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Which work is done when a parcel expands at constant pressure?

  • A.The change in internal energy
  • B.The heat supplied to the parcel, all of which appears as work when the expansion occurs at fixed pressure
  • C.Pressure multiplied by the change in specific volume
  • D.Specific volume multiplied by the change in pressure, which vanishes when the pressure is being held constant

Answer: C

The work done per unit mass is the pressure times the increase in specific volume, which is the area beneath the process on a pressure volume diagram. At constant pressure that area is a simple rectangle. Not all the supplied heat becomes work, since some raises the internal energy, which is exactly why the two specific heats differ.

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