CCEA A Level Physics 4.2 Thermal Physics Exam Questions

About CCEA A Level Physics 4.2 Thermal Physics

Thermal physics is part of Unit A2 1: Deformation of Solids, Thermal Physics, Circular Motion, Oscillations and Atomic and Nuclear Physics. You need the experimental gas laws, the ideal gas equation in both forms, pV = nRT and pV = NkT, and the assumptions of the kinetic theory of gases. You must link the average kinetic energy of molecules to absolute temperature and explain what absolute zero means. The section also covers specific heat capacity and an electrical method for measuring it. Examiners like ideal gas calculations with temperatures in kelvin, kinetic theory assumptions, and specific heat capacity experiments with sources of error.

What you need to know

• Gas laws For a fixed mass of gas: Boyle's law pV = constant at constant temperature; Charles's law V/T = constant at constant pressure; pressure law p/T = constant at constant volume. T must be in kelvin.
• Ideal gas equation pV = nRT where n is the number of moles and R = 8.31 J mol-1 K-1, or pV = NkT where N is the number of molecules and k = 1.38 x 10^-23 J K-1. R = Nk for one mole, with NA = 6.02 x 10^23 mol-1.
• Kinetic theory Assumptions: molecules are in random motion, have negligible volume, collisions are elastic, there are no forces between molecules except in collisions, and collision time is negligible. This leads to pV = 1/3 Nm<c^2>.
• Temperature and energy Average kinetic energy of a molecule = 1/2 m<c^2> = 3/2 kT, so it is proportional to absolute temperature.
• Absolute zero 0 K (about -273 °C) is the temperature at which the molecules of a substance have minimum kinetic energy. T(K) = θ(°C) + 273.
• Specific heat capacity Q = mcΔθ. Measure c with an electrical heater in an insulated block or liquid: energy supplied = VIt, record the temperature rise and calculate c = VIt / mΔθ.

Common mistakes in 4.2 Thermal Physics

• Using degrees Celsius in pV = nRT or in gas law ratios; always convert to kelvin.
• Confusing n (moles) with N (molecules) and using the wrong constant.
• Saying molecules stop moving at absolute zero; they have minimum kinetic energy.
• Writing <c^2> as the mean speed squared; it is the mean of the squared speeds.
• Ignoring energy lost to the surroundings in specific heat capacity experiments, which makes the calculated c too large.

Try three 4.2 Thermal Physics exam-style questions