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Homework Help: Tv electron and earth magnetic field

  1. Oct 21, 2007 #1
    1. The problem statement, all variables and given/known data
    The electrons in the beam of a television tube have an energy of 23.0 keV. The tube is oriented so that the electrons move horizontally from west to east. At the electron's latitude the vertical component of the Earth's magnetic field points down with a magnitude of 57.6 T. What is the direction of the force on the electrons due to this component of the magnetic field?

    2. Relevant equations

    3. The attempt at a solution
    The options are up, down, north, south, west,east.
    I chose north because B is down(middle finger), pointer finger horizontal and therefore thumb points north. Why is that wrong?
  2. jcsd
  3. Oct 21, 2007 #2


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    Staff Emeritus
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    Is an awfully strong magnetic field. The earth's field is relatively weak.

    Nevertheless, the velocity of the electron is west to east (horizontal) and the magnetic field is down (vertical) which leaves either N or S, based on

    F = q (v x B), where q is the charge, v is the velocity and B is magnetic field strength.
  4. Oct 21, 2007 #3
    With right hand rule I get north, but it is incorrect!
    Sorry it should be micro teslas!
    Last edited: Oct 21, 2007
  5. Oct 21, 2007 #4
    Actually shouldn't it be south because the negive from the charge of an electron flips the force the opposite direction.
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