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Heat Capacity

  1. Apr 25, 2007 #1
    Can someone confirm if my understanding of heat capacity is correct - particulary at constant volume and pressure?

    My understanding is that for a system to experience a rise in temperature either its pressure or its volume must remain constant. If either of these is allowed to expand naturally when heat is transferred to it then there will be no rise in temperature
  2. jcsd
  3. Apr 25, 2007 #2
    When you heat a mass of gas it will increase in volume AND/OR pressure. You can force the volume or pressure to be constant, but not both. But you can also impose any behavior to volume or pressure. You impose it by external means.
    A system can experience a rise in temperature with neither the volume nor pressure remaining constant.
    There are even very common processes called "adiabatic" where temperature, pressure and volume change simultaneously.

    But heat capacity coefficients are only defined for constant pressure or constant volume processes. Two is enough!
  4. Apr 25, 2007 #3
    You can always heat something up.
    It does not matter if you contrain it to keep its volume, or its pressure, or its shape, or its color, or its elasticity, or anything ...

    Except of course if you decide that your system must keep its temperature constant.

    I assume that by "heat up" you meant "increase temperature".
  5. Apr 25, 2007 #4


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    Staff: Mentor

    Take the case of a nearly-empty bag of air. The air occupies a certain volume and when you apply heat to it, it's temperature increases and it also expands, while the pressure remains constant.
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