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Homework Help: Electric potential

  1. Feb 9, 2006 #1

    I have this problem:

    Two charges are placed along the x axis; 3 C at 14 cm, and -4C at 15cm. Find those points along x axis where the potetial is zero.

    I dont know how to start here. Please can I have some help.
    I have the same problem with 4 charge in a plane. Is the procedure the same as this one?
    But I need to understand this one first.

  2. jcsd
  3. Feb 10, 2006 #2


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    Gold Member

    The First problem:
    First of all, you know that it has to be either to the right of -4C, to the left of 3C, or in the middle.
    You know the distance between the charges (I'll call it d). So if a point is x distance from the first charge (3) it is x+d distance from the second one if the point is to the left of 3C, x-d if it is to the right of 4C, and d-x if it's in the middle. Now for each of those cases you can make an equation were the potential of one charge equals the the potential of the other were for the distances (r) you use the pair of distances that go with that case ((x,x+d),...). In each of the cases you solve for x.

    So you just have to find an x were the forces of each charge are equal.
  4. Feb 10, 2006 #3


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    Homework Helper

    Brad Sue - you're finding where Electric Potential V = 0 , right? (Not Force).
    V is positive very near a positive charge, and is negative very near a negative charge. Far enough away from them both, the "measurer" only sees the total charge, so potential should be negative far away (tending to 0 as 1/x).
    I would sketch this function along the x-axis in 6 regions: near
    13.8, 14.2; 14.8, 15.2; 0, 25 ; connect the dots to see zero-crossings.

    Contributions to the Electric Potential (at a location) by each charge add,
    kQ/r , where r is the distance from the source charge to that location
    (as explained by Daniel)
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