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Homework Help: Current/Voltage/Resistance HELP!

  1. Oct 28, 2008 #1
    a 20cm long hollow nichrome tube of inner diameter 2.8mm, outer diameter 3.0mm is connected to a 3.0V battery. What is the current in the tube?



    I think this is a I = V/R problem. So i used R=([tex]\rho[/tex]*L)/A. Where L is the length of the wire and A is the outer area minus the inner area and rho is the resistivity of nichrome.


    A =.003^2*[tex]\pi[/tex] - .0028^2*[tex]\pi[/tex] = 3.64*10^-6
    R = ((1.5*10^-6) * (.02))/3.64*10^-6 = .0823
    I = 3/.0823 = 36.44Amps

    However the answer is 9.1A apparently. Where have i gone wrong?

    Thanks!!

    btw if the greek symbols appear superscript they are not supposed to be I cant quite figure that part out
     
  2. jcsd
  3. Oct 29, 2008 #2
    You made a simple error, 3.0mm and 2.8mm are diameters, not radii.

    Area is [tex] \pi r^2 [/tex]
     
  4. Oct 29, 2008 #3
    haha thanks! spent 45 minutes on that problem!
     
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