OCR GCSE Physics A: Dissipation, efficiency and reducing energy loss (J249) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover statements P7.2a to P7.2g, Power and efficiency, in the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026). All of the content is assessed in both tiers. Learn the ideas first from the study guide or the revision notes. Course pages: the hub and the printable checklist. Where a question needs g, it is given as 9.8 N/kg in the question.

Questions

1. Describe, with an example, what is meant by energy being dissipated. [3]

2. A battery-powered cordless drill is used to make a hole in a wall. Describe the energy transfers from the battery to the useful store, and give two ways energy is wasted. [4]

3. A hair dryer is plugged into the a.c. mains. It contains a heating element and a small fan motor. Describe how energy is transferred, and state which stores are useful and which are wasted. [4]

4. A toaster has a power rating of 1.2 kW. It is switched on for 4.5 minutes. Calculate the energy transferred, in joules. [3]

5. A 2.0 kW heater and a 0.8 kW heater are each switched on for 30 minutes. (a) Calculate the energy transferred by each heater, in kW h. [2] (b) A friend says the 2.0 kW heater must be the more efficient one. Explain why the power rating does not show this. [2]

6. An electric motor takes 540 J from its supply. It gives out 378 J of useful kinetic energy. (a) Show that the efficiency of the motor is 70%. [2] (b) Calculate how much energy is wasted. [1]

7. An LED lamp has an input power of 8.0 W and an efficiency of 0.45. It is on for 10 minutes. Calculate the useful light energy and the wasted energy. [4]

8. Lamp A takes 60 J each second and gives out 3 J of light each second. Lamp B takes 11 J each second and gives out 2.2 J of light each second. (a) Calculate the efficiency of each lamp. [2] (b) Calculate the wasted energy each second for each lamp. [2] (c) Which lamp is better to use? Give a reason. [1]

9. (a) Explain how oil on the axle of a wheel increases the efficiency of a machine. [3] (b) Explain how loft insulation reduces the energy needed to heat a house. [3]

10. Two huts are built to the same shape. Hut X has thin metal walls. Hut Y has thick brick walls. Both start at the same temperature on a cold day. (a) Describe how the thickness of the walls affects the rate of cooling. [2] (b) Describe how the thermal conductivity of the wall material affects the rate of cooling. [2] (c) A class tests this with two copper cans of hot water, one wrapped in insulation. State two things they should keep the same. [2]

11. A battery-powered winch lifts a 9.0 kg load through 3.2 m in 25 s. The battery supplies 420 J. Take g = 9.8 N/kg. (a) Calculate the gain in gravitational potential energy of the load. [2] (b) Calculate the efficiency of the winch. [2] (c) Calculate the average input power. [2] (d) Suggest two changes that would increase the efficiency of the winch. [2]

Answers

1. Energy spreads out to the surroundings [1]. It is stored in less useful ways, mainly the thermal store of the surroundings [1]. Example: friction in a bicycle chain warms the chain and the air [1]. [3] Examiner insight: Answers that say energy is “lost” or “used up” tend to lose credit. Say where it goes.

2. The battery’s chemical store transfers energy electrically through the circuit [1]. The motor transfers it to the kinetic store of the drill bit [1]. Wasted: thermal energy in the wires and motor windings because of resistance [1], and thermal energy and sound from friction in bearings and gears [1]. [4] Examiner insight: A good answer names a store or a mechanism for each wasted transfer. Writing only “heat” and “sound” is thinner.

3. The a.c. mains transfers energy electrically to the device [1]. The element transfers it to the thermal store of the air, which is useful [1]. The motor transfers it to the kinetic store of the moving air, which is useful [1]. Wasted: thermal energy in the casing and wires, and sound from the fan and vibration [1]. [4] Examiner insight: Separate the two parts of the device. Marks are easier to earn when each component has its own useful and wasted store.

4. 1.2 kW = 1200 W [1]. 4.5 minutes = 270 s [1]. E = P t = 1200 × 270 = 324 000 J [1]. [3] Examiner insight: Convert both quantities to matching units before multiplying. A wrong or missing unit on the answer is an easy way to lose credit.

5. (a) 2.0 kW × 0.5 h = 1.0 kW h [1]. 0.8 kW × 0.5 h = 0.4 kW h [1]. (b) The power rating tells you how much energy is transferred each second [1]. Efficiency is useful energy divided by input, and the rating gives no information about how much is useful [1]. [4] Examiner insight: Naming both definitions, rate of transfer and fraction useful, is a clear way to show the difference.

6. (a) Efficiency = useful output / input = 378 / 540 [1] = 0.70, which is 70% [1]. (b) Wasted = 540 − 378 = 162 J [1]. [3] Examiner insight: Dividing useful by input is the safe order. Showing the substituted fraction gives you something to be credited for if the arithmetic slips.

7. Input energy = P t = 8.0 × 600 = 4800 J [1]. Useful = 0.45 × 4800 [1] = 2160 J [1]. Wasted = 4800 − 2160 = 2640 J [1]. [4] Examiner insight: Ten minutes must become 600 s. A forgotten conversion is a common reason for a wrong answer here.

8. (a) A: 3 / 60 = 0.05 (5%) [1]. B: 2.2 / 11 = 0.20 (20%) [1]. (b) A: 60 − 3 = 57 J each second [1]. B: 11 − 2.2 = 8.8 J each second [1]. (c) Lamp B, because it is more efficient and wastes far less energy for a similar amount of light [1]. [5] Examiner insight: Use the numbers you have worked out in the final comparison, rather than a general statement.

9. (a) The oil forms a layer between the surfaces [1]. This reduces friction, so less work is done against friction [1]. Less energy is stored thermally, so more of the input goes into useful motion [1]. (b) Loft insulation is a layer of material that traps air [1]. Trapped air is a poor conductor and cannot circulate, so conduction and convection are reduced [1]. The house loses energy more slowly, so the heating needs less energy to keep the temperature up [1]. [6] Examiner insight: “Insulation keeps heat in” is too vague. Explain the mechanism and link it to the rate of energy transfer.

10. (a) Thicker walls cool more slowly [1], because energy has a longer path to travel through the wall [1]. (b) A material with high thermal conductivity, like metal, transfers energy faster, so the building cools faster [1]. A material with low thermal conductivity, like brick, slows the cooling [1]. (c) Same volume of water [1] and same starting temperature [1]. The same lid or the same room would also be accepted as sensible controls. [6] Examiner insight: Say “rate of cooling” and compare the huts directly. Answers that only say “Y is warmer” tend to leave out the reason.

11. (a) GPE = m g h = 9.0 × 9.8 × 3.2 [1] = 282 J (282.24 J) [1]. (b) Efficiency = 282.24 / 420 [1] = 0.672, so 0.67 or 67% [1]. (c) Power = E / t = 420 / 25 [1] = 16.8 W [1]. (d) Lubricate the moving parts [1] and use thicker wires with lower resistance [1]. Other sensible changes, such as reducing friction in the gears, would also be reasonable. [8] Examiner insight: Show the formula and substitution before the answer. Quote the result to 2 significant figures, as the data given support.

Where marks are usually lost

• Writing that energy is “lost” or “used up”, not dissipated or stored thermally.
• Treating a high power rating as a sign of high efficiency.
• Dividing the wasted energy by the input instead of the useful energy.
• Leaving minutes or hours unconverted when using E = P t with watts.
• Giving an efficiency above 100% or forgetting to convert a decimal to a percentage when asked.
• Describing insulation as stopping energy transfer, when it only slows it.
• Answering a cooling question without mentioning rate, thickness or conductivity.
• Skipping the controlled variables in a fair test.

Next steps

• Revise with the revision notes.
• Re-read the study guide.
• Go to the OCR GCSE Physics hub and tick off the checklist.
• Take the free 10-minute diagnostics.
• Ready for more help? Book a free trial class.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), for first assessment in 2018. Topic P7.2, Power and efficiency, statements P7.2a to P7.2g.