force (Newton, N) = mass (kg) x acceleration (m/s2)  You can see from this equation that 1N is the force needed to give

1kg an acceleration of 1m/s2.

For example, the force needed to accelerate a 10kg mass by 5m/s2 is 10 x 5 = 50N.

The same force could accelerate a 1kg mass by 50m/s2 or a 100kg mass by 0.5m/s2.

Putting it simply we can say that it takes more force to accelerate larger mass.  

Example  A truck has a mass of 2,000kg.

The driving force created by the engine is 3,000 Newton's.Calculate the acceleration caused by this force.

Answer  

1. Write down the equation.force = mass x acceleration

2. Rearrange the equation. acceleration = force/mass

3. Put in the values. acceleration = 3,000N/2,000kg

4. Work out the answer and write it down.

acceleration = 1.5m/s2

Questions and answers Here are four typical forces on which you could be asked questions:  

1. Air resistance (drag)When an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shape of the

object and its speed.  2. Contact force

This happens when two objects are pushed together. They exert equal and opposite forces on each other. The contact force from the ground pushes up on your feet even as you stand still. (This is the force you feel in your feet. You feel

the ground pushing back against your weight pushing down.)  3. Friction

This is the force that resists movement between two surfaces, which are in contact.  4. Gravity

This is the force that pulls objects towards the Earth. We call the force of gravity on an object its weight. The Earth pulls with a force of about 10

Newton's on every kilogram of mass.  

Sample questionsQuestion 1Look at the animation of the parachutist falling at a steady speed. Name the forces acting on the parachutist and state how they are acting.  

Answer 1There are just two forces acting on the parachutist. Gravity (weight) pulls the parachutist down. Air resistance (drag) pushes up on the canopy of the parachute.  

Question 2Look at the animation of the car moving at a steady speed. Name the forces acting on the car and state how they are acting.  

Answer 2There are several forces acting on the car. Gravity pulls down on the car. The contact force from the road pushes up on the wheels. The driving force from the engine pushes the car along. There is friction between the road and the tyres. There is friction in the wheel bearings. Air resistance acts on the front of the car.  

Weight and friction

Gravity is a force that attracts objects with mass towards each other. The weight of an object is the force on it due to gravity. There is a relationship between weight, mass and acceleration of free-fall (weight = mass × gravitational field strength). The gravitational field strength of the Earth is 10 N/kg.

A falling object accelerates because of the force of gravity, but as it speeds up, the frictional forces on it increase. Eventually, the weight of the object is balanced by the frictional forces and it falls at a constant speed. This is called its terminal velocity.

The stopping distance of a car depends on two things: the thinking distance and the braking distance. Several factors influence the thinking distance and the braking distance, and can increase the overall stopping distance.

WeightWeight is not the same as mass. Mass is a measure of how much stuff is in an

object. Weight is a force acting on that stuff.You have to be careful. In physics the term weight has a specific meaning, and

is measured in Newton's. Mass is measured in kilograms.The mass of a given object is the same everywhere, but its weight can change.

 Gravitational field strength

Weight is the result of gravity. The gravitational field strength of the Earth is 10 N/kg (ten Newton's per kilogram). This means that an object with a mass of 1 kg would be attracted towards the centre of the Earth by a force of 10 N. We

feel forces like this as weight.  You would weigh less on the Moon than you do on Earth because the

gravitational field strength of the Moon is one-sixth that of the Earth. But note that your mass would stay the same.  

WeightIf you drop an object on the Earth, it accelerates towards the centre of the

Earth. You can calculate the weight of an object using this equation:  weight (Newton's, N) = mass (kilograms, kg) x gravitational field

strength (newton/kilogram, N/kg)  

1. a) A person has a mass of 60 kg. How much do they weigh on Earth, if the gravitational field strength is 10 N/kg?  

weight = mass x gravitational field strengthweight = 60 kg x 10 N/kg

weight = 600N  b) How much would the same person weigh on the Moon, if the

gravitational field strength is 1.6 N/kg?  weight = mass x gravitational field strength

weight = 60 kg x 1.6 N/kgweight = 96N  

Falling objectsYou should be able to describe the forces affecting a falling object at

different stages of its fall. Usually you need to think about two forces:  The weight of the object. This is a force acting downwards because of

the object’s mass the Earth’s gravitational field. Air resistance. This is a frictional force acting in the opposite direction

to the movement of the object. Outline  

When an object is dropped, we can identify three stages before it hits the ground:

At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.

As it gains speed, the object’s weight stays the same but the air resistance on it increases. There is a resultant force acting

downwards. Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady

speed, called the terminal velocity.

Terminal velocityWhat happens if you drop a feather and a coin together? The feather and the coin have roughly the same surface area, so when they begin to fall they have about the same air resistance.  As the feather falls, its air resistance increases until it soon balances the weight of the feather. The feather now falls at its terminal velocity. But the coin is much heavier, so it has to travel quite fast before air resistance is large enough to balance its weight. In fact, it probably hits the ground before it reaches its terminal velocity.  

On the moonAn astronaut on the moon carried out a famous experiment. He dropped a hammer and a feather at the same time and found that they landed together. The moon's gravity is too weak for it to hold onto an atmosphere, so there is no air resistance. When the hammer and feather were dropped they fell together with the same acceleration.  

Stopping distancesYou should know some of the factors affecting the

stopping distance of a car.  Thinking distance

It takes a certain amount of time for a driver to react to a hazard and to start applying the brakes. During this time, the car is still moving. The faster the car is travelling, the

greater this thinking distance will be.  The thinking distance will also increase if the driver's

reactions are slower because the driver is:  •Under the influence of alcohol. •Under the influence of drugs.

•Tired

Braking distanceThe braking distance is the distance the car travels from

where the brakes are first applied to where the car stops. If the braking force is too great, the tyres may not grip the road sufficiently and the car may skid. The faster the car is travelling, the greater the braking distance will

be.  The braking distance will also increase if:

 •The brakes or tyres are worn.

•The weather conditions are poor, such as icy or wet roads.

•The car is more heavily laden, for example with passengers and luggage.

Stopping distanceThe stopping distance is the thinking distance added to the braking distance. The graph shows some typical stopping distances.                                                                    

Kinetic energy and momentum

Work done and energy transferred are measured in joules (J). The work done on an object can be calculated if the force and distance moved are known.

Objects raised against the force of gravity contain gravitational potential energy. Elastic objects gain elastic potential energy when they are squashed or stretched. Moving objects have kinetic energy. The more mass they contain and the faster they move, the more kinetic energy they contain.

A change in momentum happens when a force is applied to an object that is moving or is able to move. The total momentum in an explosion or collision stays the same.

Work, force and distanceYou should know and be able to use the

relationship between work done, force applied and distance moved.  

BackgroundWork and energy are measured in the same

unit, the joule (J). When an object is moved by a force, energy is transferred and work is

done. But work is not a form of energy, it is one of the ways in which energy can be

transferred.  

The equationThis equation shows the relationship between work done, force applied and distance moved:  work done (joule, J) = force (newton, N) x distance (metre, m)  The distance involved is the distance moved in the direction of the applied force.  For example, a force of 10N is applied to a box to move it 2m along the floor. What is the work done on the box?                                                      The work done is 10 × 2 = 20J.  

Potential energy and kinetic energyGravitational potential energy

Any object that is raised against the force of gravity stores gravitational potential energy. For example, if you lift a book up onto a shelf you have to do work against the force of gravity. The

book has gained gravitational potential energy.  Elastic potential energy

Elastic objects such as elastic bands and squash balls can change their shape. They can be stretched or squashed, but

energy is needed to change their shape. This energy is stored in the stretched or squashed object as elastic potential energy.  

Kinetic energyEvery moving object has kinetic energy (sometimes called

movement energy). The more mass an object has and the faster it is moving, the more kinetic energy it has. You should be able to

discuss the transformation of kinetic energy to other forms of energy.  

MomentumA moving object has momentum. This is the tendency of the

object to keep moving in the same direction. It is difficult to change the direction of movement of an object with a lot of momentum.  

You can calculate momentum using this equation:  momentum (kg m/s) = mass (kg) x velocity (m/s)  

Notice that momentum has:  magnitude (an amount because it depends on the object’s mass);

and direction (because it depends on the velocity of the object).

Example  What is the momentum of a 5 kg object moving at 2 m/s?  

momentum = mass x velocity= 5 kg x 2 m/s= 10 kg m/s  

Conservation of momentumAs long as no external forces are acting on the objects involved, the total momentum stays the same in explosions and collisions. We say that momentum is conserved. You can use this idea to

work out the mass, velocity or momentum of an object in an explosion or collision.  

Example  A bullet with a mass of 0.03 kg leaves a gun at 1000 m/s. If the

gun’s mass is 1.5 kg, what is the velocity of the recoil on the gun?  

momentum of bullet = mass x velocity= 0.03 kg x 1000 m/s

= 30 kg m/s  Rearrange the equation: velocity = momentum ÷ mass  

velocity of recoil on gun = 30 kg m/s ÷ 1.5 kg= 20 m/s  

Kinetic energyYou should know and be able to use, the relationship between

kinetic energy, mass and speed.  The equation

This equation shows the relationship between kinetic energy (J), mass (kg) and speed (m/s):  

kinetic energy = 1/2 × mass × speed2  An example

What is the kinetic energy of a 1000kg car travelling at 10m/s?  kinetic energy = 1/2 × mass × speed2  

kinetic energy = 1/2 × 1000 × 10² = 1/2 × 1000 × 100 = 50,000J (or 50kJ)

Static electricity

Some insulating materials become electrically charged when they are rubbed together. A substance that gains electrons becomes negatively charged, while a substance that loses electrons becomes positively charged. Charges that are the same repel, while unlike charges attract. Electrostatic precipitators, photocopiers and laser printers make practical use of electrostatic charges.

Attraction and repulsionYou should know how and why insulators can be electrically

charged.  Moving charges

When you rub two different insulating materials against each other they become electrically charged. This only works for insulated

objects – conductors lose the charge to earth.  When the materials are rubbed against each other:  

•Negatively charged particles called electrons move from one material to the other.

•The material that loses electrons becomes positively charged. •The material that gains electrons becomes negatively charged. •Both materials gain an equal amount of charge, but the charges

are opposite.

Detecting chargeIf two charged objects are brought close together, they repel each other if they have the same type of charge. That is if they are both positive, or they are both negative. They will attract each other if they have opposite charges. You can check this by clicking on the top two sentences in the diagram.  

Note that the only way to tell if an object is charged is to see if it repels another charged object. This is because charged objects will also attract small uncharged objects. You can check this by clicking on the third sentence in the diagram.  

DischargeA charged object can be discharged by connecting it to earth with a metal wire or other conductor. If the potential difference (voltage) is very large, a spark may jump across the gap between the charged object and the conductor. This can be dangerous. For example, it could cause an explosion in a petrol station.  

Using static electricityYou should be able to explain how static electricity can be useful.  Electrostatic precipitatorsMany power stations burn fossil fuels such as coal and oil. Smoke is produced when these fuels burn. Smoke comprises tiny solid particles such as unreacted carbon, which damage buildings and cause breathing difficulties. To avoid this happening, the smoke is removed from the waste gases before they pass out of the chimneys. The electrostatic precipitator is the device used for this job.  

The flow chart outlines how an electrostatic precipitator works.  

PhotocopiersThe flow chart outlines how a photocopier works. A laser printer works in a similar way.                                                                              

Circuits

Electrical circuits can be represented by circuit diagrams. The various electrical components are shown by using standard symbols in circuit diagrams. Components can be connected in series, or in parallel. The characteristics of the current and potential difference (voltage) are different in series and parallel circuits.

Circuit symbolsYou need to be able to draw and interpret circuit diagrams.  Standard symbolsThe diagram below shows the standard circuit symbols you need to know.                                               

Circuit diagramsTwo things are important for a circuit to work: •There must be a complete circuit. •There must be no short circuits. To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be able to go out of the battery, through the lamp and back to the battery.  To check for a short circuit, see if you can find a way past the lamp without going through any other component. If you can, then there is a short circuit and the lamp will not light.  Work out which of these four lamps will light when the switch is closed. Move your mouse over a circuit then left click to check your answer.  

Series and parallel connectionsYou should know the difference between series and parallel connections in circuits.  Series connectionsComponents that are connected one after another on the same loop of the circuit are connected in series. The current that flows across each component connected in series is the same.  The circuit diagram shows a circuit with two lamps connected in series. If one lamp breaks, the other lamp will not light.                                   

Parallel connectionsComponents that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel.  The circuit diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other lamp will still light.                                 

Current and potential differenceYou need to know how to measure the current that flows through a component in a circuit. You also need to know how to measure the

potential difference, also called voltage, across a component in a circuit.  

CurrentA current flows when an electric charge moves around a circuit. No current can flow if the circuit is broken, for example, when a switch is open. Click on the animation to see what happens to the charge when the switch is opened or closed.  

Measuring current: •Current is measured in amperes. •Amperes is often abbreviated to amps or A. •Current flowing through a component in a circuit is measured using an ammeter. •The ammeter must be connected in series with the component.

Potential difference (Voltage)A potential difference, also called voltage, across an electrical component is needed to make a current flow through it. Cells or batteries often provide the potential difference needed.  Measuring potential difference:  •Potential difference is measured in volts, V. •Potential difference across a component in a circuit is measured using a voltmeter. •The voltmeter must be connected in parallel with the component.                                           

Cells and circuitsYou should know what happens to the potential difference and current when the number of cells in a circuit is changed.  Potential differenceA typical cell produces a potential difference of 1.5V. When two or more cells are connected in series in a circuit, the total potential difference is the sum of their potential differences. For example, if two 1.5V cells are connected in series in the same direction, the total potential difference is 3.0V. If two 1.5V cells are connected in series, but in opposite directions, the total potential difference is 0V, so no current will flow.  

CurrentWhen more cells are connected in series in a circuit, they produce a bigger potential difference across its

components. More current flows through the components as a result.  

What will happen to the reading on the ammeter in the simulation below when the number of cells is

increased?  

Series circuitsYou should know the characteristics of the current and potential difference in series circuits.  CurrentWhen two or more components are connected in series, the same current flows through each component.

Potential differenceWhen two or more components are connected in series, the total potential difference of the supply is shared between them. This means that if you add together the voltages across each component connected in series, the total equals the voltage of the power supply.  

Parallel circuitsYou should know the characteristics of the current and

potential difference in parallel circuits.  Current

When two or more components are connected in parallel, the total current flowing through the circuit is shared between the

components.

Potential differenceWhen two or more components are connected in parallel, the potential difference across them is the same. This means that if a voltage across a lamp is 12V, the voltage across another lamp

connected in parallel is also 12V.

Resistance and resistors

Resistance is measured in ohms. It can be calculated from the potential difference across a component and the current flowing through it. The total resistance of a series circuit is the sum of the resistances of the components in the circuit.

Resistors, filament lamps and diodes produce different current-potential difference graphs. The resistance of thermistors depends upon the temperature, and the resistance of light-dependent resistors (LDRs) depends upon the light intensity.

Calculating resistanceYou should understand the relationship between potential difference, current and resistance.  Why do we get resistance?An electric current flows when electrons move through a conductor. The moving electrons can collide with the atoms of the conductor. This makes it more difficult for the current to flow, and causes resistance. Electrons collide with atoms more often in a long wire than they do in a short wire. A thin wire has fewer electrons to carry the current than a thick wire. This means that the resistance in a wire increases as: •The length of the wire increases. •The thickness of the wire decreases.

Calculating resistanceResistance is measured in ohms,   . You can calculate resistance using this equation:  potential difference (volt, V) = current (ampere, A) x resistance (ohm,  )  ExampleThe bulb in a bike light has a resistance of 3.0  . What is the potential difference across the bulb if a current of 0.6 A flows?  

Changing the resistanceYou should know how to change the resistance in a circuit and how to work out the resistance in a series circuit.  Series circuitsWhen components are connected in series, their total resistance is the sum of their individual resistances. For example, if a 2   resistor, a 1   resistor and a 3   resistor are connected side by side, their total resistance is 2 + 1 + 3 = 6  .                                 

If you increase the number of lamps in a series circuit, the total resistance will increase and less current will flow.

Variable resistorsThe resistance in a circuit can also be altered using variable resistors. For example, these components may be used in dimmer switches, or to control the volume of a CD player.

The filament lampCurrent-potential difference graphs

A graph of current (vertical axis) against potential difference (horizontal axis) shows you how the current flowing

through a component varies with the potential difference across it.  

You should be able to recognise these graphs for resistors at constant

temperature, for filament lamps, and for diodes.

Resistor at constant temperatureThe current flowing through a resistor at a constant temperature is directly proportional to the potential difference across it. A component that gives a graph like the one below is said to follow Ohm’s Law.  

The filament lampThe filament lamp is a common type of light bulb. It contains a thin coil of wire called the filament. This heats up when an electric current passes through it and produces light as a result.                

The filament lamp does not follow Ohm’s Law. Its resistance increases as the temperature of its filament increases. So the current flowing through a filament lamp is not directly proportional to the voltage across it. This is the graph of current against voltage for a filament lamp.                             

The diodeYou should be able to recognise the graph of current against voltage for a diode.  BackgroundDiodes are electronic components that can be used to regulate the potential difference in circuits and to make logic gates. Light-emitting diodes (LEDs) give off light and are often used for indicator lights in electrical equipment such as computers and television sets.                  The diode has a very high resistance in one direction. This means that current can only flow in the other direction. This is the graph of current against potential difference for a diode.  

Thermistors and LDRsYou should be able to recognise the circuit symbols for the thermistor and the LDR (light-dependent resistor), and know how the resistance of these components can be changed.  The thermistorThermistors are used as temperature sensors, for example, in fire alarms. Their resistance decreases as the temperature increases:  •At low temperatures, the resistance of a thermistor is high and little current can flow through them. •At high temperatures, the resistance of a thermistor is low and more current can flow through them.                

The LDRLDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases: •In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it. •In bright light, the resistance of an LDR is low and more current can flow through it.                

Mains electricity

The UK mains electricity supply is about 230V and it can kill if it is not used safely. Electrical circuits, cables, plugs and appliances are designed to reduce the chances of receiving an electric shock. The more electrical energy used, the greater the cost. Electrical supplies can be direct current (d.c.) or alternating current (a.c.).

Wiring a plugYou should know the features of a correctly wired three-pin mains electricity plug and be able to recognise errors in the wiring of a plug.  The cableA mains electricity cable contains two or three inner wires. Each wire has a core of copper because copper is a good conductor of electricity. The outer layers are flexible plastic because plastic is a good electrical insulator. The inner wires are colour coded:  

blueneutral

brown live

green and yellow stripes earth

The plugThe features of a plug are:  

The case is made from tough plastic or rubber because these materials are good electrical insulators.

The three pins are made from brass, which is a good conductor of electricity.

There is a fuse between the live terminal and the live pin. The fuse breaks the circuit if too much current flows.

The cable is secured in the plug by a cable grip. This should grip the cable itself and not the individual wires inside it.

Where does each wire go?There is an easy way to remember where

to connect each wire. Take the second letters of the words blue, brown and

striped. This reminds you that when you look into a plug from above:

blue goes left, brown goes right and striped goes to the top.  

EarthingYou should understand why electrical appliances are earthed.

 Earthing

Many electrical appliances have metal cases, including cookers, washing machines and refrigerators. The earth wire creates a safe route for the current to flow through, if the live

wire touches the casing.  

You will get an electric shock if the live wire inside an appliance, such as a cooker, comes loose and touches the metal casing. However, the earth terminal is connected to the metal casing, so the current goes through the earth wire instead of causing an electric shock. A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse and disconnects the appliance.  

Fuses and circuit breakersFuses and circuit breakers protect electrical circuits and

appliances.  The circuit breaker

The circuit breaker does the same job as the fuse, but it works in a different way. A spring-loaded push switch is held in the

closed position by a spring-loaded soft iron bolt. An electromagnet is arranged so that it can pull the bolt away from

the switch. If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the

push switch into the open position

The fuseThe fuse breaks the circuit if a fault in an appliance causes too much current flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.                                        

Fuses in plugs are made in standard ratings. The most common are 3 A, 5 A and 13 A. The fuse should be rated

at a slightly higher current than the device needs:  

If the device works at 3A, use a 5A fuse.

If the device works at 10A, use a 13A fuse.

Cars also have fuses. An electrical fault in a car could start a fire, so all the circuits have to be protected by

fuses.

Energy in circuitsPowerPower is a measure of how quickly energy is transferred. The unit of power is the watt (W). You can work out power using this equation:                                                           Example  An electric lamp transforms 500 J in 5 s. What is its power?                                 

                       The more energy that is transferred in a certain time, the greater the power. A 100W light bulb transfers more electrical energy each second than a 60W light bulb.  

The equation below shows the relationship between power, potential difference (voltage) and current:  power (watts) = current (amps) x potential difference (volts)  

Example 1  If the current is 5 A and the potential difference is 12 V, the power is 5 × 12 = 60 W.  This means that 60 J of energy is transferred per second.  Example 2  Which is the best fuse to use (3 A, 5 A or 13 A) with a 1.15 kW electric fire at a potential difference of 230 V?  Remember that 1.15 kW is 1150 W  Re-arrange the equation:                                             The best fuse to use would be the 13 A fuse. The 3 A and 5 A fuses would blow even when the fire was working normally.  

Direct current and alternating currentYou should know the differences between direct current (d.c) and alternating current (a.c) electrical supplies.  Direct currentIf the current flows in only one direction it is called direct current, or d.c. Batteries and cells supply d.c electricity. A typical battery may supply 1.5V. The diagram shows an oscilloscope screen displaying the signal from a d.c supply.  

Alternating currentIf the current constantly changes direction it is called alternating current, or a.c. Mains electricity is an a.c supply. The UK mains supply is about 230V. It has a frequency of 50Hz (50 hertz), which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an a.c supply.                                       

Alternating current (Higher Tier)The potential difference of the live terminal varies between a large positive value and a large negative value. However, the neutral terminal is at a potential difference close to earth, which is zero. The diagram shows an oscilloscope screen displaying the signals from the mains supply. The red trace is the live terminal and the blue trace is the neutral terminal. Note that, although the mean voltage of the mains supply is about 230V, the peak voltage is higher.                                       

Energy in circuits (Higher Tier)Charge, current and time

Electrical charge is measured in coulomb, C. The amount of electrical charge that moves in a circuit depends on the current flow and how long it

flows for.  The equation below shows the relationship between charge, current and

time:  charge (coulomb, C) = current (ampere, A) x time (second, s)  

Example  How much charge moves if a current of 10 A flows for 30s?  

charge = current × time = 10 × 30 = 300 C  Energy transferred, potential difference and charge

For a given amount of electrical charge that moves, the amount of energy transformed increases as the potential difference (voltage) increases.  

The equation below shows the relationship between energy transformed, potential difference and charge:  

energy transformed (joule, J) = potential difference (volt, V) x charge (coulomb, C)  

Example  How much energy is transformed when the potential difference is 120 V

and the charge is 2 C?  energy = potential difference × charge = 120 × 2 = 240 J  

Atoms and isotopes

An atom is made from a nucleus surrounded by electrons. The nucleus contains protons and neutrons. Isotopes are atoms that have the same number of protons, but different numbers of neutrons. The nuclei of some isotopes are unstable. They emit radiation and break down to form smaller nuclei.

An early model of the atom was the plum pudding model. It was disproved by Rutherford’s scattering experiment and replaced by the nuclear model.

Structure of the atom You should be able to describe the nuclear model of the atom.  The nuclear modelAtoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom, and the electrons are arranged in energy levels or shells around the nucleus.                                                        

The table shows the properties of these three sub-atomic particles.  

ParticleRelative mass Relative charge

Proton 1 +1

Neutron 1 0

Electron almost zero –1

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall.  

Atoms can lose or gain electrons. When they do, they form charged particles

called ions:  •If an atom loses one or more electrons it

becomes a positively charged ion. •If an atom gains one or more electrons it

becomes a negatively charged ion.

Isotopes Atomic number and mass number

The number of protons in the nucleus of an atom is called its atomic number:  

The atoms of a particular element all have the same number of protons.

The atoms of different elements have different numbers of protons.

The total number of protons and neutrons in an atom is called its mass number.  

The proton number is shown below the chemical symbol and the mass number is shown above. In the example below, the atomic number is 17 and the mass number is 35. This means that each of these atoms has:  •17 protons •17 electrons •35 – 17 = 18 neutrons

The full chemical symbol for chlorine-35

IsotopesIsotopes are the atoms of an element with different numbers of neutrons. They have the same proton number, but different mass numbers.  The table describes three isotopes of hydrogen.  

isotope

symbol                              

Radioactive decayThe nuclei of some isotopes are unstable. They can split up or

‘decay’ and release radiation. Such isotopes are called radioactive isotopes or radioisotopes. When a radioactive isotope decays, it

forms a different atom with a different number of protons  

Changing elementsWhen an atom emits alpha or beta radiation, its nucleus changes. It becomes the nucleus of a different element. This is because the number of protons in the nucleus determines which element the atom belongs to. These are the changes that occur to the number of particles in an unstable nucleus when it emits a radioactive particle:  

alpha decaybeta decay

change in number of protons –2 +1

change in number of neutrons –2 –1

In each case, a different element is left behind.  

Example 1  Uranium-230 nuclei emit alpha radiation and become nuclei of thorium-226:                          

Remember that an alpha particle is identical to a helium nucleus. Notice that:  •The mass number goes down by 4. •The atomic number goes down by 2. Example 2  Hydrogen-3 nuclei emit beta radiation and become nuclei of helium-3:                     Notice that:  •The mass number stays the same. •The atomic number goes up by 1.

Evidence for atomic structure (Higher Tier)You should be able to interpret information about Rutherford's scattering

experiment.  Plum pudding model of the atom

An early model (scientific idea) about the structure of the atom was called the plum pudding model. In this model, the atom was imagined to be a sphere of positive charge with negatively charged electrons dotted around inside it like

plums in a pudding. Scientific models can be tested to see if they are wrong by doing experiments. An experiment carried out in 1905 showed that the plum

pudding model could not be correct.  Rutherford’s scattering experiment

A scientist called Rutherford designed an experiment to test the plum pudding model. It was carried out by his assistants Geiger and Marsden. A beam of

alpha particles was aimed at very thin gold foil and their passage through the foil was detected. The scientists expected the alpha particles to pass straight

through the foil, but something else also happened.  Some of the alpha particles emerged from the foil at different angles and some

even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom. As a result of this experiment, the plum pudding

model was replaced by the nuclear model of the atom.  

Background radiation

Background radiation is all around us and we can do little to avoid it. Most background radiation comes from natural sources, while most artificial radiation comes from medical examinations, such as X-ray photographs.

Natural sourcesYou should know some of the sources of natural background radiation.  SourcesRadiation is all around us. It comes from radioactive substances including the ground, the air, building materials and food. Radiation is also found in the cosmic rays from space. Move your mouse over the diagram to check your understanding of this.  

Some rocks contain radioactive substances that produce a radioactive gas called radon. The left hand pie chart shows the average contribution of these different sources to our natural background radiation.                                                                             

Artificial radiation You should know some of the sources of artificial background radiation.  SourcesThere is little we can do about natural background radiation. After all, we cannot stop eating, drinking and breathing just to avoid it! However, human activity has added to background radiation by creating and using artificial sources of radiation.  These include radioactive waste from nuclear power stations, radioactive fallout from nuclear weapons testing and medical X-rays. Move your mouse over the diagram to check your understanding of this.  

Artificial sources account for about 15 per cent of the average background radiation dose. Nearly all artificial background radiation comes from medical procedures such as receiving X-rays for X-ray photographs.

Nuclear fission and fusion

Nuclear reactors use a type of nuclear reaction called nuclear fission. Another type of nuclear reaction, nuclear fusion, happens in the Sun and other stars.

Nuclear fission Nuclear reactors usually use a nuclear reaction called nuclear fission. Two isotopes in common use as nuclear fuels are uranium-235 and

plutonium-239.  Splitting atoms

Fission is another word for splitting. The process of splitting a nucleus is called nuclear fission. Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors because their atoms have relatively large

nuclei that are easy to split, especially when hit by neutrons.  When a uranium-235 or plutonium-239 nucleus is hit by a neutron the

following happens:  The nucleus splits into two smaller nuclei, which are radioactive.

Two or three more neutrons are released. Some energy is released.

The additional neutrons released may also hit other uranium or plutonium nuclei and cause them to split. Even more neutrons are then

released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it

going too fast.  

Nuclear fusionNuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens.  The Sun and other stars use nuclear fusion to release energy.  The sequence of nuclear fusion reactions in a star is complex, but overall hydrogen nuclei join to form helium nuclei. Here is one nuclear fusion reaction that takes place:  hydrogen-1 nuclei fuse with hydrogen-2 nuclei to make helium-3 nuclei: