Electrical power and energy transfers

Figure 8 shows an electric kettle used to boil water.

The kettle is connected to the mains electricity supply by a three-core cable and plug.

Figure 9 shows the inside of the plug.

What are the colours of the insulation around the earth, live and neutral wires inside a plug?

Choose answers from the box.

The plug in Figure 9 contains a fuse.

What is the circuit symbol for a fuse?

Tick () one box.

The electric kettle contains a heating element.

The power of the heating element is 2420 W.

The current in the heating element is 11.0 A.

Calculate the potential difference across the heating element.

Use the equation:

The power of the heating element is 2420 W.

The current in the heating element is 11.0 A.

Calculate the resistance of the heating element.

Use the equation:

Kettle A and kettle B have the same total power input.

Kettle A has a greater efficiency than kettle B.

Which two of the following describe heating water in kettle A compared with kettle B ?

Tick () two boxes.

A student built a circuit to investigate how the efficiency of a motor varies with the speed of the motor.

Figure 12 shows the circuit.

The student used the motor to lift a mass through a height of 1.0 m.

The motor took 12 seconds to lift the mass.

The charge flow through the circuit was 6.0 C.

Calculate the current in the circuit.

Use the equation:

The potential difference across the motor was 8.0 V.

The charge flow through the motor was 6.0 C.

Calculate the energy transferred to the motor.

Use the equation:

energy transferred = charge flow × potential difference

Use the Physics Equations Sheet to answer questions 07.3 and 07.4.

Which equation links efficiency, total input energy transfer and useful output energy transfer?

Tick () one box.

The efficiency of the motor was 0.70

The useful output energy transfer of the motor was 1.96 J.

Calculate the total input energy transfer to the motor.

The student adjusted the speed at which the mass was lifted.

For each speed, the student determined the efficiency of the motor.

Figure 13 shows the results.

Use Figure 13 to predict the efficiency of the motor when the speed is 0.6 m/s.

How does Figure 13 show that the efficiency of the motor is not directly proportional to the speed?

Figure 8 shows a battery-powered hand warmer that contains a heating pad.

The battery transfers energy to increase the temperature of the heating pad.

The heating pad has a mass of 0.20 kg.

When the hand warmer was switched on, the energy transferred to the heating pad was 8000 J.

specific heat capacity of heating pad = 1600 J/kg °C

Calculate the temperature increase of the heating pad.

Use the Physics Equations Sheet.

The hand warmer was powered by a 5.0 V battery.

The battery transferred 180 kJ of energy to the heating pad.

Calculate the charge flow through the battery.

Use the Physics Equations Sheet.

The circuit inside the hand warmer can be connected to a mobile phone to recharge the mobile phone.

Figure 9 shows the circuit diagram when the hand warmer is connected to a mobile phone.

The heating pad of the hand warmer is represented by the resistor symbol.

Explain how closing both switches S₁ and S₂ affects the power output of the battery compared with only closing switch S₁.

Figure 11 shows a garden water feature.

The water feature has an electric motor that pumps water from the bottom container to the top container.

The pump transfers 0.343 J of gravitational potential energy to the water each second.

gravitational field strength = 9.8 N/kg

Determine the mass of water passing through the pump each second.

Use the Physics Equations Sheet.

The power of the pump motor is 4.86 W.

The resistance of the pump motor is 6.0 Ω.

Calculate the charge flow through the motor in 30 minutes.

Use the Physics Equations Sheet.

The pump motor breaks and is replaced with a new motor.

The new pump motor has the same power input, but has a greater efficiency.

Explain how the new pump motor would affect the flow of water in the water feature.

Figure 1 shows a static electricity generator.

The rubber belt is turned by a motor.

As the rubber belt moves, charge is transferred from the rubber belt to the metal dome.

Figure 2 shows a student touching the metal dome of the static electricity generator.

The dome is negatively charged.

Complete the sentence.

Choose the answer from the box.

When the student touches the negatively charged metal dome the student’s hair

Complete the sentence.

Choose the answer from the box.

The hair on the student’s head stands up because the strands of hair experience

Which of the following diagrams shows the electric field pattern around the negatively charged metal dome?

Tick () one box.

Figure 3 shows the negatively charged metal dome and an earthed conductor.

The air between the dome and the earthed conductor is an insulator.

Complete the sentence.

Choose the answer from the box.

The air between the dome and the earthed conductor has

The earthed conductor is moved closer to the metal dome.

A spark jumps from the dome to the earthed conductor.

Complete the sentence.

Choose the answer from the box.

The spark jumps because the air around the charged dome has

Which particles are transferred when the spark jumps from the negatively charged metal dome to the earthed conductor?

Tick () one box.

The potential difference between the metal dome and earth is 300 000 V.

When the spark jumps there is a charge flow of 0.000 002 C.

Calculate the energy transferred by the spark.

Use the equation:

energy transferred = charge flow × potential difference

A student had an unknown electrical component inside a sealed box.

Figure 14 shows the circuit the student used to identify the component.

The student varied the potential difference across the component and measured the current in the component.

Table 3 shows the results when the potential difference across the component was 6.0 V.

Calculate value X in Table 3.

Figure 15 shows the results.

Calculate the power of the component when the potential difference across the component is 3.0 V.

Use Figure 15 and the equation:

power = potential difference × current

Complete the sentence.

Choose the answer from the box.

As the potential difference across the component increases, the gradient of

What is the component in the sealed box?

Tick () one box.

Figure 7 shows a student putting a coin into a vending machine that sells food.

The vending machine is connected to the mains electricity supply.

What is the frequency and the potential difference of the mains electricity supply in the UK?

The vending machine identifies the value of the coin by measuring the resistance of the coin.

The power dissipated by the coin is 340 mW when the current in the coin is 0.75 A.

Calculate the resistance of the coin.

Use the Physics Equations Sheet.

Coins that are dirty are not recognised by the vending machine.

Suggest one reason why.

Figure 8 shows part of a different circuit that is used to monitor the temperature inside

the vending machine.

The circuit symbol for a thermistor has not been included.

Draw the circuit symbol for a thermistor in the box below.

Figure 9 shows how the resistance of the thermistor varies with temperature.

The cooling system inside the vending machine turns on when the temperature of the thermistor is above 20 °C.

Determine the potential difference across the thermistor when the temperature is 20 °C.

Use the Physics Equations Sheet.

Figure 11 shows how the National Grid connects a power station to consumers.

Complete the sentences.

Use the Physics Equations Sheet to answer questions 08.2 and 08.3.

Which equation links current (I), power (P) and resistance (R)?

Tick (✓) one box.

A transmission cable has a power loss of 1.60 × 10⁹ W.

The current in the cable is 2000 A.

Calculate the resistance of the cable.

Use the Physics Equations Sheet to answer questions 08.4 and 08.5.

Write down the equation which links efficiency, total energy input and useful energy output.

The total energy input to the National Grid from one power station is 34.2 GJ.

The National Grid has an efficiency of 0.992

Calculate the useful energy output from this power station to consumers in GJ.

Figure 1 shows how the National Grid connects a power station to consumers.

Complete the sentences.

Use the Physics Equations Sheet to answer questions 01.2 and 01.3.

Which equation links current (I), power (P) and resistance (R)? Tick (✓) one box.

A transmission cable has a power loss of 1.60 × 10⁹ W. The current in the cable is 2000 A. Calculate the resistance of the cable.

Use the Physics Equations Sheet to answer questions 01.4 and 01.5.

Write down the equation which links efficiency, total energy input and useful energy output.

The total energy input to the National Grid from one power station is 34.2 GJ. The National Grid has an efficiency of 0.992 Calculate the useful energy output from this power station to consumers in GJ.

A student investigated how the current in a filament lamp varies with the potential difference across the filament lamp. Figure 6 shows the results.

Describe a method the student could use to obtain these results. You should include a circuit diagram.

Determine the resistance of the filament lamp when the potential difference across it is +3.0 V. Use the Physics Equations Sheet. Use Figure 6 on page 18.

The current in the lamp is 0.21 A when the potential difference across the lamp is 6.0 V. Calculate the energy transferred by the filament lamp in 30 minutes. Use the Physics Equations Sheet.

The power output of the lamp is 1.0 W when the potential difference across the lamp is 5.0 V. A student predicts that the power output would be 4.0 W if the potential difference was doubled. Explain why the student is not correct.

A hair dryer contains three heating elements.

Figure 7 shows the circuit diagram for the heating elements in the hair dryer.

In Figure 7 the heating elements are represented by resistor symbols.

Complete the sentence.

Which switch must always be closed for the hair dryer to work?

Tick (✓) one box.

Which switches must be closed for the hair dryer to work at maximum power output?

Tick (✓) one box.

Use the Physics Equations Sheet to answer questions 06.4 and 06.5.

Write down the equation which links energy transferred (E), power (P) and time (t).

The heating elements have a maximum power output of 1200 W.

The energy transferred to the heating elements to reach normal operating temperature is 3600 J.

Calculate the time taken for the heating elements to reach normal operating temperature at maximum power output.

The hair dryer has LEDs to indicate the power setting.

What is the circuit symbol for an LED?

Tick (✓) one box.

Figure 13 shows a large wind farm off the coast of the UK.

The mean power output of the wind farm is 696 MW, which is enough power for 580 000 homes.

Calculate the mean power needed for 1 home.

Give your answer in watts.

On one day the demand for electricity in the UK was 34 000 MW.

Suggest two reasons why wind power was not able to meet this demand.

Some of the energy from the wind used to rotate a wind turbine is wasted.

An engineer oils the mechanical parts of a wind turbine.

Explain how oiling would affect the efficiency of the wind turbine.

In most homes in the UK there are many different electrical devices.

Explain why people should be encouraged to use energy efficient electrical devices.

An engineering company has invented pavement tiles that generate electricity as people walk on them.

Figure 15 shows someone walking on the pavement tiles.

Use the Physics Equations Sheet to answer questions 11.1 and 11.2.

What equation links current (I), potential difference (V) and power (P)?

Tick (✓) one box.

When a person walks on a tile, a potential difference of 40 V is induced across the tile.

The power output of the tile is 4.4 W.

Calculate the current in the tile.

Use the Physics Equations Sheet to answer questions 11.3 and 11.4.

What equation links efficiency, total power input and useful power output?

Tick (✓) one box.

The tiles are used to power LED lights in the pavement.

An LED light has a total power input of 4.0 W.

The efficiency of the LED light is 0.85

Calculate the useful power output of the LED light.

An engineering company has invented pavement tiles that generate electricity as people walk on them. Figure 3 shows someone walking on the pavement tiles.

Use the Physics Equations Sheet to answer questions 03.1 and 03.2.

What equation links current (I), potential difference (V) and power (P)? Tick (✓) one box.

When a person walks on a tile, a potential difference of 40 V is induced across the tile. The power output of the tile is 4.4 W. Calculate the current in the tile.

Use the Physics Equations Sheet to answer questions 03.3 and 03.4.

What equation links efficiency, total power input and useful power output? Tick () one box.

The tiles are used to power LED lights in the pavement. An LED light has a total power input of 4.0 W. The efficiency of the LED light is 0.85 Calculate the useful power output of the LED light.

Figure 12 shows some hair straighteners. Hair straighteners contain heating elements.

When the hair straighteners reach normal operating temperature, an LED turns on. Draw the circuit symbol for an LED in the box.

Figure 13 shows the circuit diagram for the hair straighteners. Each resistor represents a heating element. The power output of the hair straighteners can be changed by closing different switches.

The hair straighteners have a maximum power output of 120 W. The energy transferred to the hair straighteners to reach normal operating temperature is 3.6 kJ. Calculate the time taken for the hair straighteners to reach normal operating temperature when operating at maximum power. Use the Physics Equations Sheet.

Switch S₂ and switch S₃ are then closed at the same time. Explain what happens to the power output of the power supply.

Figure 2 shows part of the National Grid linking a power station to consumers.

Name the parts of Figure 2 labelled A and B.

Electricity is transmitted through A at a very high potential difference.

What is the advantage of transmitting electricity at a very high potential difference?

Tick (✓) one box.

The power station generates electricity at a potential difference of 25 000 V.

The energy transferred by the power station in one second is 500 000 000 J.

Calculate the charge flow from the power station in one second.

Use the equation:

The electricity supply to a house has a potential difference of 230 V.

Table 1 shows the current in some appliances in the house.

Calculate the total power of all the appliances in Table 1.

Use the equation:

power = potential difference × current

Each appliance in Table 1 is switched on for 2 hours.

Which appliance will transfer the most energy?

Give a reason for your answer.

The average energy transferred from the National Grid every second for each person in the UK is 600 J.

There are 32 000 000 seconds in one year.

Calculate the average energy transferred each year from the National Grid for each person in the UK.

A student investigated how the current in a circuit varied with the number of lamps connected in parallel in the circuit.

Figure 4 shows the circuit with three identical lamps connected in parallel.

Figure 5 shows the results.

Complete the sentences.

Choose answers from the box.

Each answer can be used once, more than once or not at all.

decreased stayed the same increased

As the number of lamps increased, the current

As the number of lamps increased, the total resistance of the

circuit.

As the number of lamps increased, the potential difference across the

battery.

When there were three lamps in the circuit the ammeter reading kept changing between 0.35 A and 0.36 A.

What type of error would this lead to?

Tick (✓) one box.

Figure 6 shows a circuit with five ammeters and three identical lamps.

Complete Table 2 to show the readings on ammeters A₂ and A₅.

The resistance of one lamp is 15 Ω.

The current in the lamp is 0.12 A.

Calculate the power output of the lamp.

Use the equation: power = current² × resistance

A student investigated how the current in a series circuit varied with the resistance of a variable resistor. Figure 8 shows the circuit used.

Figure 9 shows the results.

The battery had a power output of 230 mW when the resistance of the variable resistor was 36 Ω. Determine the potential difference across the battery.

The student concluded: ‘the current in the circuit was inversely proportional to the resistance of the variable resistor.’ Explain how Figure 9 shows that the student is correct.

Explain how closing the switch would affect the current in the variable resistor.

Figure 5 shows part of a lighting circuit in a house.

What is the frequency of the ac mains electricity supply in the UK? Tick (✓) one box.

The mains electricity supply has an alternating potential difference. Which diagram shows an alternating potential difference? Tick (✓) one box.

The potential difference across the lamp is 230 V. The current in the lamp is 0.020 A. Calculate the power output of the lamp. Use the equation: power = potential difference current

The potential difference across the lamp is 230 V. Calculate the energy transferred by the lamp when 180 C of charge flows through the lamp. Use the equation: energy transferred = charge flow potential difference

An electrician needs to replace the light switch in Figure 5. Describe the possible hazard and the risk to the electrician of changing the light switch.

A student heated water in an electric kettle.

Water has a high specific heat capacity. Complete the sentence. Choose answers from the box.

The specific heat capacity of a substance is the energy needed to raise the

The kettle circuit contains a thermistor which is used to switch the kettle off when the water reaches 100 °C. What is the correct symbol for a thermistor? Tick (✓) one box.

The resistance of the heating element in the kettle is 15 Ω. The current in the heating element is 12 A. Calculate the power of the heating element. Use the equation: power = (current)² resistance

The student investigated how quickly the kettle could increase the temperature of 0.50 kg of water. Figure 10 shows the results of the investigation.

The temperature of the water did not start to increase until 10 seconds after the kettle was switched on. What is the reason for this? Tick (✓) one box.

Describe a method the student could have used to obtain the results shown in Figure 10.

The mass of water in the kettle was 0.50 kg. The temperature of the water increased from 20 °C to 100 °C. specific heat capacity of water = 4200 J/kg °C Calculate the energy transferred to the water. Use the Physics Equations Sheet.

The water in the kettle boiled for a short time before the kettle switched off. During this time 5.0 g of water changed to steam. specific latent heat of vaporisation of water = 2 260 000 J/kg Calculate the energy transferred to change the water to steam. Use the Physics Equations Sheet.

Figure 13 shows part of a mains electricity lighting circuit in a house.

A fault in the switch caused a householder to receive a mild electric shock before a safety device switched the circuit off. The mean power transfer to the person was 5.75 W. The potential difference across the person was 230 V. Calculate the resistance of the person.

An electrician replaced the switch. The electrician would have received an electric shock unless the circuit was disconnected from the mains supply. Explain why.

The current from an electric shock causes a person’s muscles to contract. The person cannot let go of the electrical circuit if the current is too high. Figure 14 shows how the maximum current at which a person can let go depends on the frequency of the electricity supply.

The UK mains frequency is 50 Hz. Explain why it would be safer if the UK mains frequency was not 50 Hz.

Figure 9 shows a circuit diagram.

In which position could a switch be placed so that both lamps can be switched on or off at the same time?

Tick (✓) one box.

Draw the circuit symbol for a switch in the box below.

In 30 seconds, 24 coulombs of charge flow through the battery. Calculate the current in the battery. Use the equation:

There is a potential difference of 3.6 V across the battery. Calculate the energy transferred by the battery when 60 coulombs of charge flows through the battery. Use the equation: energy transferred = charge flow × potential difference

A student built Circuit X and Circuit Y shown in Figure 10. The components used in each circuit were identical.

Figure 10

Circuit X Circuit Y

How would the reading on the ammeter in Circuit Y compare to the reading on the ammeter in Circuit X ?

Tick (✓) one box.

How does the total resistance of Circuit Y compare with the total resistance of Circuit X ?

Tick (✓) one box.

The student built another circuit which is shown in Figure 11.

Write down the equation which links current, potential difference and resistance.

There is a potential difference of 3.6 V across the lamp in Figure 11. The current through the lamp is 0.80 A

Calculate the resistance of the lamp.

Light bulbs are labelled with a power input.

What does power input mean?

Tick (✓) one box.

Write down the equation which links current, potential difference and power.

A light bulb has a power input of 40 W The mains potential difference is 230 V

Calculate the current in the light bulb.

Table 3 shows information about three different light bulbs.

Write down the equation which links efficiency, total power input and useful power output.

Calculate the value of X in Table 3.

In addition to power input, light bulbs should also be labelled with the rate at which they emit visible light. Suggest why.

Light bulbs are labelled with a power input.

What does power input mean?

Tick (✓) one box.

Write down the equation which links current, potential difference and power.

A light bulb has a power input of 40 W

The mains potential difference is 230 V

Calculate the current in the light bulb.

Table 1 shows information about three different light bulbs.

Write down the equation which links efficiency, total power input and useful power output.

Calculate the value of X in Table 1.

In addition to power input, light bulbs should also be labelled with the rate at which they emit visible light.

Suggest why.

Figure 7 shows a person using an electric lawn mower.

The lawn mower is connected to the mains electricity supply.

What is the frequency of the mains electricity supply in the UK?

The lawn mower has a switch on each side of the handle.

Figure 8 shows the circuit diagram for the lawn mower.

The motor in the lawn mower can only be turned on when the person using it holds the handle of the lawn mower with both hands.

Explain why.

The power input to the motor is 1.8 kW

The resistance of the motor is 32 Ω

Calculate the current in the motor.

The useful power output from the motor is 1.5 kW

Calculate the time it takes for the motor to transfer 450 000 J of useful energy.

Figure 12 shows a student after rubbing a balloon on his hair.

The balloon and hair have become charged.

Describe the force that acts on the student’s hair in Figure 12.

An earthed conductor was brought near the charged student. A spark jumped between the conductor and the student.

The potential difference between the conductor and the student was 2.5 kV The energy transferred by the spark was 0.0050 J

Calculate the charge transferred by the spark.

A defibrillator can transfer a charge to regulate a person’s heartbeat.

Figure 13 shows a defibrillator.

When the defibrillator is in use, a potential difference of 4800 V is applied across the person’s chest.

A charge of 0.16 coulombs passes through the person’s chest in 4.0 ms

Calculate the resistance of the person’s chest.

Figure 2 shows how the current through a filament lamp changes after the lamp is switched on.

The normal current through the filament lamp is 1.5 A.

For how many seconds is the current through the filament lamp greater than 1.5 A?

Tick one box.

Why might the filament inside a lamp melt when the lamp is first switched on?

The lamp is connected to a 24 V power supply. The current through the lamp is 1.5 A.

Calculate the power of the lamp.

Use the equation: power = potential difference × current

LED lamps are much more efficient than filament lamps.

What does this statement mean?

Tick one box.

The plug of an electrical appliance contains a fuse.

What is the correct circuit symbol for a fuse?

Tick one box.

The appliance is connected to the mains electrical supply. The mains potential difference is 230 V.

Calculate the energy transferred when 13 C of charge flows through the appliance.

Use the equation:

energy transferred = charge flow × potential difference

Figure 9 shows the structure of a fuse.

Write down the equation that links charge flow, current and time.

The fuse wire melts when 1.52 coulombs of charge flows through the fuse in 0.40 seconds.

Calculate the current at which the fuse wire melts.

The mass of the fuse wire is 0.00175 kg. The specific latent heat of fusion of the fuse wire is 205 000 J/kg.

Calculate the energy needed to melt the fuse wire.

Use the Physics Equations Sheet.

Figure 10 shows a hot water tank made of copper.

Figure 10

Copper has a higher thermal conductivity than most metals.

How does the rate of energy transfer through copper compare with the rate of energy transfer through most metals?

Tick one box.

The tank is insulated. When the water is hot, the immersion heater switches off.

Complete the sentences.

Compared to a tank with no insulation, the rate of energy transfer from the

for longer.

Figure 11 shows how temperature varies with time for water in a tank heated with an immersion heater.

Figure 12 shows how temperature varies with time for water in a tank heated with a solar panel.

Figure 11

Figure 12

Give one advantage and one disadvantage of heating the water using solar panels rather than an immersion heater.

Use only information from Figure 11 and Figure 12.

Advantage of solar panels

Disadvantage of solar panels

During one morning, a total of 4 070 000 J of energy is transferred from the electric immersion heater.

4 030 000 J of energy are transferred to the water.

Calculate the proportion of the total energy transferred to the water.

Write down the equation that links energy transferred, power and time.

The power output of the immersion heater is 5000 W.

Calculate the time taken for the immersion heater to transfer 4 070 000 J of energy.

Give the unit.

Figure 15 shows a coffee machine. The coffee machine uses an electric element to heat water.

Figure 15

The coffee machine has a metal case.

Why would it be dangerous for the live wire of the electric cable to touch the metal case?

The power output of the coffee machine is 2.53 kW.

The mains potential difference is 230 V.

Calculate the current in the coffee machine.

The coffee machine heats water from 20 °C to 90 °C.

The power output of the coffee machine is 2.53 kW.

The specific heat capacity of water is 4200 J/kg °C.

Calculate the mass of water that the coffee machine can heat in 14 seconds.