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Homework Help: Current / Magnetism Questions: Proton near magnetic field of moving electrons

  1. Aug 16, 2009 #1
    1. The problem statement, all variables and given/known data

    Ok guys, this problem has a few steps to it, and id appreciate any help I can get.


    Ok, so beam of electrons moving left, positive particles with same but opposite charge moving right, both moving at 1x10^8 m/s. Then there is a proton moving left on top of the beam nearby AT ALSO 1x10^8 m/s. A person standing nearby is observing, and so is a guy running ALSO 1x10^8 m/s nearby.

    1) Direction of current?
    2) Force on the proton as seen by the person standing, and the person running seperatly.
    3) Velocity/direction of the two beams from the running persons point of view?
    4) What current will the man running measure and in which direction?

    2. Relevant equations
    1) none
    2) F=qvBsin(theta) (or in this case just F=qvB since the direction of the proton is perpendicular to the magnetic field..?)
    3) none
    4) ?

    3. The attempt at a solution

    1) Current flows is opposite of current flow, so therefore in this case current flows to the right. Is this correct?

    2) Person standing still: There is a magnetic field flowing out of the screen from the view we have, if the current is indeed traveling to the right. Therefore, I simply use the equation F=qvB to solve. Is this true, or would I need to factor in the fact that the electrons are also drawing the particle towards them?

    Person running: since he is moving at the same speed and direction as the proton, it will seem to him that it is not moving.

    3) The beam moving in the same direction will seem as though it doesnt move, but the beam moving in the opposite direction will seem to move twice as fast.

    4) will he measure no current? since he is moving as fast as the electrons?

    Thanks for any help guys! I appreciate it.
    Last edited: Aug 16, 2009
  2. jcsd
  3. Aug 17, 2009 #2


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    1) I assume you meant to say that current flows in the opposite direction from electron flow? Anyway, to the right is correct.

    2) In the reference frame of the person standing, yes, use F = qvB. Think about this: if the electrons are attracting the proton towards them, why would the positive particles (in the current) not also be repelling the lone proton away from them? How do the strengths of these two effects compare?

    In the reference frame of the running person: yes, he sees it as not moving and therefore sees no magnetic force. But he sees another force acting on the proton in place of the magnetic force. Can you identify it?

    3) The beam moving along with the runner (that is, the electrons) will seem to him like it's not moving, true. The other beam will seem like it's moving almost twice as fast. (Why not exactly twice as fast?)

    4) Aren't you forgetting the beam of positive particles?
  4. Aug 17, 2009 #3


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    Have you studied relativity? I am wondering because the speeds involved are (1/3)c. If one were not to consider relativistic effects, why pick this particular speed for charge distributions and observer?
  5. Aug 23, 2009 #4

    You brought up some amazing points! Thank you so much for your help.

    For number 2, for the person running, you are referring to the force of the electric field, am I correct?

    And one other question, what direction would the electron seem to be traveling from the point of the running person?
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