AQA GCSE Physics: Conservation and Dissipation of Energy — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Energy revision notes

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Questions

1. Name four energy stores and the four transfer pathways. [4]

2. A car of mass 1200 kg travels at 15 m s⁻¹.

(a) Calculate its kinetic energy. [2] (b) Calculate the kinetic energy at 30 m s⁻¹. [2] (c) Explain what this shows about braking distance and speed. [2]

3. A 0.30 kg ball is dropped from 2.5 m. (g = 9.8 N kg⁻¹)

(a) Calculate the gravitational potential energy lost. [2] (b) Calculate the speed just before impact, assuming no air resistance. [3] (c) In reality it is slower. Explain where the missing energy has gone. [2]

4. A lamp transfers 60 J of electrical energy each second, of which 9 J becomes light.

(a) Calculate the efficiency. [2] (b) State what happens to the remaining energy. [2] (c) Explain why “wasted” is a misleading word for it. [2]

5. Explain the two factors that determine the rate of energy transfer through a house wall. [4]

6. Explain how cavity wall insulation reduces energy transfer, referring to both conduction and convection. [3]

7. A ball rolling across a rough floor gradually slows down and stops. Using ideas about energy stores, dissipation and the conservation of energy, explain what has happened to the ball’s kinetic energy, and why this does not violate the principle of conservation of energy. [4]

8. State two methods (other than better insulation) of reducing unwanted energy transfer, and explain how each works. [4]

9. Describe how Required Practical 2 investigates the effectiveness of thermal insulators. [3]

10. A student calculates an efficiency of 1.4 (140%) for a motor. Explain what this result indicates. [2]

11. State the principle of conservation of energy. [2]

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Answers

1. Any four stores: kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, magnetic, electrostatic [1] [1]. Pathways: mechanically, electrically, by heating, by radiation [1] [1].

2. (a) E = ½mv² = 0.5 × 1200 × 15² [1] = 135 000 J [1]. (b) 0.5 × 1200 × 30² [1] = 540 000 J [1]. (c) Doubling the speed quadruples the kinetic energy [1], so four times as much work must be done by the brakes and the braking distance quadruples [1].

3. (a) E = mgh = 0.30 × 9.8 × 2.5 [1] = 7.35 J [1]. (b) ½mv² = 7.35, so v = √(2 × 7.35 ÷ 0.30) [1] [1] = 7.0 m s⁻¹ [1]. (c) Some energy is transferred to the thermal store of the surrounding air by air resistance [1], and some to sound [1].

4. (a) 9 ÷ 60 [1] = 0.15 or 15% [1]. (b) It is transferred to the thermal store of the surroundings [1], heating the lamp and the air around it [1]. (c) The energy is not destroyed — it is still present [1], but it is dissipated and spread out, so it is too dilute to be useful [1].

5. The thickness of the wall [1] — a thicker wall transfers energy more slowly [1]. The thermal conductivity of the material [1] — a lower conductivity means a slower rate of transfer [1].

6. The cavity traps air, which has a low thermal conductivity, reducing conduction [1]. The insulating material prevents the air from circulating [1], which stops convection currents carrying energy across the cavity [1].

7. Friction between the ball and the floor dissipates energy from the kinetic store [1] to the thermal store of the ball and floor surfaces (and the surrounding air), and to the sound store, as the ball slows [1]. The ball’s kinetic energy has therefore decreased, not disappeared [1]. This does not violate conservation of energy, because the total energy is unchanged overall — it has simply been transferred to less useful stores, becoming spread out (‘dissipated’), rather than created or destroyed [1].

8. Lubrication — reduces friction between moving surfaces, cutting the energy dissipated as heat and sound [1] [1]. Thermal insulation — reduces the rate of energy transfer by heating, using materials of low thermal conductivity [1] [1]. A low thermal conductivity material is a good insulator, not the reverse — a common point of confusion.

9. Different materials (or insulator thicknesses) are used to wrap identical containers of hot water [1]; the temperature is measured at intervals as each cools [1], and the results are compared across materials/thicknesses to find which is the most effective insulator [1] — the material giving the smallest temperature drop over time is the best insulator.

10. Efficiency can never exceed 1 (100%), since useful output energy can never exceed total input energy [1], so a result above this signals an arithmetic or measurement error, not a genuinely efficient motor [1] — the student should re-check their input and output energy values rather than report the impossible figure.

11. Energy can be usefully transferred, stored or dissipated, but it can never be created or destroyed [1]; every system change dissipates some energy into the surroundings in a less useful form [1].

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Where marks are usually lost

• Calling heat or light an energy store.
• Saying braking distance doubles when speed doubles.
• Saying energy is “lost” rather than dissipated.
• Explaining insulation without mentioning the rate of transfer.
• Forgetting lubrication as a way to reduce unwanted energy transfer, focusing only on insulation.
• Describing Required Practical 2 without a fair-test control (identical containers, same starting temperature and volume of water).
• Accepting an efficiency calculation above 1 (100%) without recognising it signals an error.