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Homework Help: Heating coil wire resistance and length of wire required

  1. Nov 23, 2012 #1
    1. The problem statement, all variables and given/known data
    A portion of Nichrome wire of radius 2.50 mm is to be used
    in winding a heating coil. If the coil must draw a current of
    9.25 A when a voltage of 120 V is applied across its ends,
    find (a) the required resistance of the coil and (b) the
    length of wire you must use to wind the coil.

    2. Relevant equations
    RI = V
    R = I/oA

    3. The attempt at a solution
    (a) RI = V
    R = V/I = 120V/9.25A = 12.973Ohms

    R = l/oA
    l = RoA = (12.973Ohms)(.000001ohms*m)(.0025m*3.14*2) = 2.036761e-7m

    The answer to the second part of the equation is 255m and I can't figure out what I did wrong...

  2. jcsd
  3. Nov 23, 2012 #2


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    Gold Member

    Your units aren't working out well--equation for resistance seems odd.

    Try R=ρl/A

    ρ, resistivity in Ωm
    l, length in m
    A, cross-sectional area in m2.
  4. Nov 23, 2012 #3


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    2017 Award

    Did you use the correct formula for area? Did you use a value for resistivity or conductivity for the symbol "o"?
  5. Nov 23, 2012 #4
    Ah OK, that worked. I am not sure why the other formula from the book didn't show that but maybe I missed something... Thanks!
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