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Homework Help: Stuck on a few magnetic field questions

  1. Feb 26, 2006 #1
    Problem 1:
    here is what i did:

    knew to find the acceleration I needed to use a=QE/M, but since I didn't have E I had to find it. Used F(mag field)=Q*V*B*Sin(theta). I found theta from drawing the ijz axis and using trig, I got 59 degrees.

    After I solved this I plugged it in to F=QE to get E. Then plugged that into a=QE/M, and got an incorrect answer..not sure where I went wrong.

    Problem 2:
    for a:
    equations I used:

    R=MV/QB -> V= RQB/M

    plugged 2nd into 1st, M cancelled, answer wasn't correct. eh?

    for b:

    not sure i know the equation for this problem...same as in classical physics?

    thanks guys!
  2. jcsd
  3. Feb 26, 2006 #2
    1) There's nothing wrong with your first problem except that you seem to be insisting there's an electric field. There isn't. The Lorentz force says:
    [tex]\mathbf{F}=q\mathbf{E}+q\mathbf{v}X\mathbf{B}[/tex]. Since there is no E, the first term drops out. Then you calculated F, as you did. Now simply apply F=ma.

    2) Part a) looks right, I'm not sure why things aren't working. What answer did you get and what does your book say? For part b) I would assume that you aren't being relativistic or quantum mechanical, so yes use the Classical formula.

    Last edited: Feb 26, 2006
  4. Feb 26, 2006 #3
    thanks, figured the first one out!

    for 2a i got 4.94×10−18
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