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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