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Electric Charges

  1. Mar 25, 2007 #1
    1. The problem statement, all variables and given/known data
    A +10mC charge is placed 0.20 m from a -2.0mC charge. Where on the line joining the 2 charges is the a) electric field equal to zero? b) electric potential equal to zero?

    2. Relevant equations

    3. The attempt at a solution
    I am not sure, but I think that the answer to both of these questions is zero (because infinite is not an option). Am I wrong when I say that the electric field can't be zero because there are two forces pulling in the same direction, therefore they cannot cancel eachother out when between them. Also, the electric potential can't seem to equal zero because again, the forces are adding. I'm not as sure about that one though.
  2. jcsd
  3. Mar 25, 2007 #2
    Part a first: The field will be zero at a position where you could place a test charge (small positive charge) and it will experience no net force. First decide on the configuration of the "source" charges- i.e. is the +10 m C to the left of the other charge? Then figure out where the field must be zero, conceptually, i.e. to the left of them, between them, or to the right.
  4. Mar 25, 2007 #3


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    It seems you are only considering the part of the line between the two charges. Yes, inside the 2 charges, the fields will never equal 0, they will always add, but outside the 2 charges the fields point in opposite directions.

    HINT: What is the formula for an electric field? Write down an equation describing the conditions you want. After you do that, solve for the position along the line. The follow the same procedure for the potential.

    See how far you can get now. Good Luck!
    Last edited: Mar 25, 2007
  5. Mar 25, 2007 #4
    oh ok, I thought the question meant only between the 2 charges. For the electric field, I got 0.16 m past the negative charge. For the electric potential, I got 0.05m?
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