# Homework Help: Definition of Thermal Energy?

1. Apr 23, 2013

### Nexu

Hey guys.

From my understanding,

Temperature is a measure of the average kinetic energy of the molecules within a substance.
Heat is the transfer of thermal energy between a system and its surroundings
Internal Energy is the total energy (kinetic + potential) of the molecules of a substance.

What is thermal energy?

Is thermal energy the total KINETIC energy of the molecules within a substance or the total KINETIC energy and POTENTIAL energy of the molecules within a substance? If it is the latter, isn't it the same as internal energy?

I have looked for this on the web but am getting mixed responses. Some say thermal energy is the same as heat whereas others say it is the same as internal energy.

From my knowledge of the topic, I believe that Thermal Energy is the total kinetic energy of the molecules within a substance, but am not 100% sure.

2. Apr 23, 2013

### haruspex

It's the non-work component of the kinetic energy. Not sure there's a precise definition. If you have a body moving en masse relative to your frame of reference, the coherent KE is work energy, not thermal energy. If the body is some piece of machinery in which components are moving relative to each other, those aspects of motion are also work energy. Chaotic motion of molecules relative to each other is thermal energy. But there's no clear dividing line - you could have clusters of molecules as nanoscale structures in relative motion.. is that work or thermal?
The theoretical solution may be to say that the thermal energy is the part that cannot be turned into useful mechanical energy by any means, but that implies that if the body is not at a uniform temperature then a part of what you might have thought of as thermal energy is work energy after all.
Btw, I'm not sure it's right to define heat as only a transfer of energy.