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Magnetic Fields question?`

  1. Jul 18, 2011 #1
    Where did the formula r=mv/Bq come from in my answerbook for my grade 12 physics? Thanks.
     
  2. jcsd
  3. Jul 18, 2011 #2

    berkeman

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    Staff: Mentor

    Do you know the equation for uniform circular motion, given a force F that always points toward the center of the circular motion?

    Do you know the equation for the force F on a charged particle q when it is travelling at velocity v perpendicular to a magnetic field B?
     
  4. Jul 18, 2011 #3
    F=qvBsin(theta) is the equation right? and, I probably have seen the first one, but can't remember that specific one.
     
  5. Jul 18, 2011 #4

    berkeman

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    Staff: Mentor

    Correct, but what does it simplify to when the velocity and B-field are orthogonal (at right angles to each other)?


    See Uniform Circular Motion at wikipedia, and remember that F=ma:

    http://en.wikipedia.org/wiki/Uniform_circular_motion

    .
     
  6. Jul 18, 2011 #5
    Ohh okay I got it. Because the force of the magnetic field equals the net force (F=ma) so that means that qvb=ma and then since v=rw and a=vw it's qrwB=mvw, and then the w's cancel out and you rearrange. Thanks alot :biggrin:
     
  7. Jul 18, 2011 #6

    berkeman

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    Staff: Mentor

    Good job! :biggrin:
     
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