1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Find Coefficient of Performance of a refrigerator

  1. Apr 9, 2017 #1
    1. The problem statement, all variables and given/known data
    I need to find the coefficient of a refrigerator that uses a monatomic gas and only has two steps.
    I know the:
    initial pressure of the refrigerant
    initial temperature
    initial volume
    and final pressure

    Every formula I know requires 4 steps or a knowledge of the outside and inside temperature. This question is unlike anything I have seen to date. The only other problem I have seen like this was for a real refrigerator. it had four steps, and I was given a table for the saturate refrigerant.

    2. Relevant equations
    ##COP = \frac{Q_C}{W}##
    ##COP = \frac{Q_C}{Q_H-Q_C}##
    ##COP = \frac{T_C}{T_H-T_C}##

    3. The attempt at a solution

    Right now, the only thing I can think of is that I could use the equation ##COP = \frac{Q_C}{W}## by finding Q and W of the process the way I did in earlier chapters. And assume that the change in Q is Q_C .
    With this attempt, I could take advantage of the formulas that give me these values based on whether or not the gas is monatomic.

    Is this the right direction?

    edit: I did the problem this way and got a COP of 1...
    I assuming that the change in internal energy is 0. the first process is isothermal, and the second process is linear bringing it back to the same initial point. I get that Q=-W so Q/W is 1
    Last edited: Apr 9, 2017
  2. jcsd
  3. Apr 9, 2017 #2
    edit: I see. I think I see my current mistake. It isn't Q of the entire process. Just the Q_in.

    So I got a number for the coefficient and I'm onto the next part. I need to compare it into the result I would get from the carnot cycle at the same temperature difference. What I don't get is what the temperature difference is.
    All I am given is the temperature of the gas and that the first step is isothermal.
    Last edited: Apr 9, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted