
## Thermal Physics

The thermal physics module looks at the difference between heat and temperature, how the temperature of solids and liquids change as they are heated, and how they change state. Some of these ideas you will have met at GCSE, but the mathematical treatment is much more advanced. You will go on to study the experimental gas laws and how to apply newtonian physics to them to derive the equations that describe the kinetic theory of gases. This module will be tested on Paper 2 of the A level exam.

Although short, this module contains some important practical work and demonstrations. You will use the skills that you have learnt earlier in this course to measure the energy transferred to a material and the change in temperature. There is one CAP in the module, Boyle’s law and Charles’ law.

### What you need to know

Below you can read exactly what AQA want you to know for this module. You can also find the relevant section from the specification on each page of this site. You should be aware of both what you need to know, and (just as importantly) what you DO NOT need to know. It is also important to remember that you need to be able to apply these statements to a wide range of different contexts, so you must practise this by attempting lots of different questions and reading around the subject.

3.6.2.1 Thermal energy transfer

Internal energy is the sum of the randomly distributed kinetic energies and potential energies of the particles in a body.

The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it (and vice versa), eg a qualitative treatment of the first law of thermodynamics.

For a change of temperature:
$Q=mcΔθ$ where c is specific heat capacity.

Calculations including continuous flow.

Appreciation that during a change of state the potential energies of the particle ensemble are changing but not the kinetic energies. Calculations involving transfer of energy.

For a change of state $Q = ml$ where l is the specific latent heat.

3.6.2.2 Ideal gases

Gas laws as experimental relationships between p, V, T and the mass of the gas.

Concept of absolute zero of temperature.

Ideal gas equation: $pV = nRT$ for n moles and $pV = NkT$ for N molecules.

$$Work\; done = pΔV$$

Avogadro constant NA, molar gas constant R, Boltzmann constant k

Molar mass and molecular mass.

3.6.2.3 Molecular kinetic theory model

Brownian motion as evidence for existence of atoms.

Explanation of relationships between p, V and T in terms of a simple molecular model.

Students should understand that the gas laws are empirical in nature whereas the kinetic theory model arises from theory.

Assumptions leading to $$pV=\frac{1}{3}Nm\left ( c_{\mathrm{rms}} \right )^{2}$$ including derivation of the equation and calculations.

A simple algebraic approach involving conservation of momentum is required.

Appreciation that for an ideal gas internal energy is kinetic energy of the atoms.

Use of average molecular kinetic energy =

$$\frac{1}{2}m\left ( c_{\mathrm{rms}} \right )^{2}=\frac{3}{2}kT=\frac{3RT}{2N_{\mathrm{A}}}$$

Appreciation of how knowledge and understanding of the behaviour of a gas has changed over time.