# Thermal physics

The kinetic theory of gases makes the following assumptions:
• Gas consists of point particles, i.e. their volume is negligible compared with distance between particles • The particles have the same mass • The number of molecules is large enough to apply statistical treatment • The particles are in constant, random and rapid motion (Brownian motion) • All collisions are perfectly elastic • All particles are spherical in shape • Except during collisions, interactions between particles are negligible (i.e. relativistic and quantum-mechanical e ects are negligible, as a result, dynamics can be considered classically) • The average kinetic energy of the gas particles only depends on the temperature of the system • The time taken for collisions is negligible compared to the time taken between collisions
The kinetic theory of gases leads us to believe that the pressure exerted by the gas can be attributed to movement of molecules in the gas

Molar mass,M The mass (in kilograms) of a mole of a gas
Relative molecular mass,Mr The mass (in grams) of a mole of gas

1.4.2 Heat
Heat This is energy ow from a region at higher temperature to a region at lower temperature, due to the temperature di erence. In thermodynamics we deal with heat going into or out of a system. It makes no sense to speak of heat in a system.
As a result, heat enters of leaves a system through it’s boundary or container wall. We consider two systems in contact that have no heat ow between them to be in thermal equilibrium and as a a result they must have the same temperature.
1.4.3 Work
Work If the system is a gas, in a cylinder tted with a piston, the gas does work of amount p∆V when it exerts a pressure p and pushes the piston out a small way, so the gas volume increases by ∆V . Work, like heat, is energy in transit from (or to) the system.
As such: