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Magnetism Question

  1. Dec 28, 2011 #1
    1. The problem statement, all variables and given/known data

    A charged oil drop is allowed to fall through the electric field created by the plates as shown:

    + -
    l l
    l l
    l l

    In order to give the oil drop a straight trajectory, a magnetic field should be established with field lines pointing:

    A. to the right
    B. to the left
    C. out of the page
    D. into the page

    2. Relevant equations


    3. The attempt at a solution

    The back of my book says that "the electric field between the plates will push a positive charge to the right, so the magnetic field must push the drop to the left to counter the force."

    What I don't understand is how am I to know if the oil drop has a positive or negative charge since it is not stated in the question?
  2. jcsd
  3. Dec 28, 2011 #2
    You are given the charge on the plates so you should be able to know the charge on the drop from the way it is pushed..... look carefully
  4. Dec 28, 2011 #3


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    Expanding on the solution given in the book... What happens if the oil drop had a negative charge?

    (I don't have any figure.)
  5. Dec 28, 2011 #4
    The book shows the drop falling directly in between the plates and doesn't show it being pushed in any direction.

    If the drop were negative I assume it would move to the right towards the positive plate (since opposites attract).

    What I don't understand is how am I to know the charge on the oil droplet?
  6. Dec 28, 2011 #5
    In your diagram it looks, to me, as though the left plate is + and the right plate is -.
    If the drop is falling between them and is moving to the right then this tells you the charge on the drop.
    You can only know this from what it says in the back of the book !!!!!!
  7. Dec 28, 2011 #6
    That's what I think too!! It must be an error in the question because the drop is not shown to have any positive or negative orientation. I just wanted to know if it was a book error or if there was something I didn't understand.

    Thanks everyone for all your help! :)
  8. Dec 28, 2011 #7


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    I'll ask again.

    What will happen if the charge on the oil drop is negative?

    In what direction will the plates tend to push the charge?

    In what direction will the magnetic field need to point to keep the oil drop centered?
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