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B Gravitational collapse

  1. May 3, 2016 #1
    During gravitational collapse, gravitational potential energy of the gas is converted to its internal kinetic energy so the internal energy of the clous of gas is said to be increased

    But isn't gravitational potential energy included in the internal energy? Shouldn't the internal energy remains constant overall?
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  3. May 3, 2016 #2

    Simon Bridge

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    Welcome to PF;
    "Internal energy", in this context, refers to the thermodynamic definition.
    This does not usually include gravitational potential energy.
    You can see this, if you like, as one form of internal energy turning into another form.

    Total energy is conserved - internal energy does not have to be conserved.
    For instance, the object could undergo some nuclear reactions increasing internal energy or the object could radiate, decreasing internal energy.
    Last edited: May 3, 2016
  4. May 3, 2016 #3


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    Thanks Simon

    not my field of expertise ... was interested in hearing an answer as well :smile:

  5. May 4, 2016 #4

    Ken G

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    Another interesting point about gravitational collapse is that to keep it going, it generally does require a decrease in the system total energy ("internal" energy plus potential energy). Whenever gravitational energy is converted 100% into internal energy, it will eventually stop the collapse, so you need some heat loss from the system to keep it going. But it takes relatively little heat loss if the gas is relativistic, so that's why gravitational collapse happens for relativistic gases.
  6. May 4, 2016 #5
    Yes, but only if it is no potential energy in an external field.

    Yes, but only if no heat or work is exchanged over the system boundaries.
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