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Thermodynamics question

  1. Jan 9, 2013 #1
    I once was asked a question about this scenario.
    Lets say we have an insulated box with a refrigerator in it.
    Now the fridge is plugged in from outside. The box is perfectly insulated.
    The door of the fridge is open. What happens to the temperate inside the box.
    their is air in the fridge. I was told that the temperature of the box will go up because we are pumping energy into it.
    But then I thought of laser cooling. When laser cooling happens we shoot photons into
    a cloud of atoms and they Doppler shift the light an absorb them and slow down.
    We pumped energy into that system and it cooled down.
    Is it always true that if I pump energy into a system that it will heat up.
  2. jcsd
  3. Jan 9, 2013 #2


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    Staff: Mentor

    Yes, it is always true that if you pump heat energy in (or input electrical energy and immediately convert it to heat), it will heat up. That's basically the definition of "heat up".
  4. Jan 9, 2013 #3
    ok but in laser cooling we pump in energy and it cools down.
  5. Jan 9, 2013 #4


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    Gold Member

    If you cool something, something else must heat up. Laser cooling is mostly used to cool at atom level. I don't find it plausible that this is some kind of optical trapping of the atoms - forcing them to stop viberating. Laser cooling is however mostly used in quantum physics so I cant help you further regarding what heats up if the atoms cools down. At least the laser equipment itself heats up - that for sure :-)

  6. Jan 9, 2013 #5


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    Science Advisor

    In laser cooling, the laser light gets scattered and leaves the system with higher energy than the light that entered. So there is no net pumping of energy but energy removal.
  7. Jan 9, 2013 #6
    so the light gets absorbed and then remitted with more energy so it took energy away from the atom. is this similar to the Raman effect.
  8. Jan 9, 2013 #7
    If you ran the laser cooler in an opaque box, where the photons could not leave,
    the inside of the box would heat up as well.
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