Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A question about Magnetic Fields and A Charge

  1. Apr 11, 2008 #1
    [SOLVED] A question about Magnetic Fields and A Charge

    A charged particle enters in 90 degrees to the magnetic field i.e. the force on the charge is F = qvB as sin (angle) = 1. The question is which are the effects on the kinetic energy and momentum of the particle?


    A. KE => Changed
    Momentum => Changed

    B. KE => Unchanged
    Momentum => Changed

    The solution is B but I do not why? Please, help me.
  2. jcsd
  3. Apr 11, 2008 #2
    I think...

    As the force is perpendicular to the displacement no work (W=fxcos(angle)) will be done. It's kinetic energy, a scalar, remains unchanged.

    However, the force does accelerate the charged particle. This acceleration is perpendicular to it's velocity. The momentum vector changes in direction, not magnitude.

    Does this help?
  4. Apr 11, 2008 #3

    In other words, the particle will circle around and around again ie it is not actually progressing anywhere. It stays where it is and circles. Hence, can it be said that there is no work done ie. KE is unchanged?
  5. Apr 11, 2008 #4
    Yes I believe so.
  6. Apr 11, 2008 #5
    Thanks! I think I understood :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook