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Homework Help: Find the points where electric potential is zero?

  1. May 19, 2014 #1
    There are two charges 2 C and 3 C and 100 cm apart from each other.
    Find a point where electric potential is zero? Consider 2 C charge on the origin.

    I have tried out but don't get it solution...
    1) If you take point between two charges then

    2 C----------100 cm------------------3 C

    then you will get x= -200 cm that is not possible?

    2) If you take point left side of the 2 C then

    x= -40 cm that is not possible?

    3) If you take point right side of the 3 C then

    x = -60 cm that is also not possible?
  2. jcsd
  3. May 19, 2014 #2


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    That is because there is no solution. Potential at a point can be defined as the work needed to bring a unit charge from infinity to that point. As long as the 2C and 3C charges both have the same sign, the path from infinity to any finite point will be uphill against a non-zero electrical force all (or most) of the way.

    So all finite points will have positive potential.

    A more rigorous way to see it is that the potential field from the 2C charge is clearly positive everywhere. And the potential field from the 3C charge is also clearly positive everywhere. The combined field from both is just the sum. It must also be positive everywhere.
    Last edited: May 19, 2014
  4. May 19, 2014 #3
    Suppose there are 3C and -2C charges then combined charge is 1C. this is positive charge then also you get the zero potential position.
  5. May 19, 2014 #4


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    The equations that you solved the first time around need to take account of the sign of the two charges. In this version of the problem (with opposite signs on the two charges) the potential gets very large and positive near the 3C charge and verly large and negative near the -2C charge. It follows that there must a zero between them.

    There should be another solution that is not between the two charges.
  6. May 19, 2014 #5


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    Staff: Mentor

    Are you sure the question didn't ask that you find where the electric field is zero?
  7. May 19, 2014 #6

    rude man

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    Gold Member

    Or 2C and -3C or 3C and -2C?
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