IGCSE Physics: Thermal Physics (Extended) — Practice Questions (Cambridge 0625)

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 — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Tier note: every question here is marked (Extended): each one is on Supplement content of the 0625 syllabus — pV = constant (2.1.3 Supplement 3), gas pressure as force per unit area from particle collisions (2.1.2 Supplement 7), specific heat capacity (2.2.2 Supplement 3), boiling compared with evaporation (2.2.3 Supplement 6), poor conduction in gases (2.3.1 Supplement 3) and the balance of energy gained and lost by radiation (2.3.3 Supplement 4–5). Core candidates do not need them.

After each answer there is a common mistake to avoid.

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Questions

1. (Extended) A bicycle pump is sealed at the end and contains 60 cm³ of air at a pressure of 100 kPa. The handle is pushed in slowly, so the temperature stays the same, until the volume of the air is 24 cm³. Calculate the new pressure of the air. [2]

2. (Extended) A fixed mass of gas in a sealed cylinder is slowly compressed so that its temperature stays the same. Explain, in terms of the particles and the forces they exert, why the pressure of the gas increases. [3]

3. (Extended) A 60 W electric heater is placed in 0.80 kg of water. The specific heat capacity of water is 4200 J/(kg °C). Assuming all the energy from the heater goes into the water, calculate the time needed to raise the temperature of the water by 15 °C. [3]

4. (Extended) State two differences between boiling and evaporation. [2]

5. (Extended) Explain, in terms of particles, why thermal conduction in a gas is much poorer than in a solid. [2]

6. (Extended) A metal plate left in sunlight absorbs energy at a rate of 30 W and, at its present temperature, transfers energy away at a rate of 22 W. State and explain what happens to the temperature of the plate over the next few minutes. [2]

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Answers

1. (Extended) p₁V₁ = p₂V₂, so 100 × 60 = p₂ × 24 [1]; p₂ = 6000 ÷ 24 = 250 kPa [1].

Common mistake: multiplying by the volume ratio the wrong way round. A smaller volume must give a larger pressure, so check that your answer is bigger than 100 kPa.

2. (Extended) The gas particles collide with the walls, and each collision exerts a force on the wall [1]. With a smaller volume the particles hit the walls more often (more collisions per second on each unit of area) [1], so the total force per unit area, which is the pressure, is greater [1].

Common mistake: saying the particles move faster. At constant temperature their average speed does not change; the increase comes from more frequent collisions.

3. (Extended) ΔE = mcΔθ = 0.80 × 4200 × 15 = 50 400 J [1]. t = ΔE ÷ P [1] = 50 400 ÷ 60 = 840 s (14 minutes) [1].

Common mistake: stopping at the energy. The question asks for the time, so the energy must then be divided by the power.

4. (Extended) Boiling happens at one fixed temperature (the boiling point), but evaporation happens at any temperature [1]; boiling happens throughout the liquid with bubbles forming, but evaporation happens only at the surface [1].

Common mistake: describing only one of the two processes. Each difference needs both sides stated, for example “at any temperature” and “only at the boiling point”.

5. (Extended) In a gas the particles are far apart [1], so they collide with each other only rarely and energy is passed on from particle to particle much more slowly than through the closely packed, vibrating particles of a solid [1].

Common mistake: saying gas particles “do not move” or “cannot vibrate”. They move quickly; the problem is the large spacing between them.

6. (Extended) The temperature of the plate rises [1], because it receives energy faster than it transfers energy away (30 W > 22 W); as it gets hotter it transfers energy away faster, until the two rates are equal and the temperature stays constant [1].

Common mistake: saying the plate keeps getting hotter for ever. A hotter object transfers energy away faster, so it reaches a new steady temperature.

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Where marks are usually lost

• Getting the pV = constant calculation upside down; always check whether the pressure should rise or fall.
• Explaining gas pressure without mentioning force, or saying particles speed up when the temperature is constant.
• Stopping part-way through a two-step specific heat capacity calculation.
• Describing boiling and evaporation separately instead of making direct comparisons.
• Forgetting that an object reaches a constant temperature when it gains and loses energy at the same rate.