OxfordAQA International A Level Physics 3.11 Thermal Physics Exam Questions

About OxfordAQA International A Level Physics 3.11 Thermal Physics

Thermal physics opens Unit 4 (PH04), Energy and energy resources, in the OxfordAQA International A2 year. You study energy transfer by heating and by doing work, using the first law of thermodynamics, specific heat capacity and specific latent heat. You then look at conduction through materials using thermal conductivity and U-values, which links to the energy topics later in the unit. Finally you study the ideal gas laws and the kinetic theory model. Examiners often set heating and mixing calculations, method questions on the required practicals, ideal gas equation problems with unit conversions, and explanations of gas pressure in terms of molecules colliding with walls.

What you need to know

• 3.11.1 First law delta U = Q + W, where Q is energy transferred to the system by heating and W is work done on the system. Internal energy is the sum of the random kinetic and potential energies of the particles.
• Specific heat and latent heat Q = mc delta theta for a temperature change and Q = ml for a change of state at constant temperature. During melting or boiling, energy breaks bonds rather than raising the mean kinetic energy.
• 3.11.2 Conduction Rate of energy transfer by conduction = kA delta theta / delta x, where k is thermal conductivity. U-value is the rate of energy transfer per unit area per kelvin temperature difference, used to compare insulation in buildings: lower U-values mean better insulation.
• 3.11.3 Ideal gases Boyle's law pV = constant at constant temperature, Charles's law V / T = constant at constant pressure, and the pressure law p / T = constant at constant volume. pV = nRT = NkT, with temperature in kelvin and k = R / NA.
• 3.11.4 Kinetic theory Brownian motion is evidence for moving molecules. pV = 1/3 Nm(crms)^2. Assumptions include a large number of molecules moving randomly, negligible molecular volume, elastic collisions and no forces except during collisions. Average kinetic energy of a molecule = 3/2 kT = 3RT / 2NA.
• Required practicals Measure specific heat capacity or latent heat with an electrical heater, joulemeter or VI t, and correct for heat losses. Investigate the gas laws by varying one variable while holding another constant.

Common mistakes in 3.11 Thermal Physics

• Using degrees Celsius in the gas laws. Always convert to kelvin by adding 273.
• Saying temperature rises during melting. The energy supplied increases potential energy, not kinetic energy, so temperature is constant.
• Mixing up R and k. Use R with moles (n) and k with number of molecules (N).
• Writing that molecules lose energy when they collide with the container walls. In the ideal gas model collisions are elastic.
• Forgetting to use the temperature difference across a wall, not one temperature, when calculating conduction rate.

Try three 3.11 Thermal Physics exam-style questions