Topic 9: Thermodynamics — Edexcel A-Level Physics
Test yourself on Topic 9: Thermodynamics with PEARSON EDEXCEL A-Level practice questions.
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Topic 9: Thermodynamics explained
These equations describe energy transferred during heating and during a change of state.
Read the full explanation
ΔE = mcΔθ gives the energy needed to change the temperature of a mass m by Δθ, where c is the specific heat capacity (J kg⁻¹ K⁻¹). For example, heating 2.0 kg of water (c ≈ 4200 J kg⁻¹ K⁻¹) by 10 K needs ΔE = 2.0 × 4200 × 10 = 84 000 J = 8.4 × 10⁴ J. ΔE = LΔm gives the energy for a change of state, where L is the specific latent heat (J kg⁻¹) and Δm is the mass changing state; melting 0.50 kg of ice (L ≈ 3.3 × 10⁵ J kg⁻¹) needs ΔE = 3.3 × 10⁵ × 0.50 = 1.65 × 10⁵ J. During a change of state the temperature stays constant, so use the latent-heat equation, not the specific-heat one. Keep units consistent: mass in kg, temperature change in K (a change of 1 K equals 1 °C).
Your focus
- Select and apply ΔE = mcΔθ to calculate energy for a temperature change.
- Select and apply ΔE = LΔm to calculate energy for a change of state.
- Convert masses and temperature changes to consistent SI units before calculating.
Topic 9: Thermodynamics exam tips
Marking Points
- Selects ΔE = mcΔθ for a temperature change and ΔE = LΔm for a change of state.
- Substitutes mass in kg, specific heat capacity in J kg⁻¹ K⁻¹ and temperature change in K.
- Uses specific latent heat in J kg⁻¹ with the mass changing state.
- Calculates energy in joules and gives the answer to an appropriate number of significant figures.
- Recognises that temperature is constant during a change of state.
Examiner Tips
- 💡Decide first whether the temperature changes or the state changes, then pick the equation.
- 💡Convert all masses to kg and check units before calculating.
- 💡Show the substitution with units so the examiner can follow your working.
Common Mistakes
- Using ΔE = mcΔθ during melting or boiling; correction: use ΔE = LΔm because temperature does not change.
- Forgetting to convert grams to kilograms; correction: divide mass in g by 1000 before substituting.
- Mixing up specific heat capacity c and specific latent heat L; correction: c applies to temperature change, L to change of state.
- Using a temperature in °C as if it were a temperature change; correction: use the change Δθ, which is numerically the same in K and °C.