Question

Explain what happens to the average kinetic energy, thermal energy, and temperature of a substance when the particles in the substance slow down.

Answer 1

Short answer: when the particles slow down, their average kinetic energy decreases. Because temperature is proportional to the average microscopic kinetic energy (for the usual translational motion), the temperature goes down. The substance’s thermal energy (internal energy) also decreases because the kinetic-energy contribution to the internal energy is smaller; that lost energy is transferred out of the substance (for example as heat) or converted to other forms.

Details and useful distinctions:
- Average kinetic energy: decreases. For an ideal monatomic gas, average translational kinetic energy per particle = (3/2)kT, so a slower average speed means smaller KE.
- Temperature: decreases. Temperature is an intensive measure proportional to the average translational kinetic energy of the particles.
- Thermal energy (internal energy): usually decreases. Thermal energy is the total microscopic energy (sum of kinetic and potential energies of all particles) and is an extensive quantity (depends on amount of substance). If particles slow and no other energy is added, the total internal (thermal) energy falls by the energy that leaves the system (for example as heat). Note special cases: during a phase change the kinetic energies of some degrees of freedom can change while temperature stays constant because energy is absorbed or released as latent heat and goes into changing potential-energy arrangement (bonding) rather than raising/lowering temperature.