Energies Revisited
We have stressed throughout that knowing well the energy identities makes problem solving
much easier, and we have seen this in many of the problems we have tackled. It appears again
here, as we discuss processes performed on a gas.
For an isothermal expansion or compression, we wish to deal with an energy where
τ
appears as
a differential. Conventionally, the Helmholtz free energy is used. Barring any diffusive exchange,
we can see that
dF
gives us
dU

dQ
, which is exactly the work done on the system.
For an isobaric process, we wish to use the Enthalpy, for the pressure appears in the differential
there. This choice allows us to carefully account for the work done on the system and that done
on the environment in a process.
For a process that is both isobaric and isothermal, it makes sense to look at the Gibbs Free
Energy.
Therefore, while solving problems, look for what is being held constant so that you can make an
appropriate choice of energy.
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 Fall '10
 DavidJudd
 Physics, Energy

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