Kinetic Theory

Consider a cubical box, L on each side. The box contains N molecules of ideal gas, each of mass m.

All collisions are elastic. The force exerted by one molecule when it collides with a wall of the box that is perpendicular to the x-axis is, from the impulse equation:

F = 2mvx/Δt

Our single molecule collides with this wall once every:

Δt = 2L/vx

This gives an average force of:

F = mvx2/L

This is the force from a single molecule. The total force on the wall is the sum over all the molecules:

F = Σmvx2/L = (mN/L) [ Σvx2/N ]

The term in square brackets represents the average value of vx2. The square root of this average is known as the root-mean-square (rms) average of vx, so:

F = (mN/L) vx_rms2

By symmetry, all directions in the box are equivalent, so:

v2 = vx2 + vy2 + vz2 = 3vx2

This gives:

F = (mN/3L) vrms2

Dividing by the area, L2, of the wall gives the pressure:

P = (mN/3L3) vrms2

which is:

PV = (N/3)[mvrms2] = (2N/3) [½ mvrms2]

The term in square brackets is Kav, the average translational kinetic energy of the molecules in the box.

Therefore PV = (2N/3)Kav


Compare our result:

PV = (2N/3)Kav

to the ideal gas law

PV = NkT

The two equations agree when the average translational kinetic energy of the molecules is:

Kav = (3/2)kT

Here we have a fundamental connection between temperature and the average translational kinetic energy of the atoms - they are directly proportional to one another. Temperature is a measure of the average kinetic energy of the atoms.

Internal Energy and the Equipartition of Energy

The internal energy is the total of all the energy associated with the motion of the atoms or molecules in the system. This includes energy associated with translation, rotation, and vibration.

Equipartition of Energy: Each contribution to the internal energy contributes an equal amount of energy.

For a monatomic ideal gas, the only contribution to the energy comes from translational kinetic energy. The internal energy is therefore:

Monatomic ideal gas: Eint = (3/2)NkT = (3/2)nRT

Each direction (x, y, and z) contributes (1/2)NkT to the energy. This is where the equipartition of energy idea comes in - any other contribution to the energy must also contribute (1/2)NkT.

For a diatomic molecule there are three translation directions, and rotational kinetic energy also contributes, but only for rotations about two of the three perpendicular axes. The five contributions to the energy (five degrees of freedom) give:

Diatomic ideal gas: Eint = (5/2)NkT

This actually applies at intermediate temperatures. At low temperatures only the translational kinetic energy contributes, and at higher temperatures two additional contributions (kinetic and potential energy) come from vibration.