Difference between revisions of "First law of thermodynamics"

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(link to new adiabatic process)
(Q, W both "upon" the system)
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ignoring the external energy of the system, for example
 
ignoring the external energy of the system, for example
 
motion of the center of mass of the system, or the presence  
 
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]].
+
of an external field. ''Q'' is the net [[heat]] entering the
 +
system and ''W'' is the [[work]] done upon the system.
  
 
For reversible changes
 
For reversible changes
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where <math>p</math> is the [[pressure]] and <math>V</math> is the volume.
 
where <math>p</math> is the [[pressure]] and <math>V</math> is the volume.
 +
 
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)
 
then if two bodies of different [[temperature |temperatures]] are placed in contact
 
then if two bodies of different [[temperature |temperatures]] are placed in contact

Revision as of 11:09, 8 June 2009

The first law of thermodynamics is a statement of the conservation of energy for thermal processes:

\left.dU\right.=\delta Q+ \delta W

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 net heat entering the system and W is the work done upon the system.

For reversible changes

\left.dW\right.=-p~dV

and for irreversible changes

\left.dW\right. > -p~dV

where p is the pressure and V is the volume.

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 (see adiabatic process. 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.

References

  1. James Prescott Joule "On the Mechanical Equivalent of Heat", Philosophical Transactions of the Royal Society of London 140 pp. 61-82 (1850)