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Homework Help: Please check my answer on Coulomb's law

  1. Mar 12, 2015 #1
    1. The problem statement, all variables and given/known data
    Two identical conducting spheres of charges 3q and -q attract each other with a force of 36 x 10-3N. When they are separated by a distance of 0.2 m. The spheres are brought into contact with each other and then separated by a distance of 0.1 m.

    What is the magnitude of the electric force on each one exerts on the other?

    2. Relevant equations

    3. The attempt at a solution
    I just need to make sure that i can get Q1 and Q2 correctly

    So what i tried to do was rearrange the formula to fit the question by doing
    Q1 x Q2 = k x f / d2

    so, 9x109 x 36 x 10-3

    = 324 x 106
    (2 x 10-1)2

    = 324 x 108

    = 162 x 108

    so Q1 = 162 and Q2 is -54

    I think its wrong, please can anyone check
  2. jcsd
  3. Mar 12, 2015 #2


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    Staff: Mentor

    That rearrangement is not correct. Show your steps.
  4. Mar 12, 2015 #3
    What is the formula to find Q1 and Q2

    how should i properly arrange it?
  5. Mar 12, 2015 #4


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    Staff: Mentor

    What formula did you begin with? It should be elementary algebra to rearrange it. So show your steps.
  6. Mar 12, 2015 #5
    i began with F = K x Q1 x Q2 / d2

    since the force is given but the charges were not, i tried to swap them, resulting in

    Q1 x Q2 = K x F / d2

    but it was incorrect
  7. Mar 12, 2015 #6


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    Staff: Mentor

    It's incorrect because your algebra is incorrect.

    You can't just "swap" variables from one side to the other. Operations must be performed identically on each side of an equation to accomplish what you want.

    Start again with the original equation. As a first step, multiply both sides by d2.
  8. Mar 12, 2015 #7
    Alright, Thank you sir.
  9. Mar 12, 2015 #8


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    2017 Award

    Staff: Mentor

    In addition, it is useful to keep the units in the equations. You would have noted the problem then because the units do not match any more.
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