# Understanding of irreversibility

## Main Question or Discussion Point

The thermodynamic explanation of irreversibility - does it include the microscopic view of a process, i.e. the expansion of a ideal gas (the random movement of the molecules) or the melting of an ice cube?
Or is it only defined within the macroscopic world?

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The thermodynamic explanation of irreversibility - does it include the microscopic view of a process, i.e. the expansion of a ideal gas (the random movement of the molecules) or the melting of an ice cube?
Or is it only defined within the macroscopic world?
It's defined in both:

http://en.wikipedia.org/wiki/Entropy

# 2.1 Macroscopic viewpoint (classical thermodynamics)
# 2.2 Microscopic viewpoint (statistical mechanics)

Andrew Mason
Homework Helper
The thermodynamic explanation of irreversibility - does it include the microscopic view of a process, i.e. the expansion of a ideal gas (the random movement of the molecules) or the melting of an ice cube?
Or is it only defined within the macroscopic world?
Irreversibility, of course, is the opposite of reversibility. A reversible process is one that can change direction with an infinitessimal change in conditions. In reality, all processes are somewhat irreversible. An example would be the expansion of a gas caused by applying a pressure to a container wall that was infinitessimally lower than the pressure of the gas. An infinitessimal increase in pressure will reverse the expansion and result in a compression of the gas.

In thermodynamics, reversibility is definitely a macroscopic concept. It has no meaning at the molecular level. Pressure and temperature are macroscopic concepts that relate to qualities of large numbers of molecules and are not defined at the molecular level.

AM

In thermodynamics, reversibility is definitely a macroscopic concept. It has no meaning at the molecular level. Pressure and temperature are macroscopic concepts that relate to qualities of large numbers of molecules and are not defined at the molecular level.
AM
Ok, thanks. Just had to be sure.