Efficiency

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 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 16 shows a wind turbine.

Wind turbines may generate electricity when the electricity is not needed.

Two methods that can be used to store the energy from the turbine are:

Method A: Heating water to a high temperature.

Method B: Pumping water uphill into a reservoir.

Which energy store increases when water is heated?

Which energy store increases when water is pumped uphill into a reservoir?

Table 4 shows information about the two methods of storing energy.

Compare the advantages and disadvantages of the two methods of storing energy.

Include calculations in your answer.

Decreasing the amount of carbon dioxide released by different activities will help slow down climate change.

Transport and generating electricity are the two activities that released the largest amounts of carbon dioxide in the UK in 2018.

Explain one change that would reduce the amount of carbon dioxide released by each activity.

Generating electricity

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.

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.

Figure 1 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 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 10 shows a house with a solar power system.

The solar cells generate electricity.

When the electricity generated by the solar cells is not needed, the energy is stored in a large battery.

The solar cells on the roof of the house always face in the same direction.

Explain one disadvantage caused by the solar cells only facing in one direction.

The mean current from the solar cells to the battery is 3.5 A.

Calculate the charge flow from the solar cells to the battery in 3600 seconds.

Use the equation: charge flow = current × time

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

At one time in the day, the total power input to the solar cells was 7500 W.

The efficiency of the solar cells was 0.16

Calculate the useful power output of the solar cells.

The wasted energy that is not usefully transferred by the solar cells is dissipated.

What happens to energy that has been dissipated?

Tick (✓) one box.

Why is it unlikely that all the UK’s electricity needs could be met by solar power systems?

Tick (✓) one box.

During one year, 1.25 × 10¹⁸ J of energy was transferred from the National Grid. number of seconds in 1 year = 3.16 × 10⁷ Calculate the mean energy transferred from the National Grid each second. Give your answer to 3 significant figures.

Figure 5 shows a house with a solar power system. The solar cells generate electricity. When the electricity generated by the solar cells is not needed, the energy is stored in a large battery.

The charge flow through the cable between the solar cells and the battery in 24 hours was 27 000 coulombs. Calculate the mean current in the cable.

At one time, the total power input to the solar cells was 7.8 kW. The efficiency of the solar cells was 0.15 Calculate the useful power output of the solar cells.

It is unlikely that all of the electricity that the UK needs can be generated by solar power systems. Explain why.

Figure 13 shows an LED torch.

The torch contains one LED, one switch and three cells. Which diagram shows the correct circuit for the torch? Tick (✓) one box.

Write down the equation which links charge flow (Q), current (I) and time (t).

The torch worked for 14 400 seconds before the cells needed replacing. The current in the LED was 50 mA. Calculate the total charge flow through the cells.

When replaced, the cells were put into the torch the wrong way around. Explain why the torch did not work.

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

The total power input to the LED was 0.24 W. The efficiency of the LED was 0.75 Calculate the useful power output of the LED.

Figure 3 shows an LED torch.

The torch contains one LED, one switch and three cells. Which diagram shows the correct circuit for the torch? Tick (✓) one box.

Write down the equation which links charge flow (Q), current (I) and time (t).

The torch worked for 14 400 seconds before the cells needed replacing. The current in the LED was 50 mA. Calculate the total charge flow through the cells.

When replaced, the cells were put into the torch the wrong way around. Explain why the torch did not work.

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

The total power input to the LED was 0.24 W. The efficiency of the LED was 0.75 Calculate the useful power output of the LED.

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.

Electric cars have motors that are powered by a battery.

Diesel cars have engines that are powered by diesel fuel.

Table 2 compares one type of electric car with one type of diesel car.

The electric car has a range of 400 km with a fully charged battery.

The diesel car has a range of 1120 km with a full tank of diesel.

Explain the difference in the time needed to complete a 500 km journey using the electric car compared with the diesel car.

Assume both cars travel at the same speed.

Energy density is the amount of energy stored per kilogram of the energy source.

Show why the diesel car has a greater range than the electric car.

Use data from Table 2.

Assume the efficiency of the two cars is the same.

Include calculations in your answer.

Engineers have developed a way of charging electric cars while they are driving along the road.

Coils of wire buried under the road transfer energy to the car’s battery as the car is passing over the coils.

Figure 6 shows a charging lane on a motorway.

Suggest two advantages of using this method to charge electric cars compared with plugging them into the mains electricity supply.

When electric cars are not being driven, energy stored in their batteries could be used to meet sudden peaks in electricity demand.

Suggest how.

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.

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.

Complete the sentence. Choose answers from the box.

The current through an ohmic conductor is directly proportional to the

that the
remains constant.

Figure 7 shows a current – potential difference graph for a filament lamp.

Explain how the resistance of a filament lamp changes as the potential difference across it increases.

Many householders are replacing their filament lamps with LED lamps which are more energy efficient.

What does more energy efficient mean?

A Light Dependent Resistor (LDR) is used to turn on an outside lamp when it gets dark.

Part of the circuit is shown in Figure 8.

The light intensity decreases.

What happens to the potential difference across the LDR and the current in the LDR?

Potential difference

What is the resistance of the LDR when the potential difference across it is 4 V?

Give a reason for your answer.

Calculate the current through the LDR when the resistance of the LDR is 5000 Ω.

Give your answer to 2 significant figures.