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New to Electric Field

  1. Sep 1, 2011 #1
    1. The problem statement, all variables and given/known data
    Two objects, 1 and 2, are identical and small enough such that their sizes can be ignored relative to the distance between them, which is 0.200 m. In a vacuum, each object carries a different charge, Q1 and Q2 and they attract each other with a force of 1.2 N. The objects are brought into contact so the net charge is shared equally and then they are returned to their initial positions. Now it is found that the objects repel each other with a force of 1.2 N too. What is the initial charge on each object?

    2. The attempt at a solution

    The problem is that I don't even understand why the net charge is shared equally and then the objects are repelled. But anyways,

    Using Coulomb's law, force is proportional to the product of the charges. And since Q1 = Q2, I got something like this:

    1.20 = (Q1)2/ 4(pi)(permittivity of free space)(0.200)2

    Q1 = 2.31 x 10-6 C. Then I'm stuck.
  2. jcsd
  3. Sep 1, 2011 #2


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    Homework Helper

    Charges always spread out evenly over bodies, which is why they quickly equalise when the balls are touched.

    Initially the charges attracted - meaning they were opposite, and after they repelled meaning they are the same [positive or negative]: of course they are , they will be equal.

    F is proportional to charge 1 * charge 2

    The total charge is the same before and after, like 3 & 3 vs 12 & -6 but their products have the same size.

    Try a fewe numbers then develop some algebra.
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