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Homework Help: Positive & Negative Charges

  1. Jun 10, 2007 #1
    Yesterday I asked a question regarding the right hand rule and Lorentz law. I now understand how the right hand rule is applied but the examples I have looked at all involve a positive charge.

    My question is what effect does a negative charge have on the right hand rule?

    If for example a positive charge is moving in a positive x direction when it enters a magnetic field and then starts to describe a circle in an anticlockwise direction (in the xy plane), by using the right hand rule I would say that the magnetic field would be in the positive z direction... Would you agree?

    Now if a negative charge were to enter the same field and describes the same anticlockwise circle, how do I apply the right hand rule in this situation? Would I simple say, ok for a positive charge the direction of the magnetic field was in the z direction, so for a negative charge, the direction must be in the -z direction?
  2. jcsd
  3. Jun 10, 2007 #2

    Doc Al

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    Staff: Mentor

    The direction of magnetic force on a negative charge will be opposite to that on a positive charge.

    No. Using the right hand rule to compute [itex]\vec{F} = q\vec{v} \times \vec{B}[/itex] for a postive charge moving in the +x direction in a magnetic field in the +z direction I get an initial force in the +y direction--which means clockwise circle. (Viewed from above the x-y plane, where z > 0.)

    I think you have your directions mixed up in this case, but your idea is correct: To get the same direction of force on a negative charge, the magnetic field must be opposite to what it was for the positive charge.
  4. Jun 10, 2007 #3
    if u got the direction right, u can say that it is just the opposite for the negative charge.

    therefore 2nd answer is -z and 3rd answer is +z
  5. Jun 10, 2007 #4
    Ok, I think I understand.... So if we know any two of either magnetic force, magnetic field or charge, by using the right hand rule we can find the third.

    If the charge is positive then thats fine we go with the direction on the right hand, but with a negative charge it is simply the opposite direction to the right hand?

    Let me give one more example to see if I've got it...

    A negative charged particle enters a magnetic field in the positive x direction and the magnetic field is in the -z direction, the particle would experience a magnetic force in the -y direction and would describe a circle in a clockwise direction?
  6. Jun 10, 2007 #5

    Doc Al

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    Staff: Mentor


    You got it.
  7. Dec 11, 2011 #6
    Now apply Quantum wave observations to the particle in motion ^^

    Tingle, brain! Tingle! >.<
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