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Difference between enthalpy and internal energy

  1. May 12, 2010 #1
    What's the difference between enthalpy and internal energy?

    Internal energy is the average of the kinetic energy (linear and angular) of the particles of the body (or system), right?

    Could someone explain me this clearly?

    Thank you
  2. jcsd
  3. May 12, 2010 #2
    The difference is [itex]P V[/itex].
  4. May 12, 2010 #3

    Andy Resnick

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    That's a good question. First, consider the total energy E of a body or system:

    E = KE + PE + U

    Where KE is kinetic energy, PE potential energy, and U the 'internal energy'. Internal energy is energy that cannot be accounted for by specifying position, velocity, or mass. It's not a mechanical form of energy, it's (essentially) heat energy. It's related to temperature as well.

    Enthalpy H = U + PV.

    If U is the heat energy, PV is the 'work' energy. Enthalpy is also the total amount of energy available in a system or body, like E, but written using thermodynamic variables instead of mechanical variables. Changes in enthalpy occur during a thermodynamic process involving the conversion of heat into work (or vice-versa).
  5. May 12, 2010 #4
    Thank you, now it's very clear.
  6. May 13, 2010 #5
    I think this is a very good quote, if you exclude the mass.

    Andy, are you sure you want to include mass, isn't internal energy an extensive property, ie twice the mass means twice the internal energy, all other things being equal?
  7. May 13, 2010 #6

    Andy Resnick

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    'mass' goes to the kinetic (e.g. 1/2 mv^2) and potential (e.g. mgh) energies. But yes, internal energy is an extensive property (as is enthalpy).
    Last edited: May 13, 2010
  8. May 13, 2010 #7


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    Last edited by a moderator: Apr 25, 2017
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