Definition: Thermal energy
The kinetic energy associated with the random motions of the molecules of a material or object; often used interchangeably with the terms heat and heat energy. Measured in joules, calories, or Btu.
- In thermodynamics, thermal energy refers to the internal energy present in a system due to its temperature. The concept is not well-defined or broadly accepted in physics or thermodynamics, because the internal energy can be changed without changing the temperature, and there is no way to distinguish which part of the internal energy is "thermal". Instead, the appropriate thermodynamic concept is heat, defined as a transfer of energy (just as work is another type of transfer of energy). Heat and work therefore depend on the path of transfer and are not state functions, whereas internal energy is a state function.