Paper 4 Thermal Physics A Answers

Q1 Electrical heating practical

1. Axes and units [1]; suitable scale [1]; accurate plots [2]; best-fit straight line [1]. [5]

2. The fitted gradient is approximately $0.0417\mathrm{K}{\mathrm{s}}^{-1}$. With $P=mc(\mathrm{d}\theta /\mathrm{d}t)$,

$$c=\frac{18.0}{0.480(0.0417)}\approx 9.00\times 10^2\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}.$$

Accept follow-through from the candidate’s line. [3] 3. Some electrical input is transferred to the surroundings, so the measured temperature-rise rate is smaller than it would be without loss. Using the full $P$ in $c=P/(m\mathrm{d}\theta /\mathrm{d}t)$ therefore overestimates $c$. [2] 4. Use insulation or lagging; add a lid where appropriate; use a polished/reflective shield; reduce draughts; use a correction based on a cooling curve. Any two. [2] 5. Close placement makes the reading more representative of the nearby block temperature and reduces delay, but contact with the heater would bias the thermometer towards heater temperature rather than the block’s bulk temperature. [2]

Q2 Latent heat practical

1. Heater voltage/current/time or power/time, mass of water evaporated during timed interval. [3]
2. Water may evaporate before steady boiling/timing starts, so mass loss not due to measured energy interval must be excluded. [2]
3. Use eye protection, avoid steam burns, secure hot apparatus, or use low-voltage electrical safety. [1]