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Refrigerator Problem- Could you check my answer?

  • Thread starter bcjochim07
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  • #1
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Homework Statement


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?


Homework Equations





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

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
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sorry, didn't mean to post the same question twice
 
  • #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?
 
  • #4
alphysicist
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Hi Hootenanny,

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?
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).
 
  • #5
Hootenanny
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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).
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:

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