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Homework Help: Finding the rate of heat transfer

  1. Sep 14, 2011 #1
    Hi there, I have this complicated question and Im not sure that im converting everything corectly. The question is:
    "For conduction, the rate of heat transfer is given by [itex]\Delta[/itex]Q/[itex]\Delta[/itex]t=-kA([itex]\Delta[/itex]T/[itex]\Delta[/itex]x)

    Data given is:

    k=806 W.m^-1.K^-1

    "calculate the rate of energy transfer [itex]\Delta[/itex]Q/[itex]\Delta[/itex]t, and give its correct S.I unit

    I just substituted all of the data into the equation, but didnt know if I had to convert meters^2 into meters?
    so this is what I had:
    [itex]\Delta[/itex]Q/[itex]\Delta[/itex]t= -(806)(2.53)[(-54.6)/(5.50x10^-3)]
    and my answer was 20243496=2.02 x 10^7 W.m^2.K-1(J.s^-1.m^-2.K^-1)- is there a simple unit name for the answer?
  2. jcsd
  3. Sep 14, 2011 #2


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    You cannot convert m^2 into m, this is nonsensical.

    As for the final answer, you should end up with something that has dimensions of power (i.e. energy/time). This is clear from looking at the left-hand side: delta Q/delta t. Therefore, your answer should just have units of watts, and if it doesn't, then you've made a mistake.
  4. Sep 15, 2011 #3
    oh okay thanks:) but in the working out, is everything right? I mean, converted correctly to get a number of 2.02 x 10^7 ?
  5. Jul 22, 2012 #4
    Let us look at the dimensions of the right hand side of your equation

    the thermal conductivity K has the units of W/mK

    A is the area available for heat transfer and thus has a unit of m^2

    dT/dX has the unit of K/m

    so in all, the RHS has the units of W (Joules/Second)

    the LHS must also have the same dimension !!
  6. Jul 22, 2012 #5
    As a followup, your substitutions seem to be OK, just that your units do not match up
  7. Jul 22, 2012 #6


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    Sri_Govind: this thread is 10 months old, and it is highly unlikely that the OP will check it any longer. Resurrecting dead threads for no reason (called "necro-posting") is strongly frowned upon. Don't do it.
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