A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Internal Energy and Heat Transfer — mark scheme explained
The short answer
Understanding internal energy and heat transfer is crucial in the study of thermodynamics, which forms a significant part of A-Level Physics. Internal energy is the sum of the randomly distributed kinetic energies and potential energies of the particles within a system.
The question
A 2 kg block of ice at -10°C is heated until it completely melts. The specific heat capacity of ice is 2100 J/kg°C, and the specific latent heat of fusion for ice is 334,000 J/kg. Calculate the total energy required. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Calculate the energy required to raise the temperature of the ice from -10°C to 0°C using Q = mcΔθ.
- S2
Q 1 = 2 kg × 2100 J/kg°C × (0°C - (-10°C))
- S3
Q 1 = 2 kg × 2100 J/kg°C × 10°C
- S4
Q 1 = 42,000 J
- S5
Calculate the energy required to melt the ice at 0°C using Q = ml.
- S6
Q 2 = 2 kg × 334,000 J/kg
- S7
Q 2 = 668,000 J
- S8
Add the two energies to find the total energy required.
- S9
Total Q = Q 1 + Q 2
- S10
Total Q = 42,000 J + 668,000 J
- S11
Total Q = 710,000 J
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Calculate the energy required to raise the temperature of the ice from -10°C to 0°C using Q = mcΔθ.
- S2
Q 1 = 2 kg × 2100 J/kg°C × (0°C - (-10°C))
- S3
Q 1 = 2 kg × 2100 J/kg°C × 10°C
- S4
Q 1 = 42,000 J
- S5
Calculate the energy required to melt the ice at 0°C using Q = ml.
- S6
Q 2 = 2 kg × 334,000 J/kg
- S7
Q 2 = 668,000 J
- S8
Add the two energies to find the total energy required.
- S9
Total Q = Q 1 + Q 2
- S10
Total Q = 42,000 J + 668,000 J
- S11
Total Q = 710,000 J
Final answer: 710,000 J
Common mistakes
- Confusing specific heat capacity with specific latent heat. — Review the definitions: Specific heat capacity is used for temperature changes (Q = mcΔθ), while specific latent heat is used for phase changes (Q = ml).
- Forgetting to convert units, especially when dealing with mass and time. — Always check that the units for mass (kg), temperature (°C), and time (s) are consistent. Convert if necessary.
- Using the wrong formula for continuous flow systems. — Remember to use the correct continuous-flow approach: P = (m/t)cΔθ + H, eliminating heat loss by comparing two different flow rates (P₁ − P₂ = ((m₁ − m₂)/t)cΔθ).
- Neglecting to account for phase changes in energy calculations. — Always check if the problem involves a phase change and use the appropriate formula (Q = ml) for that part of the calculation.
- Failing to identify and correct systematic errors in experiments. — Calibrate equipment, use more accurate instruments, and correct for known biases. Always check for consistency in measurements.
- Incorrectly applying the first law of thermodynamics. — Ensure a clear understanding of the first law: The change in internal energy is equal to the heat added minus the work done by the system. Practice applying this concept in various scenarios.
Where the marks go
- Full worked solution (all marking points)5 marks