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Homework Help: Easy resistance problem

  1. Oct 11, 2007 #1
    1. The problem statement, all variables and given/known datathe filament of a 230V light bulb is 0.72M long and has a radius of 6.0 x 10^-2mm. The resistivity of the filamen metal is 1.2 x 10^-5 ohm metres. Work out the resistance of the bulb

    2. Relevant equations I have tried working out the area using pi x radius square but am having trouble converting 6.0x10^-2mm into metre square.

    i am using R=pL/A so i did 1.2x10^-5 x 0.72/1.1 x 10^-8m^2 which equals 785 ohms but the answer is 760 ohms can anyone please help me out?

    3. The attempt at a solution
  2. jcsd
  3. Oct 11, 2007 #2
    can anybody help?
  4. Oct 11, 2007 #3


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    Homework Helper

    I think the problem is with your rounding...

    I get 763.94ohms... which when taken as 2 sig figs is 760ohms.

    convert 6.0*10^-2mm to m first

    6.0*10^-2mm * 1m/(1000mm) = 6.0*10^-5m

    area is pi*(6.0*10^-5m)^2 = 1.13097*10^-8 m^2
  5. Oct 11, 2007 #4
    hmmm thanks when i enter the pi x (6.0x10^-5)^2 i keep getting 1.1 x 10^-8 i dont get why
  6. Oct 11, 2007 #5
    could it be a problem with my calculator??
  7. Oct 11, 2007 #6


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    Homework Helper

    set your calculator to scientific notation. then you should see the other digits.
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