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Homework Help: Finding Current in a Wire

  1. Dec 7, 2015 #1
    1. The problem statement, all variables and given/known data
    The terminals of a 0.70 Vwatch battery are connected by a 70.0-m-long gold wire with a diameter of 0.100 mm.
    What is the current in the wire?

    2. Relevant equations
    V = IR
    R = pL/a

    3. The attempt at a solution

    I try to find resistance first, resistivity of gold is 2.2 * 10 ^-8

    Area of the wire is a = pi * r ^ 2
    r = ( 0.1 * 10^-3 ) /2

    So I found a = 7.85 * 10^-9

    R = 2.2 * 10^-8 * 70 / (7.85 * 10^-9) = 196.1

    I plug this in to V/R = I, .70/196.1 = .00357 = 3.57 mA, but this answer ends up being wrong. Any help on where I went wrong would be appreciated, thanks.
  2. jcsd
  3. Dec 7, 2015 #2


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  4. Dec 7, 2015 #3
    I'm putting exactly 3.57 mA into MasteringPhysics, but it keeps saying my answer is incorrect. http://imgur.com/SZN22sv is an image of what the question is, did I type anything wrong? http://imgur.com/pl1MQbC here are the answers I have already tried.
  5. Dec 7, 2015 #4


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    Where does that value for resistiviuty of gold come from?
  6. Dec 7, 2015 #5
    I searched online for the value of the resistivity of gold wire
  7. Dec 7, 2015 #6


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    Does Mastering Physics provide a reference table of constants to use? Perhaps they have a value for the the resistivity of gold that they expect you to use. The value that you found has just two significant figures and you're told to supply a result with three. This could make a difference in your least significant figure.
  8. Dec 7, 2015 #7
    No, I don't think there is one. I tried a value to three significant figures, 2.25 * 10^-8 but it still wasn't right. MasteringPhysics is usually pretty good about saying it's right as long as you are close, so that must mean my answer is a good bit off. Are there any numbers I put in wrong?
  9. Dec 7, 2015 #8


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    The value should be in the neighborhood of 3.5 mA. Using various values I've seen for the resistivity of gold I get a range from about 3.51 mA to 3.55 mA.

    Could be the question is "broken" in Mastering Physics.
  10. Dec 7, 2015 #9
    Mastering Physics is a stickler for significant figures. Your textbook has a table of resistivities.
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