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Electric Potential Energy Question - Distance Between Charges and Ratio?

  1. Jan 28, 2007 #1
    1. The problem statement, all variables and given/known data

    A positive charge +q1 is located to the left of a negative charge -q2. On a line passing through the two charges, there are two places where the total potential is zero. The first place is between the charges and is 4.00 cm to the left of the negative charge. The second place is 7.00 cm to the right of the negative charge. (a) What is the distance between the charges? (b) Find q1/q2, the ratio of the magnitudes of the charges.

    2. Relevant equations

    Not sure which one to use. the EPE one or kq1/x = kq2/d-x

    3. The attempt at a solution

    Ok...This might be one of the most confusing questions I've ever encountered in Physics! I drew a diagram, but i had no clue what i was doing. I know the equation V = Kq/r is supposed to be used, but i don't know what to substitute in the problem!!! please show me how you do this problem. appreciate all the help i can get! WHAT IS Q1 AND Q2??? my main problem

    Book Answer: I only have part b, which is 3.67
    Last edited: Jan 28, 2007
  2. jcsd
  3. Jan 29, 2007 #2


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    Homework Helper

    The only equation you need is

    [tex]V = k \frac{q}{r}[/tex]

    Some hints to solve the problem:
    1. The total potential is the algebraic sum of the potentials as a result of the two charges.
    2. A negative charge produces a negative potential while a positive charge results in a positive potential at a point (a positive test charge need to be pushed up against a positive charge).
    3. You can eliminate k from the equations due to the fact that the sum is zero in both cases.
    4. You can insert the distances in centimeters in the equations since the conversion factor, [tex]\times 10^{-2}[/tex], can be eliminated due to the fact that sum is zero.
    5. Compare the ratios of the charges!
  4. Mar 4, 2010 #3
    hey uhh can u tell me how u found the answer to part b?
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