Paper 4 Thermal Physics Route Answers

Q1 Selecting a thermal experiment

1. Measure mass, voltage, current, heating time, and temperature rise. [3]
2. Insulation reduces energy transfer to surroundings. [1]
3. If all electrical energy is assumed useful, heat loss makes $VIt$ larger than energy gained by the sample, so calculated $c=VIt/(m\Delta T)$ is too large. [2]

Q2 Gas-law graph

1. Axes and units [1]; scale that accommodates the data and extrapolation [1]; accurate plots [2]; best-fit straight line [1]. [5]

2. The temperature-axis intercept of the fitted line gives approximately $-272{}^{\circ }\mathrm{C}$, consistent with absolute zero at about $-273{}^{\circ }\mathrm{C}$ within the scatter and long extrapolation. Accept a value consistent with the candidate’s line. [3]
3. The intercept lies far outside the measured range, so small uncertainties in gradient or intercept produce a much larger uncertainty in the extrapolated crossing. [2]
4. A real gas may deviate from ideal behaviour and condense before zero pressure is reached; the straight-line ideal-gas relation cannot be tested all the way to absolute zero. [1]