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What is meant by heat capacity at constant pressure?

  1. Feb 22, 2016 #1
    Generally in chemistry and physics, we learned that a material has a heat capacity at constant pressure. However, I am confused at whether this refers to a constant external pressure or a constant system internal pressure?

    Last edited: Feb 22, 2016
  2. jcsd
  3. Feb 22, 2016 #2


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    Hello half integer spin, :welcome:

    Thermodynamics is about equilibrium. Internal pressure and external pressure are supposed to be equal.
  4. Feb 23, 2016 #3
    To elaborate on what BvU said, heat capacity is a physical property of a material, and does not depend on the process being applied to the material. Heat capacity per mole at constant pressure is defined as $$C_p=\left(\frac{\partial H}{\partial T}\right)_P$$
    where H is the molar enthalpy of the material. If a process is irreversible so that the internal pressure and temperature of the material are not uniform, this equation still applies locally.
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