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Homework Help: Coloumbs Law Find the Force

  1. Jan 26, 2013 #1
    1. The problem statement, all variables and given/known data
    Part 1:

    A charge of 2 µC is at the origin, and a charge
    of 6 µC is on the x axis at x = 1 m.
    Find the force on charge q2. The Colulomb
    constant is 8.98755 × 10^9 N · m2/C2.

    Answer in units of N

    Part 2:

    Find the force on q1.
    Answer in units of N

    2. Relevant equations

    (Kc * Q2 * Q1)/R^2
    Kc = 8.98x10^9

    3. The attempt at a solution

    So basically, for part one, i did..

    (Kc * 2 * 6)/(1^2) = 1.07850E11
    answer is positive so they should be repelling each other, but the program says its wrong, and im not sure where to go for part 2.

  2. jcsd
  3. Jan 26, 2013 #2
    When doing physics always, always mind your units, you should have put
    -(Kc*2E-6*6E-6) / (1^2) = -0.108N

    The second part should be the same answer, if not it will be the answer to Part 1 in its negated form (0.108N) or it will be basically the same equation with one of the point charges removed, i.e.

    (Kc*q2) / (R^2)

    But that is the expression for and electric field of a point charge, so Part 2 should just be the negative answer of part 1.
    Last edited: Jan 26, 2013
  4. Jan 26, 2013 #3
    What i dont understand is, my professor did a similar problem in class, except with 3 charges, and used the units in microcoloumbs and got the right answer? Or should i be doing all the problems in coloumbs instead?
  5. Jan 26, 2013 #4


    User Avatar

    You should convert the charges to coulombs from micro coulombs. You can verify this by paying careful attention to the units, particularly the units of Coulomb's constant. If your teacher used micro coulombs, perhaps he had adjusted the constant beforehand; otherwise he couldn't have gotten the right answer.
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