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: Refrigerator and Heat Pump

  1. Nov 9, 2011 #1
    1. The problem statement, all variables and given/known data
    A freezer has a coefficient of performance of 6.30. It is advertised as using electricity at a rate of 457kWh/yr.
    a)On average how much energy does it use in a single day?
    b) What maximum mass of water at 20.0 degree celsius could the freezer freeze in a single day.

    NOTE: One kilowatt hour is an amount of energy equal to running a 1-kW appliance for one hour

    2. Relevant equations
    COP = Q / W

    3. The attempt at a solution
    I tried to use comparison like so;

    if 457kWh is the rate for 365 days
    then xkWh is the rate for 1 day

    This gives me 1.25 J which is very wrong.

    Please can i get clues on how to start or what to do on the "A" part of the question. It's really twisting my head..thanks a lot for your help..
  2. jcsd
  3. Nov 9, 2011 #2

    I like Serena

    User Avatar
    Homework Helper

    Hi fiziks09! :smile:

    What you have is 1.25 kWh, which is 1250 Watt times 1 hour.
    This is not 1.25 J, which is 1.25 Watt times 1 second.

    You'll need to convert the unit...
  4. Nov 9, 2011 #3
    but converting the unit still gives a wrong answer...
  5. Nov 9, 2011 #4

    I like Serena

    User Avatar
    Homework Helper

    How did you convert it?
  6. Nov 9, 2011 #5
    okay i might have misunderstood. i'm so sorry..what conversion is being made?
    is it from kWh to Joules or what? if so how so??
  7. Nov 10, 2011 #6
    Allright i got it now..
    since 1kWh = 3.6MJ (which is 3600000J)

    1.25kWh X 36000000 = 4.5 X 10^6 Joules..

    thanks for the help...
  8. Nov 10, 2011 #7

    I like Serena

    User Avatar
    Homework Helper

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook