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Coulomb's law

  1. Aug 20, 2008 #1
    1. The problem statement, all variables and given/known data
    Two small charges q1 = +2.65 nC and q2=-4.75nC are at a certain distance from each other. Q1 is affected by a force of 55μN from q2.
    a) how large is the force on q2? Draw a figure that shows the electric forces that work on q1 and q2.
    b) determine the distance between the charges.

    2. Relevant equations
    a) coulomb's law: F= (k*qi*q2)/d^2
    b) coulomb's law: F= (k*qi*q2)/d^2

    3. The attempt at a solution
    a)Now I know that these questions are generally rather simple and straightforward, but what I cannot understand is how I'm going to solve the problem with two unknowns (the force on q2 and the distance). Because I only have a set of data I cannot really make a simultaneous equation out of it. I tried using E=F/q for q2 but then I only get E for one of the charges.. so how would I go on from there? I already drew a picture showing the two attracting forces with arrows toward each other (as one charge is positive and the other one negative).
    b) Once I find the force on q2, I just add the forces together and make them equal to F in coloumb's law.. right? So then I just rearrange the variables and solve for d?

    I could really use some help, thanks!!
  2. jcsd
  3. Aug 20, 2008 #2
    a) Think of Newton's laws. The answer is really simple when you see it.
  4. Aug 20, 2008 #3
    Oh, you mean for every action there is an equal and opposite reaction? So if q2 works on q1 by 55μN, this means that q1 works on q2 by 55μN? So then in b) I just add them together to make 110μN, which I then plug into coulomb's law:

    110 μN = (k*q1*q2)/d^2
    And solve for d?
  5. Aug 20, 2008 #4


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    No, do not add the forces. Just use 55 uN for the force.

    That equation gives the force F exerted by one charge on the other. They told you this force is 55 uN.
  6. Aug 20, 2008 #5
    Oh, I see! Thanks :)
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