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Heat Loss

  1. Jul 29, 2009 #1
    1. The problem statement, all variables and given/known data
    A metal (doesn't say what metal) is in a heater that is set to 700 K.
    If the temperature of the ball is 900 K then the rate of heat loss = 0.10 J/min.
    What is the rate of heat loss when the ball's temperature = 800 K.
    The ball's emissivity doesn't change appreciably with temperature.


    2. Relevant equations
    Q = emissivity * Boltzman Law * Temperature (to the 4th power in K) * Area * time


    3. The attempt at a solution
    I solved for A using the given information then plugged it in to the equation above to solve for Q/t using 800 K instead of the 900 K. My answer doesn't check out. Am I suppose to convert 0.10 J/min into J/seconds? If so, how?
     
  2. jcsd
  3. Jul 29, 2009 #2

    tiny-tim

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    Hi Cheddar! :smile:
    Yes, all your measurements must be in SI units (kg, second, joule, etc).

    Calculations can go horribly wrong if you don't do that!

    To convert J/min into J/second, just do it the same way you'd convert feet/min into feet/second. :wink:
     
  4. Jul 30, 2009 #3
    Thank you. I did that and the answer still doesn't match up though.
    I get an area of 4.48018196 * 10(-8 power). Plugging this into:
    Q/t = emissivity * Boltzman's * Temperature(4th power) * Area
    leaves me with 0.0010404918, which doesn't check out.
     
  5. Jul 30, 2009 #4

    Redbelly98

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    This equation isn't quite right. Question: what would the rate of heat loss be if the ball were at the same temperature as the heater in which it is located?
     
  6. Jul 30, 2009 #5
    So, instead of T(4th power) it should maybe be the difference between the ball's temp and the environments temp(to the 4th power)??
     
  7. Jul 30, 2009 #6

    Redbelly98

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    Close. It would be (Tball4 - Tenvir4)
     
  8. Jul 31, 2009 #7
    I think I have it.
    0.40 J/min
     
  9. Jul 31, 2009 #8

    Redbelly98

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    The heat loss rate should be less at a lower temperature.
     
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