Systems have different motions and kinetic energies {degrees of freedom, partition}, such as translations, rotations, and vibrations.
translation
All particles can have translations. Average random translational kinetic energy determines temperature.
rotations
Spherically symmetric molecules cannot have net rotational motion. Linear molecules can have one rotational motion state. Two-dimensional molecules can have two rotational motion states. Three-dimensional molecules can have three rotational motion states.
vibrations
Molecules with chemical bonds can have vibration states. Vibrations can involve one bond and be along bond axis. Vibrations can involve two bonds and be across bond axes. Molecule symmetries can cancel vibration states.
partition
Heat can go equally into all available energy states {partition of energy, heat}|. If molecule has more rotation and/or vibration states, raising temperature requires more energy, because some heat does not become average random translation kinetic energy.
partition: heat capacity
Material heat capacity depends on molecular-motion degrees of freedom. Molecules with more rotation and/or vibration states have higher heat capacity.
partition: equipartition
Motion-type average kinetic energies must be the same {equipartition, energy} {energy equipartition} {principle of equipartition of energy}, because energy transfers freely among states by collisions.
amount
Partition average kinetic energy KE is half Boltzmann constant k times temperature T: KE = 0.5 * k * T.
Physical Sciences>Physics>Heat>Kinetics
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Date Modified: 2022.0224