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: Thermodynamics help

  1. Feb 8, 2009 #1
    1. The problem statement, all variables and given/known data

    A household refrigerator with a coefficient of performance of 2.1 removes energy from the refrigerated space at a rate of 600 Btu/h. Evaluating electricity at $ 0.08 per kW · h determine the cost of electricity in a month when the refrigerator operates for 360 hours.

    2. Relevant equations

    coefficient of performance = Q_in / W_cycle
    W_cycle = Q_out - Q_in

    3. The attempt at a solution

    I assumed that Q_out was 600 Btu/hr, since it said that was the energy removed. The coefficient of performance is obviously 2.1, and I tried to find Q_in using the above equations but had some trouble. I also know that you need to find the work and then you can just use the given "$0.08 per kW" to find the money needed, which is the answer. But I am really confused with this one.
  2. jcsd
  3. Feb 9, 2009 #2
    I didn't think Q/W could be greater than 1. Since 1 represents 100% effeciency. If an engine had a Q/W value of 2.1, wouldn't it be working at 210% efficiency??
  4. Feb 9, 2009 #3
    thermal efficiency is different than coefficient of performance
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook