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Physics pop-quiz! Let's go!

  1. Jun 2, 2009 #1
    Here's a riddle-ish physics problem I came up with:

    Picture a room with a refrigerator in it.
    The refrigerator is running and its doors are open.
    There is a constant supply of energy to the fridge from outside the room.
    Other than that, nothing enters or leaves the room.

    What happens to the average temperature of the room over time?


    Not really a riddle, but a pretty cool example of one of the first things you learn when studying physics.
     
  2. jcsd
  3. Jun 2, 2009 #2

    atyy

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    It will drop until the desired temperature is attained?
     
  4. Jun 2, 2009 #3
    Won't it stay the same?
    It lowers the temperature 'in' the refrigerator, but raises it in the 'back' of the refrigerator.
     
  5. Jun 3, 2009 #4
    I think that the room temperature may actually increase, because a fridge works by the reapeated evaporation and condensation of refrigerant which depends of small heaters on the back of fridges to release the heat from the inside. If the fridge is open then the heat from the air will just constantly travel through these heaters and this will cause the room temperature to increase.
     
  6. Jun 3, 2009 #5
    It'll raise. There's a motor running and it's getting it's power from outside, thus there is energy being added to the room.
     
  7. Jun 3, 2009 #6
    Hi there,

    The room's temperature will definetily increase. The energy needed to cool down the fridge can be compensated by the heat production of the cooler. In a perfect world, the two would exactly compensate. Since we don't live in this world, there will be more heat produced from the cooler than the energy needed to cool down the fridge.

    Furthermore, the cables that carry the electricity to the fridge to be cooled down have a certain resistance, e.g. producing heat.

    At the end of the day, you will have a room that heats up.

    Cheers
     
  8. Jun 3, 2009 #7

    djeitnstine

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    The room has become both heat sink and exhaust. Since no fridge is has perfect efficiency, of course there will be excess energy in the room so the temperature must increase.
     
  9. Jun 3, 2009 #8
    You are very close on this one. The energy required to compress the coolant and put it through the cycle of decompression (not evaporation) to inititate the temperature change creates a heat discharge from the soft or 'decompressed side' of the coolant loop so the heat that was removed from the air being cooled during the cycle is approximately the same as the the as the ambient temperature in the room the fridge is in. The 'drain' or net effect of added heat is soley because of the electrical motor.
     
  10. Jun 3, 2009 #9
    Good job all! Thank you for all the responses!

    As a lot of you realized, this was a thermo dynamics question in disguise, or more specifically, "Conservation of Energy".

    All you have to know is:
    1. Energy is being added to the system.
    2. Energy is NOT leaving the system.

    If both statements are true, you know that:
    3. The energy in the room is increasing.

    Finally, if these are true:
    4. There is no process that converts one form of energy into another without losing some to heat.
    5. There is no process that converts heat energy into another form without putting off more heat.

    Then,
    6. You cannot cool an isolated system. The temperature of the room increases.

    --------------------------------------------------------------------
    THE FOLLOWING IS A TANGENT. IT JUST GOES A LITTLE DEEPER.
    YOU DON'T NEED TO READ IT:
    Depending on the apparatus, not all the energy would necessarily be converted to heat. If some of it could be converted to a non-heat form, the temperature would not increase as quickly. My question involved a refrigerator, which does almost nothing but "move" heat energy around. The few other conversions that take place (a little sound, a few moving parts, maybe a light) are dissipated as heat anyway. But, if instead of a refrigerator, there was an elevator that lifted a weight, the energy would be converted into a combination of gravitational potential energy and heat. Assuming the rate of energy input was the same as in the refrigerator example, the elevator room would actually heat up slower than the refrigerator room. By the same token, a space heater room would heat up at the same rate as the refrigerator room.
    --------------------------------------------------------------------
     
  11. Jun 3, 2009 #10

    Mapes

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    I think this could be better worded as "You cannot cool all parts of an otherwise isolated system by adding energy with all other constraints fixed."

    The first problem with the original statement is that one can't heat or cool an entire completely isolated system at all because its energy is constant. Note that the room in your thought experiment has a power cord running into it.

    Second, one can certainly cool parts of an isolated room; consider the inside of a closed refrigerator. One can't cool all parts of the room, however; the area around the refrigerator get hotter, as you pointed out. So let's specify the entire room.

    Third, one can often (not always) cool a system by increasing its volume, even with some influx of energy. So let's also specify that the walls stay fixed.
     
  12. Jun 3, 2009 #11

    djeitnstine

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    This is physics forum. Many of us specialize in this stuff, are doing this in course work at the moment, or even use these concepts every day at work.

    That is, this forum is full of physicists, engineers and scientists alike.

    I am stumped as to think why people come in here thinking stuff like this are even challenges or even allow us to use more than 10 seconds of thinking.

    Post this in a liberal arts forum or some other forum where you'll get more than one page of people trying to get the answer right....
     
  13. Jun 3, 2009 #12

    Mapes

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    And students at all levels. We are all learning more about Nature and physical laws.

    I'm sure you wouldn't appreciate more advanced colleagues demeaning your earnest questions. If you find a puzzle's solution obvious, hooray for you. Move on or participate with constructive comments.
     
  14. Jun 3, 2009 #13
    Thanks for posting. Can't believe I got it wrong, I thought the temperature would stabilize :/.
     
  15. Jun 3, 2009 #14

    mgb_phys

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    It's usually given to students in the form of "what would you notice when you open the door of the room"
    Correct answer of course is the heat. Bonus answer is the huge pile of bills from the electricity company - what happens to the efficency of the refirgeration cycle as the heat sink temperature rises.
     
  16. Jun 3, 2009 #15
    You're totally right. My use of the word "isolated" was irresponsible, I should have stated that I was referring to "average temperature" and I wasn't very explicit about the constraints.

    All good, important catches on your part.

    My Dad is a professional opera singer. Regardless of whether he's listening to a karaoke singer at a bar or watching "American Idol", he refuses to ever say anything bad about another vocalist.

    "Because there's no place for it," he says, "It's not proffessional; it's not graceful. It serves no purpose. It's just a bad habit that a lot young singers have."

    I agree with him entirely.
     
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