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Homework Help: Why isn't current negative in this example?

  1. Jan 31, 2015 #1
    1. The problem statement, all variables and given/known data
    The figure gives the electric potential V(x) along a copper wire carrying uniform current, from a point of higher potential Vs = 9.00 μV at x = 0 to a point of zero potential at xs = 3.20 m. The wire has a radius of 1.50 mm, and copper has a resistivity of 1.69 × 10-8 Ω·m. What is the current in the wire?


    2. Relevant equations


    p = 1.69*10^-8 (ohm * meter) for copper

    3. The attempt at a solution

    I solved it by just plugging values into the above equation give. It is clear that A>0 (cross section), L > 0 (length), p > 0 (given constant). And V < 0 because Vf = 9*10^-6 and Vi = 0.

    The answer is +1.176e-3, but I got a negative value. As you can see, every value is positive except delta V, because Vf < Vi, so the current should be negative, should it not?
  2. jcsd
  3. Jan 31, 2015 #2


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    Homework Helper
    Gold Member
    2017 Award

    Probably the formula was intended to give only the magnitude of the current. So, all of the quantities in the formula should be considered as magnitudes (positive). You can always get the direction of the current from the rule that current in a conductor flows from higher to lower potential.
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