The equipartition theorem is a principle of classical (non-quantum) statistical mechanics which states that the internal energy of a system composed of a large number of particles at thermal equilibrium will distribute itself evenly among each of the quadratic degrees of freedom allowed to the particles of the system.
For a molecule of gas, each component of velocity has an associated kinetic energy. This kinetic energy is, on average,
In general, for any system with a classical Hamiltonian of the form:
- where and are constant with respect to all and ,
- and are spatial coordinates and their conjugate momenta,
each degree of freedom and will contribute a total of to the system's total energy, resulting in a total of equipartition energy.
The equipartition theorem is valid only in the classical limit of an energy continuum. The equipartition theorem breaks down in the limit of large gaps between quantum energy levels, because it becomes more difficult to excite degrees of freedom which are highly quantized, such as electronic excitations in non-metals, vibrational modes with a large ratio of force constant to reduced mass, or rotational degrees of freedom about an axis with a low moment of inertia.
- Degrees of freedom (physics and chemistry)
- Kinetic theory
- Heat capacity
- Statistical mechanics
- Quantum statistical mechanics