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Electron in a cloud chamber

  1. Jan 13, 2009 #1
    An electron enters a cloud chamber which has a magnetic field B but the path of the electron is not perpendicular to the magnetic field. I understand that the electron path will describe a spiral.

    Does the v in the equation F = q(v x B) refer to the component of the electron's velocity which is perpendicular to the B field? Can I assume that an electron's velocity parallel to the B field remains constant and is unaffected by the B field?
    Last edited: Jan 13, 2009
  2. jcsd
  3. Jan 13, 2009 #2


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    Staff: Mentor

    The [itex]\vec v[/itex] in that equation is the vector velocity of the electron, including all three (x, y, z) components in general. However, you're correct that the component of [itex]\vec v[/itex] parallel to [itex]\vec B[/itex] does not contribute to [itex]\vec F[/itex]. So if, for example, [itex]\vec B[/itex] is in the z-direction (i.e. [itex]B_x = B_y = 0[/itex]), then it doesn't matter what [itex]v_z[/itex] is, as far as [itex]\vec F[/itex] is concerned:

    [tex]F_x = q (v_y B_z - v_z B_y)[/tex]

    [tex]F_y = q (v_z B_x - v_x B_z)[/tex]

    [tex]F_z = q (v_x B_y - v_y B_x)[/tex]
  4. Jan 13, 2009 #3
    Thank you. That completely answers my question
  5. Jan 14, 2009 #4


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    Science Advisor

    But it is a helix, not a spiral.
  6. Jan 14, 2009 #5
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