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Equivalent Electri Force, Find the charge.

  1. Feb 12, 2013 #1
    1. The problem statement, all variables and given/known data
    Two objects are identical and small enough that their sizes can be ignored relative to the distance between them, which is 0.3 m. In a vacuum, each object carries a different charge, and they attract each other with a force of 2.3 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 one another with a force whose magnitude is equal to that of the initial attractive force. What is the initial charge on each object? (Note: there are two possible pairs of answers, but assume q1 to be the larger number.)


    2. Relevant equations
    F=kq1q2/r2


    3. The attempt at a solution
    So I know that since force is equal Fr2/k is a constant. Also since the charge in the 2nd case is equally shared I can re-write the equation for force as F=kq2/r2. Solving for q gets me 4.8e-6.


    From here since the charge was shared equally between q1 and q2, q=q1+q2/2. Using substitution to solve for q2, I came up with the following quadratic:

    -q12+9.6e-6(q1)-9.6e-6. This results in an invalid quadratic equation because b2-4ac gives me a negative number. I think I made math errors or I'm missing a step, but the underlying concept makes sense.
     
  2. jcsd
  3. Feb 12, 2013 #2

    SammyS

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    Be careful with parentheses. That should be written,
    q = (q1+q2)/2​
    It's likely that the signs of the charges is what's causing your problems.

    The magnitude of the force is the same for both situations, so
    |q1q2| = q2
    but because q1q2 is negative,
    |q1q2| = -q1q2 .​
     
  4. Feb 12, 2013 #3
    ok now I just got a completely wrong answer. My new quadratic equation is q22-9.59e-6+4.8e-6. I got answer of like .00348
     
  5. Feb 12, 2013 #4

    SammyS

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    Coulombs?

    What equations are you putting together to get your answers ?
     
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