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Homework Help: Radiation: simple, yet

  1. Apr 9, 2006 #1
    ...im getting it wrong.

    Calculate the heat flow through a 1.000m long copper bar with a cross sectional area of 2.00 cm2. One end at 2.0 degrees Celsius and the other end at 99.0 degrees Celsius. Rate of heat flow is Q/t. In this problem the coefficient of copper is 390.

    P=(ka (Thot-Tcold))/L

    so I input 390*(2/100)*97 and get 756.6=wrong:frown:
     
  2. jcsd
  3. Apr 9, 2006 #2

    lightgrav

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    You seem to be ignoring UNITS ...
    some coefficient of copper is 390 ... WHATs?
    cm? /cm? /m? J/K?

    By the way, this is CONDUCTION , not radiation.
    ... and Conductivity (k) is not usually refered to as a coefficient.
     
  4. Apr 9, 2006 #3
    390 is thermal conductivity (J/s*m*deg C)

    redoing with "UNITS" would be 390J/s*m*deg C*(2cm^2/100cm/m)*97degK
     
  5. Apr 9, 2006 #4

    lightgrav

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    so, which of the cm are you converting ... the height, or the width?
     
  6. Apr 9, 2006 #5
    so, which of the cm are you converting?

    the area
     
  7. Apr 9, 2006 #6

    chroot

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    Hint: To convert cm^2 into m^2, you don't just divide by 100. Think about this for a second: how many squared centimeters are in a squared meter?

    - Warren
     
  8. Apr 9, 2006 #7
    ^ yeah, that was it. I followed a similar example to a "T" too...
     
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