IGCSE Physics: Thermal Physics — 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: Thermal Physics revision notes

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Questions

1. Describe the arrangement and motion of particles in a gas. [2]

2. Convert 20 °C to kelvin. [1]

3. A fixed mass of gas at constant temperature has a pressure of 200 kPa and a volume of 0.5 m³. The gas is compressed to a volume of 0.2 m³ at the same temperature. Calculate the new pressure. [3]

4. A 2 kg block of metal is heated, raising its temperature from 20 °C to 45 °C, using 22 500 J of energy. Calculate the specific heat capacity of the metal. [3]

5. Explain, in terms of particles, why evaporation causes cooling of the remaining liquid. [3]

6. State two factors that increase the rate of evaporation of a liquid. [2]

7. A metal spoon and a wooden spoon are both left in a room overnight. Explain why the metal spoon feels colder to the touch, even though both are at the same temperature. [3]

8. Explain why a matt black surface is a better emitter of infrared radiation than a shiny silver surface, and describe one everyday application of this fact. [3]

9. Describe, using ideas about density, how convection currents form in a room heated by a radiator. [3]

10. State and explain the order in which solids, liquids and gases expand when heated by the same temperature rise, and give one everyday application of thermal expansion. [3]

11. Describe how you would observe Brownian motion using smoke particles in air, and state what this observation provides evidence for. [3]

12. Explain why metals are much better conductors of heat than non-metallic solids such as wood or glass. [2]

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Answers

1. Particles in a gas are far apart, arranged randomly [1], and move quickly in random directions, only interacting during collisions [1].

2. T (K) = θ (°C) + 273 = 20 + 273 = 293 K [1].

3. p₁V₁ = p₂V₂ [1] → (200 × 0.5) = p₂ × 0.2 [1] → p₂ = 100/0.2 = 500 kPa [1].

4. c = ΔE / (mΔθ) [1] = 22500 / (2 × 25) [1] = 22500/50 = 450 J/(kg °C) [1].

5. Evaporation is the escape of the most energetic particles from the surface of a liquid [1]. Removing these highest-energy particles lowers the average kinetic energy of the particles left behind [1], which lowers the liquid’s temperature [1].

6. Any two of: higher temperature; larger surface area; air movement/draught over the surface; lower humidity [2 — one mark each, max 2].

7. Metal is a much better thermal conductor than wood [1]. When you touch it, heat conducts away from your hand into the metal much faster than into the wood [1]. Your skin senses the rate of heat loss rather than an object’s actual temperature, so the faster heat loss to the metal makes it feel colder even though both spoons are genuinely at the same temperature [1].

8. A matt black surface absorbs and emits infrared radiation much better than a shiny silver surface, which mostly reflects radiation instead [1]. This is because dull, dark surfaces are efficient emitters and absorbers, while light, shiny surfaces are poor emitters and good reflectors [1]. Application: e.g. car radiators/heating panels are often painted matt black to radiate heat away efficiently; vacuum flasks have shiny silvered inner surfaces to minimise radiation loss [1, any valid example].

9. Air near the radiator is heated, so it expands and becomes less dense than the surrounding cooler air [1]. This warmer, less dense air rises [1]. Cooler, denser air sinks to take its place, is then heated in turn, setting up a continuous convection current that circulates warm air around the room [1].

10. Gases expand the most, then liquids, then solids [1], because of differences in how strongly the particles are held together and how far apart they already are — gas particles are weakly attracted and far apart, so they spread out much more for the same rise in kinetic energy, while a solid’s particles are held rigidly in place and can only vibrate slightly further [1]. Application: e.g. expansion gaps are left in bridges and railway tracks, or a bimetallic strip is used in a thermostat, so that the structure is not damaged as it expands [1, any valid example].

11. View smoke particles in a glass cell under a microscope, illuminated from the side [1]. The smoke particles are seen to move in a continuous, random, erratic (zig-zag) path [1]. This is evidence for the particle model of matter: the visible smoke particles are being struck unevenly by fast-moving, randomly-moving air molecules that are too small to see directly [1].

12. In addition to particle vibrations passing energy to neighbouring particles (as happens in any solid), metals contain free (delocalised) electrons [1]. These free electrons move quickly through the metal, carrying thermal energy along with them much faster than vibrations alone can, which is why metals conduct heat far better than non-metallic solids [1].

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Where marks are usually lost

• Giving the pV = constant relationship without stating the condition “at constant temperature, for a fixed mass of gas.”
• Confusing evaporation with boiling — evaporation happens at any temperature and only at the surface; boiling happens at one fixed temperature throughout the liquid.
• Explaining conduction in metals without mentioning free electrons, or explaining it as if all solids conduct the same way.
• Describing thermal expansion without linking the size of the effect to how strongly the particles are held together, in gases, liquids and solids respectively.

Work through the Thermal Physics revision notes alongside these questions: the notes set out the kinetic particle model and the three heat-transfer mechanisms in table form, while these questions test whether you can apply the underlying particle reasoning to a specific, unfamiliar situation rather than just recall a definition.