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Homework Help: Finding the potential difference across two wires

  1. Mar 29, 2009 #1
    Hello everyone, as you can tell I'm new and just wondering if I could get some help with this question as I'm not sure where exactly I've gone wrong:

    1. The problem statement, all variables and given/known data

    Two wires are used to connect a lamp to a power supply of negligible internal resistance. The potential difference across the lamp is 12V and its power is 36W. Calculate the potential difference across each wire. The two wires in question are made of copper and have a resistivity of 1.7 x 10^-8 Ohm meter.

    2. Relevant equations

    Not 100% sure but:

    V=W/Q, R=V/I, I=P/V, E= I (R + r)

    And I'm not sure about an others.

    3. The attempt at a solution

    I = 36/12, I = 3
    R = 12/3, R = 4
    E = 3 (4 + 1.7x10^-8) = 12V

    Therefore the potential difference across each wire is 12V

    I know I've gone wrong some where and am hoping someone can put me right.

    Many thanks,
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 29, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Good. You found the current, which you'll need.
    That's the resistance of the lamp. OK, but not needed.

    Hint: What's the resistance of each wire? How long are they?
  4. Mar 29, 2009 #3
    Thanks for the help. :smile:

    Using 0.6m as length and 0.001m as the cross sectional area I have derived an answer of 3.06 x 10^-5 after first findinng the resistance of both wires (as you stated) then using the resistance found in the equation V = IR to find the potential difference.

    Thanks again.
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