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Homework Help: Long straight wire: Ampere/Biot Savart

  1. Apr 10, 2013 #1
    1. The problem statement, all variables and given/known data
    A long straight wire carries a current of 2.0A. An electron moves parallel to it at a distance of 0.01m (I'm assuming id doesn't matter which way it moves) at a velocity, v = 100ms-1
    Calculate magnitude & direction of force acting on it.

    2. Relevant equations
    B = μ0 L / 2∏r

    F = q(v X B)

    I know the second is the Lorentz force law & that the first relates to Biot-Savart but for the Biot-Savart I also have,
    F12 = μ0/4∏ * I1 δl1 * ( I2 δl2 * rhat 12) / r122

    So how does one relate to the other, please.

    3. The attempt at a solution

    Surely it can't be this easy!

    B = μ0 L / 2∏r
    B = 4∏x10-7 * 2.0 / 2∏(0.01)
    B = 3.95x10-8 T

    F = q(v X B)
    F = (-1.6x10-19)(100x103)(3.95x10-8
    F = -6.32x10-22 N

    Magnitude 6.32x10-22 N
    Direction (radially) away from wire
  2. jcsd
  3. Apr 10, 2013 #2

    rude man

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    Homework Helper
    Gold Member

    Surely it can!
    Except the direction could be towards or away from the wire, depending on the relative direction of v and i and the position of q.

    BTW I've never seen L used for current before ...
  4. Apr 11, 2013 #3
    L . . . oops!
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