1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar

    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


    User Avatar
    Science Advisor

    But it is a helix, not a spiral.
  6. Jan 14, 2009 #5
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook