First law of thermodynamics: Difference between revisions
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|Meaning=The total [[internal energy]] ''U'' of an [[isolated system]] is constant. | |||
|Explanation=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 <blockquote>[[File:ams2001glos-Fe5.gif|link=|center|ams2001glos-Fe5]]</blockquote> 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 <blockquote>[[File:ams2001glos-Fe6.gif|link=|center|ams2001glos-Fe6]]</blockquote> 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. | |||
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Latest revision as of 11:19, 30 March 2024
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.
