Degrees of freedom and the equipartition principle

This is not mentioned at A level, but is an important concept.

Energy is spread equally between different degrees of freedom.

A degree of freedom is a way of absorbing energy. There are three important types of degrees of freedom:
Translational: the ways the centre of mass of the molecule can move. There are often three translational degrees of freedom; in the x, y, and z directions.

Rotational: the ways the molecule can rotate. A linear molecule has two rotational degrees of freedom, and a nonlinear molecule has three.

Vibrational: all the other motion of the molecule. This is related to the molecule bending and stretching. Linear molecules have 3N-5 vibrational degrees of freedom and nonlinear molecules have 3N-6. N is the number of atoms in the molecule.

Each degree of freedom has:


Where kB is the Boltzmann constant.

Back to Contents: Physics: Thermodynamics

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