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Constant pressure and constant volume?

  1. Jun 30, 2011 #1
    Can a thermodynamic system simultaneously exist under both constant volume and constant pressure at a particular instant. I mean , when the system is under constant pressure condition, can we impose constant volume on the system. Please clear my doubt. I don't know if it's a silly question, but eating my brain.
  2. jcsd
  3. Jun 30, 2011 #2


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    It's possible, but pretty uncommon. Equivalently this says that the thermal expansion (∂V/∂T)P vanishes. If you Google "zero thermal expansion" you'll find a few articles where such a material has been created. Otherwise the only time the thermal expansion vanishes is at absolute zero.
  4. Jun 30, 2011 #3


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    Sure. It just means that you also have constant temperature.
  5. Jun 30, 2011 #4
    Wouldn't a sealed uncooled canister of gas, such as a helium tank at a party store, be at a constant pressure and constant volume? Or is there something else to the problem I don't see?
  6. Jun 30, 2011 #5
    If by "constant" you mean constant in time, the specification underlined will make it a nonsense.
    Either you mean "uniform" (that means no spatial variation) or you mean a process and not an instant.

    A process with no change in pressure or volume may be possible, for a general system. Other parameters may change. For ideal gas you may have a system which variable number of particles.
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