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GCSE · Physics

P1b Efficiency, Power and Energy Resources

AQA 8464 · Calculator allowed · about 75 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Efficiency is an important idea in physics whenever energy is transferred from one store to another.

(a)State what is meant by the efficiency of a device.(1)

(b)Give one reason why no device can be 100% efficient.(1)

(Total for Question 1 is 2 marks)

A kettle is supplied with 240000 J of electrical energy. It transfers 180000 J of this energy usefully to heat the water in the kettle. The rest of the energy is wasted to the surroundings. efficiency = useful output energy transfer / total input energy transfer

(a)Calculate the efficiency of the kettle. Give your answer as a decimal.(2)

(b)Calculate the efficiency of the kettle as a percentage.(1)

(Total for Question 2 is 3 marks)

An electric fan heater transfers 540000 J of energy to a room in 6 minutes. power = energy transferred / time

(a)Calculate the power output of the heater, in watts.(3)

(Total for Question 3 is 3 marks)

A car engine's moving metal parts are coated with a layer of oil (a lubricant). Many UK homes have loft spaces filled with a thick layer of mineral wool insulation.

(a)Explain, in terms of energy, how the oil reduces the amount of energy wasted between the moving parts in the engine.(2)

(b)Explain how the mineral wool insulation reduces the rate of energy transfer by heating from the house.(2)

(Total for Question 4 is 4 marks)

Many countries, including the UK, are trying to change the mix of resources used to generate electricity.

(a)State three reasons why many countries, including the UK, are trying to reduce the proportion of their electricity generated using fossil fuels (coal, oil and natural gas).(3)

(Total for Question 5 is 3 marks)

An electric motor is supplied with 500 J of energy. It transfers 350 J of this energy usefully by doing work. The rest of the energy supplied is wasted. efficiency = useful output energy transfer / total input energy transfer

(a)Calculate the efficiency of the motor as a percentage.(2)

(b)Calculate the energy wasted by the motor.(1)

(c)State the main way in which the wasted energy from the motor is dissipated to the surroundings.(1)

(Total for Question 6 is 4 marks)

An LED lamp has a power input of 15 W. The lamp is 80% efficient.

(a)Calculate the power wasted by the lamp.(3)

(Total for Question 7 is 3 marks)

The table shows the thickness and thermal conductivity of three loft insulation materials. A lower thermal conductivity means a material is a better thermal insulator (it transfers energy by heating more slowly). Material, thickness (mm), thermal conductivity (W/mK): mineral wool, 270, 0.040; sheep's wool, 250, 0.038; rigid foam board, 100, 0.022.

(a)State which of the three materials is the best thermal insulator. Use data from the table to support your answer.(2)

(b)Mrs Osei wants to insulate her loft using the smallest possible thickness of material. Suggest, with a reason, which of the three materials she should choose.(2)

(Total for Question 8 is 4 marks)

Here are six energy resources used to generate electricity in the UK: coal, wind, natural gas, solar, nuclear, biofuel.

(a)State which three of these resources are non-renewable.(2)

(b)Describe the general trend in the mix of energy resources used to generate the UK's electricity since 1990.(3)

(Total for Question 9 is 5 marks)

Aaliyah investigates how the thickness of an insulating material affects the rate of cooling of hot water. She wraps identical metal cans, each containing 200 cm3 of water starting at 80 degreesC, in different thicknesses of the same insulating material, and records the temperature of the water in each can every 2 minutes for 10 minutes. With 1 cm of insulation, the water cools from 80 degreesC to 60 degreesC in 10 minutes.

(a)Identify: (i) the independent variable, (ii) the dependent variable, (iii) one variable that Aaliyah should control (keep the same) in this investigation.(3)

(b)Calculate the mean rate of temperature decrease, in degreesC per minute, for the can with 1 cm of insulation.(2)

(c)Suggest one improvement Aaliyah could make to her method to increase the validity of her comparison between different thicknesses of insulation.(2)

(Total for Question 10 is 7 marks)

Kwame investigates the effect of a lubricant on friction. He pulls a wooden block across a bench using a newtonmeter and records the maximum force needed to start the block moving. He then repeats the test after coating the underside of the block with a thin, even layer of oil.

(a)Identify one variable Kwame should keep the same to make this a fair test.(1)

(b)Explain, in terms of energy, why the reading on the newtonmeter is lower once the oil has been applied.(2)

(c)Kwame obtains the following maximum force readings, in newtons, for three repeats without oil: 6.2, 6.6, 6.0. Calculate the mean force.(1)

(d)Give one reason why taking repeat readings and calculating a mean improves the validity of Kwame's conclusion.(1)

(Total for Question 11 is 5 marks)

A different electric motor has a power input of 800 W and an efficiency of 65%. efficiency = useful power output / total power input

(a)Calculate the useful power output of the motor.(3)

(Total for Question 12 is 3 marks)

An electric shower has a power rating of 9.5 kW. It is used for 5 minutes. power = energy transferred / time

(a)Calculate the total energy supplied to the shower during this time. Give your answer in joules.(3)

(b)The shower is 92% efficient at transferring energy usefully to heat the water. Calculate the useful energy transferred to the water.(2)

(Total for Question 13 is 5 marks)

Wind power stations and gas-fired power stations are both used to generate electricity in the UK.

(a)Compare the reliability of these two ways of generating electricity, and explain why gas-fired power stations are often used to provide 'back-up' generation.(4)

(Total for Question 14 is 4 marks)

The UK is changing the balance of resources it uses to generate electricity.

Evaluate the extent to which the UK should generate more of its electricity from renewable resources rather than non-renewable resources. In your answer, consider environmental impact, reliability of supply and the finite nature of fossil fuels.

(Total for Question 15 is 6 marks)

A washing machine motor has a useful power output of 184 W. The motor has an efficiency of 0.80. efficiency = useful power output / total power input

(a)Calculate the total power input to the motor.(3)

(Total for Question 16 is 3 marks)

An immersion heater has a power rating of 3 kW. It is switched on for 15 minutes and has an efficiency of 95%. power = energy transferred / time

(a)Calculate the total energy supplied to the heater. Give your answer in joules.(2)

(b)Calculate the useful energy transferred to the water.(2)

(c)Calculate the percentage of the total energy supplied that is wasted, and state the energy store to which most of this wasted energy is transferred.(2)

(Total for Question 17 is 6 marks)

A household replaces an old filament lamp with an LED lamp. The filament lamp has a power input of 60 W and is 15% efficient. The LED lamp has a power input of 9 W and is 80% efficient. Both lamps produce a similar amount of useful light output.

(a)Calculate the useful (light) power output of the filament lamp.(2)

(b)Calculate the useful (light) power output of the LED lamp.(2)

(c)The two lamps produce a similar amount of useful light power, but the LED lamp wastes far less energy each second. Calculate the difference in power wasted between the two lamps.(3)

(Total for Question 18 is 7 marks)

Mark scheme · P1b Efficiency, Power and Energy Resources

Question 1

(a) B1 the proportion (fraction or percentage) of the total energy input to a device that is transferred usefully, oe
(a) Answer: The proportion of the total input energy that is usefully transferred.
(b) B1 some energy is always wasted (dissipated) to the surroundings, usually as heat or sound, due to a named cause such as friction or electrical resistance, oe
(b) Answer: Some energy is always wasted to the surroundings, for example as heat due to friction.

Question 2

(a) M1 180000 / 240000 (correct substitution)
(a) A1 0.75 cao
(a) Answer: 0.75
(b) B1 75%, ft their value from (a) * 100
(b) Answer: 75%

Question 3

(a) M1 converts time to seconds: 6 * 60 = 360 s
(a) M1 540000 / 360 (correct substitution)
(a) A1 1500 W cao
(a) Answer: 1500 W

Question 4

(a) B1 the layer of oil reduces friction between the moving surfaces, oe
(a) B1 so less energy is transferred (dissipated) to the thermal energy store of the surroundings/engine casing, i.e. less energy is wasted as heat and/or sound, oe
(a) Answer: The oil reduces friction between the moving parts, so less energy is wasted (transferred to the thermal energy store of the surroundings) by heating.
(b) B1 the insulation traps pockets/layers of air within it, oe
(b) B1 air is a poor conductor of heat (thermal energy), so this reduces the rate of energy transfer by conduction through the roof/ceiling, oe
(b) Answer: The insulation traps pockets of air, and air is a poor thermal conductor, so it reduces the rate of energy transfer through the loft.

Question 5

(a) B1 fossil fuels are non-renewable (finite) and will eventually run out, oe
(a) B1 burning fossil fuels releases carbon dioxide, which contributes to global warming/climate change, oe
(a) B1 burning fossil fuels can release other pollutants, e.g. sulfur dioxide or particulates, that cause acid rain/air pollution/harm health, oe (any three distinct reasons)
(a) Answer: Any three of: fossil fuels are finite and will run out; burning them releases carbon dioxide which causes global warming; burning them releases pollutants that cause acid rain or air pollution.

Question 6

(a) M1 350 / 500 (correct substitution)
(a) A1 70% cao
(a) Answer: 70%
(b) B1 150 J (500 - 350), ft from total minus useful
(b) Answer: 150 J
(c) B1 as heat (thermal energy), mainly due to friction in the motor's bearings/moving parts (accept sound as an additional minor pathway)
(c) Answer: Mostly as heat, transferred to the thermal energy store of the surroundings, due to friction in the motor.

Question 7

(a) M1 calculates useful power output: 0.80 * 15 = 12 W, or wasted fraction = 1 - 0.80 = 0.20
(a) M1 correct method to find wasted power: 15 - 12, or 0.20 * 15
(a) A1 3 W cao
(a) Answer: 3 W

Question 8

(a) B1 rigid foam board
(a) B1 it has the lowest thermal conductivity (0.022 W/mK), so transfers energy by heating most slowly, oe (must reference the value)
(a) Answer: Rigid foam board, because it has the lowest thermal conductivity (0.022 W/mK).
(b) B1 rigid foam board, ft from (a)
(b) B1 because it has the lowest thermal conductivity, a smaller thickness of it can achieve the same (or better) insulating effect than the other materials, oe
(b) Answer: Rigid foam board, because its low thermal conductivity means a thinner layer still insulates effectively.

Question 9

(a) B1 any one of coal, natural gas, nuclear correctly identified
(a) B1 all three of coal, natural gas and nuclear identified, with no renewable resource incorrectly included
(a) Answer: Coal, natural gas and nuclear.
(b) B1 the proportion of electricity generated from fossil fuels, especially coal, has decreased significantly, oe
(b) B1 the proportion of electricity generated from renewable resources, e.g. wind and solar, has increased, oe
(b) B1 this trend is linked to targets to reduce carbon dioxide emissions/reach net zero emissions by 2050, oe
(b) Answer: Since 1990 the UK has used much less coal and other fossil fuels and much more renewable energy, especially wind and solar, to generate electricity, partly to meet targets to cut carbon dioxide emissions.

Question 10

(a) B1 independent variable: the thickness of the insulating material
(a) B1 dependent variable: the temperature of the water (after a fixed time)/rate of cooling
(a) B1 control variable: any one of - volume/mass of water, starting temperature of water, type of insulating material, type/size of can, whether the can has a lid
(a) Answer: Independent: thickness of insulation. Dependent: temperature of the water. Control (any one): starting temperature of the water.
(b) M1 (80 - 60) / 10 (correct method)
(b) A1 2 degreesC/min cao, unit required
(b) Answer: 2 degreesC per minute
(c) B1 a suitable improvement, e.g. use a lid on each can, use a draught-free room, or use a data logger for continuous readings
(c) B1 a linked reason, e.g. a lid reduces uncontrolled energy loss by evaporation/convection from the open top, so the comparison isolates the effect of the insulation thickness alone
(c) Answer: Use a lid on each can, so that energy loss from the open top of the can does not affect the comparison between different insulation thicknesses.

Question 11

(a) B1 any one of - the mass of the block, the surface material of the bench, the area of the block in contact with the bench, the speed at which the block is pulled
(a) Answer: For example, the mass of the block.
(b) B1 the layer of oil separates the two solid surfaces, reducing the friction between them, oe
(b) B1 less energy is transferred (wasted) to the thermal energy store of the surroundings by heating, so less force is needed to overcome friction and start the block moving, oe
(b) Answer: The oil reduces friction between the block and the bench, so less energy is wasted as heat and less force is needed to move the block.
(c) B1 6.27 N, accept awrt 6.3 N
(c) Answer: 6.27 N (or 6.3 N to 2 sf)
(d) B1 it reduces the effect of random error/anomalous readings, giving a more accurate and reliable value to compare with the 'with oil' result, oe
(d) Answer: It reduces the effect of random error, giving a more reliable value to use in the comparison.

Question 12

(a) M1 converts 65% to a decimal: 0.65
(a) M1 0.65 * 800 (correct substitution)
(a) A1 520 W cao
(a) Answer: 520 W

Question 13

(a) M1 converts power to watts: 9.5 kW = 9500 W
(a) M1 converts time to seconds: 5 * 60 = 300 s, and substitutes: 9500 * 300
(a) A1 2850000 J cao, accept 2.85 * 10⁶ J or 2.85 MJ
(a) Answer: 2850000 J (2.85 MJ)
(b) M1 0.92 * 2850000, ft their value from (a)
(b) A1 2622000 J cao, accept 2.622 MJ, ft from (a)
(b) Answer: 2622000 J (2.622 MJ)

Question 14

(a) B1 the output of a wind power station depends on wind speed, so it is intermittent/cannot be guaranteed at all times, oe
(a) B1 a gas-fired power station can generate electricity on demand, whenever it is needed, regardless of the weather, oe
(a) B1 this makes gas-fired generation more reliable in the short term than wind, oe (comparative statement)
(a) B1 gas-fired power stations can be used to 'fill the gap' and quickly meet demand when renewable output is low, e.g. on a still day, oe (any four distinct points)
(a) Answer: Wind power is intermittent (it depends on wind speed) so is less reliable than gas, which can generate electricity on demand whenever needed; gas stations are therefore used as reliable back-up when wind output is low.

Question 15

Level 3 (5-6): A balanced and detailed discussion of renewable and non-renewable resources is given, with points that are logically linked. A clear, justified conclusion is reached that is well supported by the evidence discussed throughout the answer.
Level 2 (3-4): Relevant points for and/or against increased use of renewable resources are given, with some linkage between ideas. A judgement may be attempted but is only partly supported by the argument given.
Level 1 (1-2): Basic, isolated statements are made about renewable and/or non-renewable resources, with little or no linkage between ideas. The answer only partially addresses the question.
Level 0 (0): No relevant content.
Indicative content:
- Renewable resources (e.g. wind, solar, tidal, hydroelectric) do not release carbon dioxide when generating electricity, so contribute less to global warming/climate change than fossil fuels.
- Renewable resources will not run out, whereas fossil fuels are finite (non-renewable) and will eventually run out.
- Many renewable resources are intermittent/unreliable, e.g. wind and solar output depends on the weather, so cannot always meet demand on their own.
- Non-renewable resources such as gas and coal can generate electricity on demand, reliably, regardless of weather or time of day.
- Burning fossil fuels releases carbon dioxide and other pollutants, e.g. sulfur dioxide causing acid rain, harming the environment and human health.
- Nuclear power (a non-renewable fuel) gives reliable, low-carbon electricity, but produces radioactive waste and has high build/decommissioning costs.
- Some renewable resources have high set-up costs, require specific locations, or can damage local habitats, e.g. hydroelectric dams or tidal barrages.
- A mixed approach, increasing renewables while retaining some reliable back-up (e.g. gas or nuclear) and/or investing in energy storage, may be needed to balance reliability with reducing emissions.
- A justified conclusion, either way, that is clearly supported by the points made.

Question 16

(a) M1 rearranges: total power input = useful power output / efficiency
(a) M1 184 / 0.80 (correct substitution)
(a) A1 230 W cao
(a) Answer: 230 W

Question 17

(a) M1 correct unit conversions and substitution: 3000 * (15 * 60) = 3000 * 900
(a) A1 2700000 J cao, accept 2.7 * 10⁶ J or 2.7 MJ
(a) Answer: 2700000 J (2.7 MJ)
(b) M1 0.95 * 2700000, ft their value from (a)
(b) A1 2565000 J cao, accept 2.565 MJ, ft from (a)
(b) Answer: 2565000 J (2.565 MJ)
(c) M1 wasted energy = 2700000 - 2565000 = 135000 J, then (135000 / 2700000) * 100
(c) A1 5% AND thermal energy store of the surroundings/casing of the heater (transferred by heating), both required
(c) Answer: 5% is wasted, mostly to the thermal energy store of the surroundings.

Question 18

(a) M1 0.15 * 60 (correct substitution)
(a) A1 9 W cao
(a) Answer: 9 W
(b) M1 0.80 * 9 (correct substitution)
(b) A1 7.2 W cao
(b) Answer: 7.2 W
(c) M1 filament lamp wasted power: 60 - 9 = 51 W, ft from (a)
(c) M1 LED lamp wasted power: 9 - 7.2 = 1.8 W, ft from (b)
(c) A1 difference = 51 - 1.8 = 49.2 W, ft from their values
(c) Answer: 49.2 W

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