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Homework Help: Resistance and temperature problem; answer does not agree with book

  1. Jul 5, 2008 #1
    1. The problem statement, all variables and given/known data

    What is the fractional change in the resistnace of an iron filament when its temperature changes from 25.0 degrees Celsius to 50 degrees Celsius?

    2. Relevant equations

    R = R[tex]_{0}[/tex](1 + [tex]\alpha[/tex](T - T[tex]_{0}[/tex]))

    3. The attempt at a solution
    R = R[tex]_{}0[/tex](1+5x10^3x25)
    R = 1.125R[tex]_{}0[/tex]
    1.125R[tex]_{}0[/tex]/R = 1.125

    The book's answer is 0.125, and i have 1.125, what did I do wrong?

    Sorry; I can't figure out how to make the 0's in the R's go into subscript, not superscript. Those 0's are supposed be subscripted.
  2. jcsd
  3. Jul 5, 2008 #2


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    Homework Helper

    Hi clairez93,

    In this case, this is not the quantity they are asking for. They want the fractional change. So your fraction has to have the change in resistance in it. Do you see what it needs to be?

    About the subscripts: put the entire equation between the tex tags, and don't put the {} brackets you have right before the zero subscript.
  4. Jul 6, 2008 #3
    So I'm guessing I have to subtract 1.125R_0 - R, so that would be 0.125, and that is the change?
  5. Jul 6, 2008 #4


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    Homework Helper

    No, not exactly. Since R=1.125 R_0, then 1.125R_0 - R would just equal zero.

    The change in anything is the final value minus the initial value. So first find the change in resistance.

    Then, since they want the fractional change, use the change in resistance that you found in your fraction. The fraction will cause an unknown variable to vanish.
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