A Level Physics: Temperature — 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: Temperature revision notes and the full study guide, which covers specific heat capacity and specific latent heat in more depth.

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Questions

1. State what temperature is a measure of, and explain how this differs from thermal energy. [3]

2. Define thermal equilibrium, and state the direction of energy flow between two bodies. [3]

3. Convert: (a) 37 °C to K, (b) 195 K to °C, (c) a temperature rise of 25 °C to K. [3]

4. Explain why the thermodynamic scale is considered superior to an empirical scale. [3]

5. A resistance thermometer has resistance 200.0 Ω at the ice point, 280.0 Ω at the steam point, and 254.0 Ω when placed in a liquid whose temperature is to be measured.

(a) State the formula used to convert a thermometric property to a temperature on an empirical scale. [1] (b) Calculate the empirical temperature of the liquid on this scale. [2] (c) State one reason why this value might differ from the temperature found using a different thermometric property. [1]

6. A student must measure a rapidly changing temperature.

(a) State which thermometer is most suitable. [1] (b) Give two reasons. [2] (c) State one disadvantage of this choice. [1]

7. Explain what happens to the molecules of a substance at absolute zero. [2]

8. A large bath of warm water contains more thermal energy than a small cup of boiling water. Explain how this is possible. [3]

9. State the equation linking energy transferred, specific heat capacity and temperature change, then calculate the energy required to raise the temperature of 500 g of water (c = 4200 J kg⁻¹ K⁻¹) from 20 °C to 80 °C. [3]

10. A pure substance is cooled steadily and its temperature is plotted against time. Explain why the temperature remains constant while the substance changes state, even though energy continues to be removed from it, and name the equation used to calculate this energy. [3]

11. The specific latent heat of fusion of ice is 3.34 × 10⁵ J kg⁻¹. Calculate the energy required to melt 200 g of ice at 0 °C. [2]

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Answers

1. Temperature is a measure of the mean kinetic energy of the molecules [1]. Thermal energy is the total energy of all the molecules [1], so it depends on the amount of substance as well as the temperature [1].

2. Two bodies are in thermal equilibrium when there is no net flow of thermal energy between them [1], which occurs when they are at the same temperature [1]. Energy flows from the body at higher temperature to the one at lower temperature [1].

3. (a) 310 K [1]. (b) −78 °C [1]. (c) 25 K — an interval is the same size on both scales [1].

4. It is defined independently of the properties of any particular substance [1]. Two empirical thermometers using different thermometric properties can agree at the fixed points but disagree between them [1], whereas the thermodynamic scale gives a unique value at every temperature, making it the standard against which every empirical scale is calibrated [1].

5. (a) θ = (X_θ − X₀) ÷ (X₁₀₀ − X₀) × 100, where X is the thermometric property (here, resistance) [1]. (b) θ = (254.0 − 200.0) ÷ (280.0 − 200.0) × 100 [1] = 67.5 °C [1]. (c) Different thermometric properties vary non-linearly with each other, so empirical scales built on different properties agree only at the fixed points and disagree between them [1].

6. (a) A thermocouple [1]. (b) It has a small thermal capacity, so it responds quickly [1], and it absorbs very little energy from the system, so it barely disturbs what it is measuring [1]. (c) Its response is non-linear and it requires calibration against known fixed points before use [1].

7. They have the minimum possible internal (kinetic) energy [1] — since mean translational kinetic energy is proportional to absolute temperature, this minimum is zero at 0 K, the lowest temperature theoretically attainable [1].

8. Temperature measures the mean kinetic energy per molecule, which is higher in the boiling water [1]. Thermal energy is the total over all molecules [1], and the bath contains very many more molecules, so its total exceeds that of the cup despite the lower temperature [1].

9. Q = mcΔθ, where Q is energy transferred, m is mass, c is specific heat capacity and Δθ is the temperature change [1]. Q = 0.500 × 4200 × (80 − 20) [1] = 126 000 J (126 kJ) [1].

10. The energy removed during a change of state is latent heat, calculated from Q = mL [1]. This energy changes the arrangement/separation of the molecules (breaking or forming intermolecular bonds), not their kinetic energy [1], so the mean kinetic energy of the molecules — and hence the temperature — stays constant until the change of state is complete [1].

11. Q = mL, where L is the specific latent heat [1]. Q = 0.200 × 3.34 × 10⁵ [1] = 66 800 J (66.8 kJ) [1].

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Where marks are usually lost

• Converting a temperature interval by adding 273.
• Saying molecules have some residual motion left at absolute zero — per the syllabus relation, mean kinetic energy is proportional to temperature, so it is zero at 0 K.
• Confusing temperature with thermal energy.
• Forgetting to use the correct differences (X_θ − X₀ over X₁₀₀ − X₀) when calculating an empirical temperature.
• Using Q = mcΔθ across a change of state — it only applies while the substance stays in a single state; a change of state needs Q = mL instead.
• Forgetting Δθ is the same size in kelvin or Celsius, so temperature changes never need converting, even though absolute temperatures do.