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Homework Help: Two Charges - Electric Potential Question

  1. Jan 19, 2012 #1
    1. The problem statement, all variables and given/known data

    Two charges q = 4.0μ C are fixed in space a distance d = 4.0 cm apart, as shown in the figure. With V = 0 at infinity, what is the electric potential at point C?

    You bring a third charge q = 4.0μC from infinity to C. How much work must you do?

    What is the potential energy U of the three-charge configuration when the third charge is in place?

    Diagram shows (let me use x-y coordinate terms to describe this). Charge q at the origin. Another charge q at (4 cm, 0 cm) and point C at (2 cm, 2 cm)

    2. Relevant equations

    Electric Potential = kQ/r

    3. The attempt at a solution

    That's the right equation for the first part correct? I get 1.27*10^4 for the electric potential for one of the charges on point C. When you double that, you get 2.5 * 10^4 V and that should be the answer? (online homework system says that it is wrong).
    What am I doing wrong here?

    Also I don't understand the second part. If you are moving a charge from infinity, how can you calculate the work? You don't know the distance?
  2. jcsd
  3. Jan 19, 2012 #2


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    Staff Emeritus
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    Hello pierretong. Welcome to PF !

    I see that this is your 2nd post & that you posted your first thread yesterday?

    I hope DocAl's explanation was helpful, and that you were you able to solve your problem.

    As for this thread:
    4 cm = 4× 10-2 m, not 4× 10+2 m.​
    Your answer is in error by some power of 10.
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