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Refrigerator Problem- Could you check my answer?

  1. May 12, 2008 #1
    1. The problem statement, all variables and given/known data
    1 L of 20 degree C water is placed in a refrigerator. The refrigerator's motor must supply an extra 8 W to chill the water to 5 degrees C in 1 hr. What is the refrigerator's coefficient of performance?


    2. Relevant equations



    3. The attempt at a solution

    8W = 8 J/s * 3600 s = 28,800 J -- this is the work in - Win

    1L of H20 has mass 1kg and c=4190 J/kgK

    so Q=(1Kg)(4190)(-15) = -62850 J --- this is the heat extracted from cold reservoir - Qc

    K= Qc/Win so K = 62850J/28800J K= 2.18
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. May 12, 2008 #2
    sorry, didn't mean to post the same question twice
     
  4. May 12, 2008 #3

    Hootenanny

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    From what I can see your solution is correct, however the answer is nonsensical: a 200% efficiency?! Are you sure that the question is quoted correctly?
     
  5. May 12, 2008 #4

    alphysicist

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    Hi Hootenanny,

    The question was not asking for efficiency; it asked for the coefficient of performance, which can be greater than 1 for a refrigerator (and should be greater than 1 for a good refrigerator).
     
  6. May 12, 2008 #5

    Hootenanny

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    Oh my word, it's been a while since I've worked with any thermo :blushing:. My apologies, of course the COP isn't the same as the thermal efficiency.

    I guess I should stop posting so late at night :zzz:.
     
    Last edited: May 12, 2008
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