First law of thermodynamics: Difference between revisions

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For reversible changes
For reversible changes


:<math>\left.dW\right.=-PdV </math>
:<math>\left.dW\right.=-p~dV </math>


and for irreversible changes
and for irreversible changes


:<math>\left.dW\right. > -PdV </math>
:<math>\left.dW\right. > -p~dV </math>


For an adiabatic system (i.e. a system in which no heat enters or leaves)
For an adiabatic system (i.e. a system in which no heat enters or leaves)

Revision as of 12:12, 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.