Elementary Thermodynamics Question

[Steven925]

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

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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!

 

[xopenx]

hrmmm. I tried doing mcAt = mcAt...

but it doesn't seem to work. lol

 

[Andrew Mason]

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!

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