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Homework Help: Why is the charge negative?

  1. Apr 13, 2008 #1
    [SOLVED] Why is the charge negative?

    A uniform magnetic field is directed into the page. A charge q moves in a clockwise direction in the plane of the page. The charge q is?

    Answer: Negative.

    Can someone clarify why it is negative? I know it has something to do with the left hand rule, but I'm not sure. Thanks.
  2. jcsd
  3. Apr 13, 2008 #2


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    Left-hand rule applies for negative charges, right-hand rule applies for positive. Just try it and you'll see it only works the way described with the left hand, i.e negative charge.
  4. Apr 13, 2008 #3
    the question u framed is wrong..... if in an uniform magneticfield a charge{+ve or-ve} is projected into the field it then circulates. A stationary charge never disturbs magnetic field only moving charge is influenced by magnetic field.. according to u the circulation is clockwise or anti clockwise it can be shown by "right hand thumb rule"

    for eg current carrying wires only produces magnetic field..... stationary charge never produce magnetic field... if any clarifications PM me
  5. Apr 13, 2008 #4


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    Umm... The charge isn't producing the magnetic field in this problem, it's an external magnetic field?
  6. Apr 14, 2008 #5
    Okay, so basically if I am unable to use my right hand and it only works with the left hand rule, it's always going to be negative?
  7. Apr 14, 2008 #6
    If the charge is moving in a circular direction, then you know the magnetic force is centripetal and should point towards the center. Take any instant during the charge's clockwise motion and use the right hand rule to determine the direction of the force. Since you know the force is centripetal, you should know its technical direction; negative charges produce a force in the direction opposite of what the right hand rule implies with v x B (or as Nabeshin pointed out, you could simply use your left hand for negative charges).
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