Editing First law of thermodynamics

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Where ''U'' is the [[internal energy]] of the system, i.e.
Where ''U'' is the [[internal energy]] of the system, i.e.
ignoring the external energy of the system, for example
ignoring the external energy of the system, for example
motion of the centre 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 net [[heat]] entering the
of an external field. ''Q'' is the [[heat]] and ''W'' is the [[work]].
system and ''W'' is the [[work]] done upon the system. <math>\delta</math> represents an [http://mathworld.wolfram.com/InexactDifferential.html inexact differential], indicating that the resulting value depends on the path taken.


For reversible changes, where the system evolves through a succession of equilibrium states, one has:
For reversible changes


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


and for irreversible changes
and for irreversible changes


:<math>\left.dW\right. > -p~dV </math>
:<math>\left.dW\right. > -PdV </math>
 
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 temperatures are placed in contact
and then separated then the sum of the heat within the system is unchanged (see
and then separated then the sum of the heat within the system is unchanged.
[[adiabatic process]].
This is conservation of energy. The first law of thermodynamics does not
This is conservation of energy. The first law of thermodynamics does not
provide information on the direction of the heat transfer (if any).
provide information on the direction of the heat transfer (if any).
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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]].
==References==
#[http://dx.doi.org/10.1098/rstl.1850.0004 James Prescott Joule "On the Mechanical Equivalent of Heat", Philosophical Transactions of the Royal Society of London '''140''' pp. 61-82 (1850)]
#[http://ailis.lib.unair.ac.id/opac/detail-opac?id=12591 Kenneth Wark "Thermodynamics"]
[[Category: Classical thermodynamics]]
[[Category: Classical thermodynamics]]
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