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Homework Help: Ranking velocities of particles in uniform magnetic field

  1. Mar 12, 2010 #1
    1. Five equal-mass particles (A–E) enter a region of uniform magnetic field directed into the page. They follow the trajectories illustrated in the figure.


    Rank from largest to smallest velocity

    2. qvBsin(theta) = ma
    qvB = ma = m(v^2/r)
    v = qBr/m

    3. Based on their radii, it should be A > B > C=E

    But what about D? It is neutral, so has no radius because it doesn't go in a circle, but clearly has a velocity despite r = 0. Is the answer indeterminate, then?
  2. jcsd
  3. Mar 12, 2010 #2


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    I think you're meant to assume that all particles have the same charge; otherwise, the problem would be unsolvable. So assuming they have the same charge, D appears to go straight because it's barely affected by the magnetic field. What does that tell you about its velocity?
  4. Mar 12, 2010 #3
    The answer actually was that it couldn't be determined.

    Now I am asked to rank the speeds largest to smallest of A, B, C, and E. I thought it was clearly A > B > C=E, but it is not and it tells me this:

    The radius of the trajectory of a particle depends on more than just its speed. Be sure not to make assumptions about the relative magnitudes of the charges.

  5. Mar 13, 2010 #4


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    The velocity depends on the charge, and you are not given any information about the charges. Therefore the problem is not solvable.
  6. Apr 4, 2011 #5
    Last edited: Apr 4, 2011
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