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Homework Help: 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.

    Thanks!
     
  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

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    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.

    AM
     
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