1. Not finding help here? Sign up for a free 30min 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!

Elementary Thermodynamics Question

  1. Jan 1, 2007 #1
    The prompt for the question I am seeking help on is:

    "A freezer contains 20 kg of food with a specifit heat of 2x10^3 J/kg*c. The temperature inside the freezer is initially -5*C. The freezer motor them operates for 10 minutes, reducing the temperature to -8*C."

    I have found solutions to the first three portions of the question:
    a)How much heat is removed from the food during this time? The freezer operates at 400W

    Q = -120,000J using Q = CM(t(f) - t(i))

    b)How much energy is delivered to the freezer motor suring the 10 minute period?

    P = W/T = 240,000J

    c)During this time, how much total heat is ejected into the room in which the freezer is located?

    Q = Q + W = 360,000J


    D is what i need help on:

    "Determine the temperature change in the room if the specific heat of air is 700J/Kg*C. assume there are 80 kilograms of air in the room, the volume of the air is constant, and there is no heat loss from the room"

    I have tried using the standard Q = cm(delta)t:

    360,000J = 700J/Kg*c * 80 (delta)t

    but that comes out to approximately 6.2 degrees celcius. My instructor says the answer is 7.3*C and i'm unsure of what I may be doing incorrectly. Any guidance would be greatly appreceiated.

  2. jcsd
  3. Jan 2, 2007 #2
    hrmmm. I tried doing mcAt = mcAt....

    but it doesn't seem to work. lol
  4. Jan 2, 2007 #3

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    I don't see anything wrong with your method. I get 6.4 deg. C which to the correct significant figures should be 6 deg. C.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Elementary Thermodynamics Question