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Homework Help: Vector evaluation & Lorentz force law

  1. Oct 10, 2012 #1
    1. The problem statement, all variables and given/known data
    An electron in a magnetic field B=2.0T(ex-ez) has velocity v=(2.5x107 ms-1 (ex-ey)
    a) calculate magnetic force on electron at that instant
    b) what is the magnitude of this force

    2. Relevant equations
    I am using the Lorentz force law.
    F = q(vXB)
    & evaluating the directions by vector

    My argument is that the ey term is negative, but another person evaluates this as positive. Please confirm whether I'm right or wrong.

    This affects the outcome of magnitude of force, too.

    3. The attempt at a solution
    F = -e 2(2.5x107) (ex-ez)* (ex-ey)

    I ex ey ez I
    I 1 . -1 . 0 I
    I 1 . 0 . -1 I

    = ex((-1x-1)-(0x0)) + ey((1X-1)-(0x1)) + ez ((1x0)-(-1x1))
    = ex - ey + ez)

    F = -8x10-12 (ex - ey + ez)
  2. jcsd
  3. Oct 10, 2012 #2

    rude man

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    Better believe your friend about the sign of the j term! (Re-check your determinant).
    Last edited: Oct 10, 2012
  4. Oct 11, 2012 #3
    Ah! Having looked at it, I found that I didn't change the sign for the second term.
    So simple it's annoying.
    Thank you.
  5. Oct 11, 2012 #4

    rude man

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    Welcome to the club! :smile:
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