# Can't understand what's thermal equilibrium

1. Oct 10, 2011

### Tosh5457

In wikipedia it says: "Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space."

But isn't temperature defined because of the thermal equilibrium concept? It's defined because of the zeroth law, and the zeroth law uses thermal equilibrium concept.

2. Oct 10, 2011

### atyy

Yes, first we define thermal equilibrium, then we define temperature.

3. Oct 12, 2011

### Low-Q

Because temperature is relative?

4. Oct 12, 2011

### johng23

If you have two systems which can exchange energy, but neither can do work on the other, thermal equilibrium is the state in which no net energy transfer takes place. It turns out one quantity has to be equal in both systems for this to occur, and we call this quantity the temperature.

5. Oct 12, 2011

### patricks

Given two systems in "thermal contact" (aka, two systems that can exchange energy) and constraints on the combined system (for example, a constant total energy = U) we know that the entropy, aka the number of states, must always increase ignoring slight fluctuations.

The condition that an exchange of energy between the two systems has no effect on the total entropy is DEFINED to be thermal equilibrium. Temperature is then DEFINED by this thermal equilibrium condition.

The uniqueness of thermal equilibrium (aka, a single peak in the combined multiplicity functions) and therefore temperature is an assumption based on the properties of systems with very large numbers of particles.