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Homework Help: Easy Question on Magnetic Force of Electron

  1. Feb 21, 2010 #1
    1. The problem statement, all variables and given/known data

    An electron that has a velocity with x component 1.7 x 106 m/s and y component 3.9 x 106 m/s moves through a uniform magnetic field with x component 0.029 T and y component -0.15 T. (a) Find the magnitude of the magnetic force on the electron. (b) Repeat your calculation for a proton having the same velocity.

    3. The attempt at a solution

    I tried taking the magnitude of the magnetic field, √ 0.0292 + 0.152 = 0.1527 T

    Then the magnitude of the velocity, (√ 1.72 + 3.92 ) x 10^3 = 2.609 x 10^3 m/s

    So for a) I got: F = Bqv = 0.1527 * - 1.6 x 10^-19 * 2.609 x 10^3 = - 0.637 x 10^-16 N

    and for b) I just made it positive and got .637 x 10^-16 N

    But both of those answers are incorrect :(

    Please help me :(
  2. jcsd
  3. Feb 21, 2010 #2


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    Science Advisor
    Homework Helper
    Gold Member

    First you write the velocity and magnetic field vectors in unit vector notation.
    Then you find the magnetic force vector from F = q v x B, i.e. take the cross product between the two vectors correctly. Note that, for an electron, q = -e.
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