AQA GCSE Physics: Energy Changes in a System — 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 available free from your board.

Related: Energy Changes in a System study guide | Energy Changes in a System revision notes

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Questions

1. State the three routes by which a system’s energy can change. [3]

2. A cyclist and bicycle of total mass 75 kg travel at 6.0 m/s.

(a) Calculate the kinetic energy. [2] (b) The cyclist speeds up to 12 m/s. Calculate the new kinetic energy. [2] (c) Without recalculating, state how the kinetic energy has changed and why. [2]

3. A spring with a spring constant of 40 N/m is stretched by 0.15 m, within the limit of proportionality.

(a) Calculate the elastic potential energy stored. [2] (b) State the condition under which this equation applies. [1]

4. A crate of mass 12 kg is lifted 2.5 m onto a shelf. (g = 9.8 N/kg)

(a) Calculate the gravitational potential energy gained. [2] (b) The crate later falls off the shelf and lands on a lower ledge 1.0 m below the shelf (not the floor). Calculate the g.p.e. lost during the fall (not the full 2.5 m). [2]

5. In an experiment, 1.5 kg of water is heated using an electrical heater. The specific heat capacity of water is 4200 J/kg°C.

(a) Calculate the thermal energy needed to raise the temperature of the water by 20°C. [2] (b) Name the required practical this method belongs to, and describe briefly how it is carried out. [3] (c) State one source of error in this experiment and explain its effect on the result. [2]

6. A motor lifts a load and transfers 900 J of energy in 6.0 seconds.

(a) Calculate the power of the motor. [2] (b) A second motor transfers the same 900 J in 3.0 seconds. Which motor is more powerful, and why? [2]

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Answers

1. Heating [1], work done by a force [1], work done when a current flows [1].

2. (a) Ek = ½ × 75 × 6.0² [1] = 1350 J [1]. (b) Ek = ½ × 75 × 12² [1] = 5400 J [1]. (c) It has quadrupled [1], because kinetic energy is proportional to speed squared, and the speed has doubled [1].

3. (a) Ee = ½ × 40 × 0.15² [1] = 0.45 J [1]. (b) The equation only applies within the limit of proportionality — beyond this point, force is no longer proportional to extension [1].

4. (a) Ep = 12 × 9.8 × 2.5 [1] = 294 J [1]. (b) Ep = 12 × 9.8 × 1.0 [1] = 117.6 J [1] — the height used is the height fallen, not the full shelf height.

5. (a) ∆E = 1.5 × 4200 × 20 [1] = 126 000 J [1]. (b) Required practical activity 1 [1]: an immersed heater transfers a known amount of electrical work into the water [1], and the resulting temperature rise is measured to calculate the specific heat capacity [1]. (c) Some thermal energy is dissipated to the surroundings (the container, the air) rather than all going into the water [1], which would make the calculated specific heat capacity too high if not accounted for [1].

6. (a) P = 900 ÷ 6.0 [1] = 150 W [1]. (b) The second motor [1] — it transfers the same energy in less time, so its rate of energy transfer (power) is greater [1].

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Where marks are usually lost

• Forgetting the ½ in the kinetic energy and elastic potential energy equations.
• Not linking a doubled speed to a quadrupled kinetic energy — stating the change without the reasoning loses the explanation mark.
• Using the full drop height rather than the height actually fallen in a g.p.e. calculation.
• Describing Required practical activity 1 without naming what it measures (specific heat capacity) or the method (linking electrical work done to temperature rise).
• Treating power and energy as the same quantity — power questions always involve a time.

How this sub-topic relates to Conservation and Dissipation

Exam questions do not ask you to name the specification sub-topic they come from, and the command word is not a reliable guide to it: the two sub-topics are divided by subject matter. Sub-topic 4.1.1 covers energy stores and systems (including describing how energy is stored before and after a change), kinetic, elastic potential and gravitational potential energy, specific heat capacity and power. Sub-topic 4.1.2 covers energy transfers in a closed system, dissipation, reducing unwanted transfers (for example lubrication and thermal insulation) and efficiency — and efficiency is itself a calculation, so a question that asks for a value is not automatically about this page’s equations. What matters in the exam is recognising which physical ideas a question needs, then applying them.

Approaching multi-step calculation questions

Several questions above ask for a value in one part and a comparison or explanation in the next. Calculate the number first, write it down clearly with units, and only then answer the “why” or “which” part by referring back to that number — examiners award the explanation mark independently of the calculation mark, but only if the reasoning actually uses the figures you found, rather than repeating the rule in words alone.