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Homework Help: Please explain these questions

  1. May 28, 2005 #1
    1. A charged particle moves in a straight line through a certain region of space. The magnetic field in that region
    a) has a magnitude of zero
    b) has a zero component perpendicular to the particle's velocity
    c) has a zero component parallel to the particle's velocity

    2. The north-pole end of a bar magnet is held near a stationary positively charged piece of plastic. Is the plastic
    a) attracted
    b) repelled
    c) unaffected by the magnet

    The answers are 1) either a or c, 2) c

    I don't understand why. Can someone explain.
  2. jcsd
  3. May 28, 2005 #2


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    I'd say it's "a" and "b" for #1.

    HINTS:1.What's the expression of the magnetic Lorentz force and what kind of
    mathematica operation does it contain?
    2.For the second problem,what's the direction of the magnetic field...?

  4. May 28, 2005 #3
    for 1. I don't know what Lorentz force is
    2. The direction of the magnetic field should be towards the south-pole end.
  5. May 28, 2005 #4


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    Lorentz Force is the name of the the force acting on a charged particle in an electric and magnetic field. Only the magnetic field is involved in this problem. Only the component of the field perpendicular to the velocity results in a force. That's why the answer is a or b and not a or c.


    For the second one, the important thing is that there is no relative motion of the charges and the magnet, so relative to the magnetic field, v is zero.
  6. May 29, 2005 #5
    For number 1, 'b' would be the correct answer. As having 'no magnetic field' is a case of having 'no magnetic field in the perpendicular direction' when considered in context of the question and its answers.
  7. May 29, 2005 #6


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    Good point. a) might be true, but it is not certainly true. Same can be said about c). b) has to be true
    Last edited: May 29, 2005
  8. May 29, 2005 #7
    The lorentz force is one of the first interactions between fields and particles learned in magnetism, it is also a very important one. You should know this one like the back of your hand.

    Its also one of the easier ones :)

    Good evening folks :)
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