First law of thermodynamics: Difference between revisions
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The '''first law of thermodynamics''' is a statement of the conservation of energy for thermal processes: | |||
:<math>\left.dU\right.=\delta Q+ \delta W</math> | :<math>\left.dU\right.=\delta Q+ \delta W</math> | ||
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of an external field. ''Q'' is the [[heat]] and ''W'' is the [[work]]. | of an external field. ''Q'' is the [[heat]] and ''W'' is the [[work]]. | ||
For reversible changes | |||
:<math>\left.dW\right.=-PdV </math> | :<math>\left.dW\right.=-PdV </math> | ||
irreversible | and for irreversible changes | ||
:<math>\left.dW\right. > -PdV </math> | :<math>\left.dW\right. > -PdV </math> | ||
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In other words, the common experience that the hotter object gives | In other words, the common experience that the hotter object gives | ||
heat to the colder object, the direction of heat flow from hot to cold, | heat to the colder object, the direction of heat flow from hot to cold, | ||
is left to the second law of thermodynamics. | is left to the [[second law of thermodynamics]]. | ||
[[Category: Classical thermodynamics]] | [[Category: Classical thermodynamics]] | ||
Revision as of 11:29, 21 June 2007
The first law of thermodynamics is a statement of the conservation of energy for thermal processes:
Where U is the internal energy of the system, i.e. ignoring the external energy of the system, for example motion of the center of mass of the system, or the presence of an external field. Q is the heat and W is the work.
For reversible changes
and for irreversible changes
For an adiabatic system (i.e. a system in which no heat enters or leaves) then if two bodies of different temperatures are placed in contact and then separated then the sum of the heat within the system is unchanged. This is conservation of energy. The first law of thermodynamics does not provide information on the direction of the heat transfer (if any). In other words, the common experience that the hotter object gives heat to the colder object, the direction of heat flow from hot to cold, is left to the second law of thermodynamics.