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Electrical Resistivity Equation

  1. Feb 20, 2005 #1
    Is the correct equation that relates to the electrical resistivity of a conducting material to its resistance:

    p = RS/l

    p = is the electrical resistivity
    R = is the resistance of the material
    S = is the cross-sectional area
    l = is the length of the specimen

    if I'm using the right formula, would my answer to this question be correct?

    A potential difference of 1.5V exists across the ends of a copper wire of length 2.0m and uniform radius 0.40mm. Calculate the current in the wire.

    I've been given the resistivity of copper as 1.7 x 10^-4 ohms.meter


    p = RS/l
    = (1.7 x 10^-4) x 0.40mm / 2.0m
    = 0.0007 x 0.0004 / 2.0
    = 0.00000014

  2. jcsd
  3. Feb 20, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    That's correct.

    Only if you use it correctly!

    Not correct. (If you checked the units of your calculation, you'd see that things don't match.)

    First, you need to calculate the resistance (R) of that piece of copper wire, not the resistivity ([itex]\rho[/itex], which is a fixed property of copper). You are given the resistivity, not the resistance. Rearrange that formula as so:
    [tex]R = \rho L / S[/tex]
    Once you find the resistance, then you can use Ohm's law to find the current.
  4. Feb 20, 2005 #3
    Thanks Doc!

    Using the rearranged formula I've calculated:

    R = pl/S

    R = 0.0007 x 2.0 / 0.0004

    R = 0.0014 / 0.0004

    R = 3.5 Ohms

    Using Ohms Law

    I = V/R

    I = 1.5V (as stated in the question) / 3.5 ohms

    I = 0.42857..

    I = 0.43
  5. Feb 20, 2005 #4


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    Staff Emeritus
    Science Advisor
    Gold Member

    You are given the radius of the wire, not its cross-sectional area.
  6. Feb 20, 2005 #5
    Thanks Gokul! I've really got to start reading questions more thoroughly.. I'm losing marks like this all of the time :rolleyes:

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