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Homework Help: Electron moving through a uniform magnetic field

  1. Jun 22, 2009 #1
    The Problem:
    An electron moves with speed 9.00 105 m/s in a uniform magnetic field of 2.0 T, pointing south. At one instant, the electron experiences an upward magnetic force of 2.00 10-14 N. In what possible directions might the electron be moving at that instant? Give your answers as angles clockwise from south (from 0° to 360°), in increasing degrees.

    The only equation needed should be F= lql * VBsin(theta)

    I used the above equation and found the angle to be 3.98° counterclockwise of South. So my first answer was 360-3.98=356.02° (which was correct). My problem came with finding the angle at the second position. I initially thought that it would be 180-3.98, but, through guess work, I found that the correct answer is 180+3.98= 183.98. I'm not sure why this is right.

    Can someone help me understand the concept behind the second possible position of the electron?

    Thank you!
  2. jcsd
  3. Jun 22, 2009 #2


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    Homework Helper

    Welcome to PF!

    Hi bioradical! Welcome to PF! :smile:
    Because what matters is sinθ.

    sin(180º + θ), which was your first try, is minus sinθ (so the force would have to be down) …

    but sin(180º - θ) is sinθ :wink:
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