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Simple heat transfer problem

  1. Feb 16, 2016 #1
    1. The problem statement, all variables and given/known data
    A hot reservoir with a temperature of 653 K is 0.7 m away from a cold reservoir with a temperature of 399 K. The two reservoirs are insulated from each other except for a rod of brass (k = 109 W/m-K) that has a cross-sectional area of 0.07 m2. The entire system is allowed to reach a steady-state condition.
    1)How much energy is transferred between the hot reservoir and the cold reservoir by heat in ten minutes?

    2. Relevant equations
    thermal current=change in temp*thermal resistance, I=ΔT/R
    3. The attempt at a solution
    R=.7/(109*.07)=.091743 K/W
    I=(653-399)/.091743=2768.6 W
    for 10 minutes: 2768.6 J/S * 60 S/min * 10 min=1,661,162.16 J
    Sorry for the simple question, but the homework software says that answer is incorrect and I can't really see what could be wrong with such a straightforward problem, I imagine I'm missing something stupid but can't find it.
  2. jcsd
  3. Feb 16, 2016 #2


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    Try to write the result in normal form, with three significant digits.
  4. Feb 16, 2016 #3


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    Hello and welcome to PF!

    I think your work is correct. I get the same answer.
  5. Feb 16, 2016 #4


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    To expand on what ehild said: You are using way too many significant digits. Based on the precision of the inpot, there is no way that you can obtain an answer with that kind of precision. This means that most of your significant digits are junk and should not appear in the final answer. Keeping the digits throughout computation is a good thing to avoid rounding errors propagating, but in the final answer you simply do not have precision. Based on the input, giving any more than two significant digits would be overstating the precision.
  6. Feb 17, 2016 #5
    Thanks for the replies, but it's smartphysics/flipitphysics. It doesn't mark things off for overdoing significant figures, it usually requires more than would be in line with the problem's parameters. I did try the answer to 2 and 3 figures just to be sure, but it seems like the problem is bugged.
  7. Feb 17, 2016 #6


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    Did you give the unit as J? What about using kJ or MJ?
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