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Math Help - Finding the unknown using a formula help

  1. #1
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    Finding the unknown using a formula help

    Hi,

    I need help using a formula to find the length of an electrical conductor.

    (A) The cross sectional area of the conductor is 2.5mm2 which I have been told : 2.5 x 10^-6 (power of minus 6).
    (R) The resistance is 0.86 (in ohms).
    (P) The resistivity of the conductor is 17.24 nanohm metres which is : 17.24 x 10^-9 (power of minus 9).
    (L) The length is unknown.

    The formula was R: PL/A, now that length is unknown I believe the formula changes to L: RA/P?

    So I came up with 0.86 x (2.5 x 10^-6) / 17.24 x 10^-9

    Which gives me 1.25 (this ain't the right answer), did I go wrong in formula transposing or something?

    Thanks
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  2. #2
    MHF Contributor
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    Re: Finding the unknown using a formula help

    Hey cliffgnome.

    You need to careful about the brackets (you need 17.24 x 10^-9 in one single bracket).

    By using the brackets in the right place we get (from R) the answer to be:

    > 0.86*2.5/17.24*1000
    [1] 124.71

    since 10^-6 / 10^-9 = 10^3.
    Thanks from cliffgnome
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