OxfordAQA International GCSE Physics: Energy transfers, efficiency, Sankey diagrams, resources (9203) – 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 sections 3.2.2 and 3.2.3 of the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards. Where needed, use g = 9.8 N/kg. Explanations are in the study guide and the revision notes. Course pages: OxfordAQA IGCSE Physics hub and printable checklist. Not sure where to start? Try the free diagnostics.

Questions

1. A pendulum bob is pulled to one side and released.

(a) Describe the energy transfers as the bob swings from its release point to the lowest point. [2] (b) Explain why each swing is slightly lower than the one before. [2]

2. A pendulum bob of mass 0.40 kg is released from rest 0.15 m above its lowest point.

(a) Calculate its gravitational potential energy at release, relative to the lowest point. [2] (b) Calculate its speed at the lowest point if no energy is dissipated. [2] (c) The speed measured at the lowest point is 1.6 m/s. Calculate the energy dissipated. [2]

3. Name the main energy transfer in each case. [3]

(i) A stretched bowstring is released and fires an arrow. (ii) A ball rolls up a slope and stops. (iii) A cyclist brakes to a stop.

4. A motor takes in 240 J of energy and transfers 156 J usefully.

(a) Calculate the efficiency as a decimal. [1] (b) Give the efficiency as a percentage. [1] (c) Calculate the energy dissipated. [1]

5. A hoist is 0.45 efficient and its useful power output is 72 W.

(a) Calculate the power supplied to the hoist. [2] (b) Calculate the power wasted. [1]

6. An electric fan takes in 80 J of electrical energy. It transfers 48 J as kinetic energy of the air and 4 J as sound. The rest is dissipated as thermal energy.

(a) Calculate the thermal energy dissipated. [1] (b) A Sankey diagram uses a scale of 1 mm for 2 J. Calculate the width of each of the four arrows. [2] (c) Calculate the efficiency as a percentage. [1] (d) State what feature of the diagram shows that energy is conserved. [1]

7. A Sankey diagram for a small generator uses a scale of 1 mm for 40 J. The input arrow is 45 mm wide, the useful electrical arrow is 12 mm wide and the sound arrow is 3 mm wide. The remaining arrow shows thermal energy.

(a) Calculate the input energy. [1] (b) Calculate the width of the thermal arrow and the thermal energy it shows. [2] (c) Calculate the efficiency as a percentage to 3 significant figures. [1]

8. Data supplied for three fuels:

Fuel	State at room temperature	Energy content (MJ/kg)	Hazard
A	Solid	29	Burns slowly, smoky
B	Liquid	46	Flammable vapour
C	Gas	142	Ignites very easily, stored under high pressure

(a) Calculate the mass of fuel B needed to release 920 MJ. [2] (b) Calculate the mass of fuel C needed to release 920 MJ. [1] (c) Explain why fuel C is not the obvious choice for a small portable stove even though its energy content is highest. [2]

9. A boiler burns 3.5 kg of a fuel with energy content 32 MJ/kg. The water gains 84 MJ.

(a) Calculate the energy released by the fuel. [1] (b) Calculate the efficiency of the boiler as a percentage. [2] (c) State where the rest of the energy goes. [1]

10.

(a) State what is meant by a renewable energy resource. [1] (b) Give one environmental impact of extracting a fossil fuel and one of burning it. [2] (c) Give one environmental impact of solar power, and state at which stage (extraction, use or disposal) it occurs. [2]

11. A coastal town has hot springs nearby, large waves most of the year, and long cloudy winters. It needs electricity at night as well as by day. Options are solar panels, wave power and geothermal power.

(a) A solar array has area 25 m². Sunlight gives 480 W for each square metre and the array is 16% efficient. Calculate the useful power output. [3] (b) Calculate the energy transferred in 6.0 hours at this output, in MJ. [2] (c) Recommend an option for night-time supply, with one advantage and one drawback of your choice. [3]

12. A car of mass 900 kg travelling at 20 m/s brakes to rest.

(a) Calculate the kinetic energy before braking. [2] (b) State what happens to this energy during braking. [2] (c) The engine is 24% efficient. Calculate the chemical energy needed from the fuel to give the car 180 kJ of kinetic energy. [2] (d) The fuel has energy content 46 MJ/kg. Calculate the mass of fuel in grams to 3 significant figures. [2]

Answers

1. (a) Gravitational potential energy is transferred [1] to kinetic energy [1]. (b) Air resistance and friction at the pivot dissipate energy [1] by heating the surroundings, so less is available to lift the bob on each swing [1]. Examiner insight: “Energy is lost” does not describe what happened to it; naming dissipation to the surroundings by heating is what separates a full answer.

2. (a) GPE = mgh = 0.40 x 9.8 x 0.15 [1] = 0.588 J [1]. (b) 0.5 x 0.40 x v^2 = 0.588 [1], so v^2 = 2.94 and v = 1.7 m/s (1.71 to 3 s.f.) [1]. (c) Kinetic energy = 0.5 x 0.40 x 1.6^2 = 0.512 J [1]. Dissipated = 0.588 - 0.512 = 0.076 J [1]. Examiner insight: Show the substitution before the answer: visible working lets an examiner see your method even if the final value slips.

3. (i) Elastic potential energy to kinetic energy [1]. (ii) Kinetic energy to gravitational potential energy (and some dissipated by friction) [1]. (iii) Kinetic energy to thermal energy in the brakes and surroundings [1]. Examiner insight: Name both stores or types of energy in each answer; “energy changes” with no types named is too vague to credit.

4. (a) 156/240 = 0.65 [1]. (b) 65% [1]. (c) 240 - 156 = 84 J [1]. Examiner insight: Match the form requested: a decimal when asked for a decimal, a percentage when asked for a percentage, and do not add a unit.

5. (a) Input power = useful power / efficiency [1] = 72/0.45 = 160 W [1]. (b) 160 - 72 = 88 W [1]. Examiner insight: Dividing by the efficiency, not multiplying, is the usual slip; check that the input comes out larger than the output.

6. (a) 80 - 48 - 4 = 28 J [1]. (b) Input 80/2 = 40 mm [1]. Kinetic 24 mm, sound 2 mm, thermal 14 mm [1]. (c) 48/80 = 0.60, so 60% [1]. (d) The widths of the output arrows add up to the width of the input arrow (24 + 2 + 14 = 40 mm) [1]. Examiner insight: A Sankey answer is stronger with a scale, labels and values on every arrow; unlabelled arrows are hard to credit.

7. (a) 45 x 40 = 1800 J [1]. (b) Thermal width = 45 - 12 - 3 = 30 mm [1], so energy = 30 x 40 = 1200 J [1]. (c) 480/1800 = 0.2667, so 26.7% [1]. Examiner insight: The useful energy is the electrical arrow only, so do not include the sound arrow in the numerator.

8. (a) Mass = energy / energy content = 920/46 [1] = 20 kg [1]. (b) 920/142 = 6.5 kg (6.48 kg) [1]. (c) Fuel C is a gas stored under high pressure, so the container is heavy and bulky [1], and it ignites very easily, which is a safety risk when portable [1]. Examiner insight: A comparison needs two linked points: a data-based comparison of mass or energy content, and a storage or safety reason.

9. (a) 3.5 x 32 = 112 MJ [1]. (b) Efficiency = 84/112 [1] = 0.75, so 75% [1]. (c) The remaining 28 MJ is transferred to the surroundings (for example in hot flue gases) [1]. Examiner insight: The input to the efficiency is the energy released by the fuel, not the mass of fuel.

10. (a) A resource that is replaced naturally as fast as it is used [1]. (b) Extraction: for example habitat damage from mining, or oil spills [1]. Burning: carbon dioxide released, contributing to global warming (or sulfur dioxide giving acid rain) [1]. (c) For example the manufacture or disposal of panels has an environmental cost, or large areas of land are covered [1]. The matching stage is given, such as disposal or extraction of materials [1]. Examiner insight: “Pollution” alone is too general; name the substance or the damage, and tie it to the stage asked about.

11. (a) Input power = 25 x 480 = 12 000 W [1]. Output = 0.16 x 12 000 [1] = 1920 W [1]. (b) E = P x t = 1920 x 6.0 x 3600 [1] = 41 472 000 J = 41 MJ [1]. (c) Geothermal power is the best match [1]. Advantage: it gives steady output day and night using the nearby hot springs [1]. Drawback: drilling and building are costly (or some sites release gases) [1]. Examiner insight: A recommendation is stronger when linked to the scenario given, here the need for night-time supply and cloudy winters, rather than a general list.

12. (a) Ek = 0.5 x 900 x 20^2 [1] = 180 000 J (180 kJ) [1]. (b) The kinetic energy is transferred by friction in the brakes [1] as thermal energy that heats the brakes and surroundings, so it is dissipated [1]. (c) Input = useful / efficiency = 180/0.24 [1] = 750 kJ [1]. (d) 750 kJ = 0.750 MJ, so mass = 0.750/46 [1] = 0.01630 kg = 16.3 g [1]. Examiner insight: Keep units consistent (kJ with kJ, MJ with MJ) and convert the final mass to the unit requested.

Where marks are usually lost

• Describing energy as “lost” or “used up” rather than dissipated to the surroundings.
• Dividing the input by the output when finding efficiency.
• Giving a percentage when a decimal was asked for, or the other way round.
• Sankey arrows whose widths do not add up, or with no scale or labels.
• Including wasted energy in the useful output.
• Mixing MJ and kJ, or joules and watts, in one calculation.
• Vague environmental points such as “pollution” with no stage or substance named.
• Not linking a recommended energy source to the conditions in the question.

Next steps

• Revision notes
• Study guide
• OxfordAQA IGCSE Physics hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OxfordAQA International GCSE Physics (9203) specification, Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. Sections 3.2.2 and 3.2.3.