Ranking velocities of particles in uniform magnetic field

  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.

    [​IMG]

    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. ideasrule

    ideasrule 2,321
    Homework Helper

    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. 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.

    WTF?
     
  5. Redbelly98

    Redbelly98 12,045
    Staff Emeritus
    Science Advisor
    Homework Helper

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