Physics › Thermal physics

Thermal physics

Heating, temperature and the behaviour of gases, explained by the motion of molecules.

Year 13 · 3 topics.

What thermal physics covers

Heating and temperature treated properly, then the behaviour of gases, then the molecular model that explains why the gas laws take the form they do. The unit contains one of the derivations A-level asks you to reproduce, so the assumptions behind kinetic theory need learning as carefully as the result.

The main ideas

  • Solids, liquids and gases described by particle spacing, ordering and motion, and internal energy as the total inside a body.
  • Why temperature does not change during a change of state, and calculations with specific heat capacity and latent heat taken stage by stage.
  • The electrical determination of a specific heat capacity or latent heat, and how the continuous-flow method cancels the losses.
  • The three gas laws with the fixed quantity named each time, and the extrapolation that locates absolute zero.
  • The equation of state in both forms, in moles and in molecules, and work done at constant pressure.
  • The assumptions of kinetic theory, the derivation that follows, and temperature as a measure of mean molecular kinetic energy.

The equations it turns on

Q=mcΔθQ = mc\Delta\theta
energy for a temperature change, with no change of state
Q=mlQ = ml
energy for a change of state, at constant temperature
pV=nRTpV=NkTpV = nRT \qquad pV = NkT
the equation of state, in moles and in molecules
W=pΔVW = p\Delta V
work done by a gas expanding at constant pressure
pV=13Nm(crms)2pV = \tfrac{1}{3}Nm(c_{rms})^{2}
the result of the kinetic theory derivation
12m(crms)2=3kT2\tfrac{1}{2}m(c_{rms})^{2} = \frac{3kT}{2}
what temperature measures, molecule by molecule

Where it usually goes wrong

  • Heat and temperature are different quantities. During melting or boiling the energy supplied separates the particles instead of speeding them up, so the temperature holds still while energy continues to flow in.
  • Every gas law calculation runs in kelvin. Degrees Celsius give an answer wrong by a factor, and absolute zero is located by extrapolation rather than defined.
  • The root mean square speed is not the mean speed. The squares are averaged first and the root taken afterwards, and the order matters.
  • The kinetic theory derivation is examinable in full, assumptions included, so it needs practising as a written argument rather than recognised.

Where to start

Thermal energy transfer first, then ideal gases and the gas laws. Take molecular kinetic theory last, since it explains the empirical laws you have just used, and give the derivation a session of its own.