# What exactly is equilibrium in thermodynamics?

1. Feb 26, 2005

### chingkui

In thermodynamics, what exactly is equilibrium? Does that mean everything inside a system has equal temperature? In case like fluid flow between two parallel plates of different temperatures, energy transfer from the plate to the fluid, and different point has different temperature, is the system not in equilibrium? Is temperature defined only for equilibrium system? And if so, how can we talk about non-uniform temperature?
Can someone clarify this, and give me some examples of equilibrium and non-equilibrium systems? Thanks.

2. Feb 26, 2005

### dextercioby

Since thermodynamics as a whole is nothing else than statistical thermodynamics,which is a nonstatistical subtheory of statistical physics,the notion of equilibrium is much more general.According to statistical physics,a statistical system is in equilibrium if the Hamiltonian does not depend on time explicitely,equivalently,the statistical system is placed under time independent external conditions...

Daniel.

P.S.For further details,i infer you to theorem & equation of Liouville (classical statistical physics) or to the equation of von Neumann (quantum statistical physics).

3. Mar 4, 2005

### tarunjot

well.. equilibrium need not always be related to temperature. its the thermal equilibrium when we talk of temperatures of the system..
and for the whole of the thermodynamic system to be in a state of equilibrium, we need to have all the thermodynamic parameters to be in a state of equilibrium, viz.. pressure etc.
and equilibrium does not merely mean the equivalence of temperature or pressure at everytime.. all the time..
a system might be in a state of equilibrium and yet its temp. and pressure keep changing.. BUT that change must be infinitesimally small or slow.

regarding your parallel plate problem.. i guess the whole of the system can be considered to be in thermal equilibrium.. if there is no, net heat loss and the temperature remains constant for the system. read about the system boundaries and all n u might feel more comfortable with the whole concept.