Efficiency

Figure 5 shows a plastic kettle.

The kettle is connected to the mains electricity supply.

What is the frequency of the UK mains electricity supply?

Tick () one box.

The cable that connects the kettle to the plug contains two insulated wires.

Draw one line from each wire to the colour of the insulation on the wire.

Use the Physics Equations Sheet to answer questions 04.3 and 04.4.

Write down the equation that links current (I), potential difference (V) and power (P).

The power of the heating element in the kettle is 2100 W.

The potential difference across the heating element is 230 V.

Calculate the current in the heating element.

Give your answer to 2 significant figures.

The kettle is designed to switch off when the water reaches boiling point.

Before the kettle switches off, 0.0080 kg of water changes state.

The energy transferred to change the state of the water is 18 000 J.

Calculate the specific latent heat of vaporisation of the water.

Use the Physics Equations Sheet.

Specific latent heat of vaporisation = J/kg

A different kettle is used to heat some water.

The useful output energy transferred to the water is 75 600 J.

The total input energy transferred by the kettle is 84 000 J.

Calculate the efficiency of the kettle.

Use the equation:

Explain one advantage of electrical appliances having a high efficiency.

A system is designed to store excess energy from the National Grid.

• Excess energy from the National Grid powers a motor that lifts a large cube.

• To transfer the energy back to the National Grid, the cube is allowed to fall.

• The falling cube turns a generator which produces electricity.

Figure 6 shows the cube before it is lifted and after it is lifted.

The cube has a density of 4200 kg/m³.

Determine the mass of the cube.

Use the Physics Equations Sheet.

Explain why the cube is made from a material with a high density.

The system is designed to store excess energy from the National Grid.

Explain how the efficiency of the system could be determined.

The system in Figure 6 on page 20 needs to be able to store excess energy as quickly as possible.

Explain one change to this system that would allow the same amount of energy to be stored more quickly.

Do not refer to efficiency in your answer.

Energy can be stored using a Compressed Air Energy Storage (CAES) system.

When more electricity is generated than is needed, a CAES system uses the surplus electricity to compress air in a storage tank.

When there is a greater demand for electricity, the CAES system releases the compressed air to generate electricity.

Figure 7 shows a diagram of the CAES system.

The CAES system has two processes:

• compressing air to store energy. This process has an efficiency of 0.72

• releasing the air to generate electricity. This process has an efficiency of 0.86

Calculate the efficiency of the CAES system.

Give your answer to 2 significant figures.

The generator works when forces from the compressed air cause a turbine to rotate.

When particles of air collide with the turbine they transfer energy to the turbine.

In warm weather the temperature of the compressed air increases, increasing the pressure inside the storage tank.

Explain how the motion of the particles in warmer air causes an increase in the power transferred to the turbine.

The volume of the storage tank is 5.0 × 10⁵ m³.

The density of the compressed air is 48 kg/m³.

specific heat capacity of air = 1100 J/kg °C

The temperature of the compressed air increases from 12 °C to 27 °C.

Calculate the energy transferred to the compressed air in the storage tank.

Use the Physics Equations Sheet.

Give your answer in standard form.

The UK government wants the amount of carbon dioxide released into the atmosphere to be reduced.

Explain why efficient energy storage is important in reducing the amount of carbon dioxide released when generating electricity.

Figure 3 shows a lift near the bottom of a building.

The lift is attached by a cable to a counterweight.

An electric motor moves the lift.

The lift is moving up.

As the lift moves up, how far does the counterweight move down?

Tick (✓) one box.

What happens to the gravitational potential energy of the counterweight as it moves down?

Tick (✓) one box.

Calculate the change in gravitational potential energy of the lift when it moves up 4.0 m

The mass of the lift is 1300 kg

gravitational field strength = 9.8 N/kg

Use the equation:

gravitational potential energy = mass × gravitational field strength × height

Complete the sentences.

Choose answers from the box.

Friction between the brakes and the cable causes the speed of the lift to decrease.

energy of the lift.

energy of the brakes.

The motor transfers different amounts of energy each time people use the lift.

Which factors affect the amount of energy transferred by the motor as the lift moves?

Tick (✓) two boxes.

The weight of the lift and the counterweight stretch the cable by 0.015 m

The cable acts like a spring with a spring constant of 880 000 N/m

Calculate the elastic potential energy of the stretched cable.

Use the equation:

elastic potential energy = 0.5 × spring constant × (extension)²

A lift system using a counterweight is more efficient than a lift system that does not use a counterweight.

How does having a more efficient system affect the energy transferred by the motor?

Tick (✓) one box.

Solar intensity is a measure of the radiation received from the Sun at the surface of the Earth.

Figure 7 shows how the mean solar intensity changes with the distance from the equator.

The city of Athens is 4200 km from the equator.

What is the mean solar intensity in Athens?

Solar water heaters use radiation from the Sun to heat water.

The heated water is stored in a water tank.

Figure 8 shows a solar water heater on the roof of a building.

Cities closer to the equator have many more buildings with solar water heaters than cities further away from the equator.

Suggest why.

The use of solar water heaters may reduce the need to burn fossil fuels.

Complete the sentence.

Choose the answer from the box.

Burning fossil fuels contributes to global warming because there is an increase in

The efficiency of the solar water heater is 0.61

Calculate the useful power output when the total power input to the solar water heater is 1100 W.

Use the equation:

useful power output = efficiency × total power input

Different solar water heaters have different sized heating panels.

Suggest how the size of the heating panels affects the input power to a solar water heater.

Water has a high specific heat capacity.

What is meant by the specific heat capacity of water?

Tick (✓) one box.

The water tank contained 80 kg of water.

The change in thermal energy of the water was 8 400 000 J.

specific heat capacity of water = 4200 J/kg °C

Calculate the temperature change of the water.

Use the Physics Equations Sheet.

The water tank is thermally insulated.

How does thermal insulation affect the rate of energy transfer from the water in the tank?

Tick (✓) one box.

Table 1 shows information about different materials.

Which material in Table 1 is the best thermal insulator?

Tick (✓) one box.

An air source heat pump transfers energy from the air outside a building to increase the temperature of the air inside the building. Figure 13 shows an air source heat pump.

The compressor is connected to the mains electricity supply. The pipe in the heat pump contains a substance called coolant. In the evaporator, energy is transferred from the air outside the building to the liquid coolant. The temperature of the coolant increases and it evaporates.

Explain what happens to the internal energy of the coolant as its temperature increases.

What name is given to the energy needed to change the state of the liquid coolant?

What happens to the mass of the coolant as it evaporates and becomes a vapour? Tick (✓) one box.

The compressor increases the density and temperature of the coolant vapour inside the pipe. Explain why the pressure in the pipe increases.

The condenser transfers energy from the coolant to the air in the building. When the total energy input to the heat pump system is 1560 kJ the temperature of the air in the building increases from 11.6 °C to 22.1 °C. The efficiency of the heat pump system is 87.5%. The mass of the air inside the building is 125 kg. Calculate the specific heat capacity of the air in the building. Give your answer in standard form.

The air in the building gains 400 J for every 100 J of energy transferred from the mains electricity supply to the compressor. An advertisement claims that the heat pump system has an efficiency of 400%. Explain why the advertisement is not correct.

An eco-house is designed to be environmentally friendly. Figure 4 shows a picture of an eco-house.

The solar panels and a wind turbine are used to generate electricity for the eco-house. Solar and wind are both renewable energy resources. What does renewable energy resource mean? Tick (✓) one box.

Biomass, nuclear and natural gas are three other energy resources. Complete the table to show whether each energy resource is renewable or non-renewable.

Moving air makes the wind turbine spin. The wind turbine generates electricity which is used to charge a battery. Complete the sentences. Choose answers from the box.

energy.

battery.

The roof of the eco-house is covered with soil. Covering the roof with soil decreases the thermal conductivity of the roof. What are the advantages of having a roof with a lower thermal conductivity? Tick (✓) two boxes.

The average power transferred to the solar panels by sunlight is 26 000 W Calculate the average energy transferred to the solar panels in 30 seconds. Use the equation: energy transferred = power × time

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

The solar panels on the roof of the eco-house have an efficiency of 0.15 The average power input to the solar panels is 26 000 W Calculate the average useful power output from the solar panels.

Explain why it is a good idea for the eco-house to have both a wind turbine and solar panels.

A student investigated how the area of a solar panel affected the output potential difference of the solar panel.

The student placed different sized solar panels under a lamp.

Figure 3 shows a solar panel under a lamp.

Which variable should be controlled?

Tick (✓) one box.

The student measured the output potential difference using a voltmeter.

When the voltmeter was not connected, the reading on the voltmeter was 0.7 V

What name is given to this type of error?

Tick (✓) one box.

Table 1 shows the results of the investigation.

The readings for which solar panel show an anomalous result?

Tick (✓) one box.

The student did not have a solar panel with an area of 40 cm²

Determine the most likely value for the mean output potential difference of a 40 cm² solar cell.

The total input energy transfer to one of the solar panels was 8.0 joules.

The useful output energy transfer was 0.96 joules.

Calculate the efficiency of the solar panel.

Use the equation:

Solar power is a renewable energy resource.

Complete the sentence.

Choose the answer from the box.

Some homes have solar panels which generate electricity.

On a sunny day the potential difference across a solar panel is 31 volts.

A charge of 490 coulombs flows through the solar panel.

Calculate the energy transferred by the solar panel.

Use the equation:

energy transferred = charge flow × potential difference

Give your answer to 2 significant figures.

Why do solar panels on homes help reduce the environmental impact of using electrical devices?

Tick (✓) one box.