First law of thermodynamics

From Glossary of Meteorology

first law of thermodynamics

The total internal energy U of an isolated system is constant.

If a thermodynamic system is not isolated, its internal energy may change because of two distinguishable macroscopic processes: working (a force exerted through a distance) and heating (energy exchange by virtue of a temperature difference between the system and its surroundings). The first law may be written
where Q is the rate of heating and W is the rate of working on the system. For a simple system in which working is solely a consequence of volume changes, the rate of working is given by
where p is pressure and V is volume, provided volume changes at a sufficiently slow rate (quasi- static process) that the pressure is approximately uniform.