Refrigerator Problem- Could you check my answer?

[bcjochim07]

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

 

[bcjochim07]

sorry, didn't mean to post the same question twice

 

[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?

 

[alphysicist]

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).

 

[Hootenanny]

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 . 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:.