| Astrophysics (Index) | About |
Thermodynamic equilibrium (TE) is the state of a thermodynamic system (e.g., a gas cloud) that is in thermal equilibrium, chemical equilibrium, mechanical equilibrium and radiative equilibrium. Often the term is used when the term thermal equilibrium might suffice, i.e., the state of no net heat flow, e.g., everything in it being the same temperature, which is the internal characteristic of a black body, i.e., the condition that produces black-body radiation. These are theoretical states, never quite perfect in the real world, but are useful in modeling, e.g., when heat flow is minimal.
A variant, local thermodynamic equilibrium (LTE) is the state where conditions in region(s) within the system are such that the temperature is virtually the same over the distance that a particle or photon is likely to reach in a single hop. Given the complications of including the effects of heat flow, presuming LTE can simplify models such as those for radiative transfer, making them tractable. It can be reasonable to presume LTE for some aspects of processes that only involve interactions within these limited distances.