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Ranking velocities of particles in uniform magnetic field

by Linus Pauling
Tags: field, magnetic, particles, ranking, uniform, velocities
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Linus Pauling
Mar12-10, 05:20 PM
P: 190
1. Five equal-mass particles (AE) 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?
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Mar12-10, 07:44 PM
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ideasrule's Avatar
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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?
Linus Pauling
Mar12-10, 09:43 PM
P: 190
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.


Mar13-10, 08:27 AM
Redbelly98's Avatar
P: 12,064
Ranking velocities of particles in uniform magnetic field

Quote Quote by Linus Pauling View Post
v = qBr/m
The velocity depends on the charge, and you are not given any information about the charges. Therefore the problem is not solvable.
Apr4-11, 05:14 PM
P: 1

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