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Homework Help: Coulombs's law problem, asked to find an unknown charge.

  1. Sep 7, 2015 #1
    1. The problem statement, all variables and given/known data
    Two small, positively charged spheres have a combined charge of 23.0 × 10-5 C. If each sphere is repelled from the other by an electrostatic force of 0.800 N when the spheres are 2.00 m apart, what is the charge on the sphere with the smaller charge?

    2. Relevant equations
    Coulomb's law F=k q1q2/r2

    3. The attempt at a solution
    im very stuck on this one.

  2. jcsd
  3. Sep 7, 2015 #2
    You do not need to keep indicating they are absolute values since it is clarified in the problem that they are positively charged.

    Since you have two equations about q1 and q2, you can now solve them easily!
  4. Sep 7, 2015 #3


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    You have a good start. Now try to use the fact that the two charges have a combined charge of 23.0 x 10-5C.
    [EDIT: Sorry, C. Lee. You posted while I was still typing my post.]
  5. Sep 7, 2015 #4
    I tried to solve for q1 and q2, and i got q1=0 and q2=23E-5. my answers are true but not the correct solution to this problem.
  6. Sep 7, 2015 #5


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    How did you come up with these figures? If q1 = 0, then the force = 0, since the product of the charges = 0.

    You'll have to post your work
  7. Sep 7, 2015 #6
    I don't get how you ended up there. If q1 = 0, q1*q2 cannot have nonzero value.
  8. Sep 7, 2015 #7
    I'll try again to solve, this are my 2 equations: q1+q2=2.3E-4 and q1q2=3.5605E-10.
    Last edited: Sep 7, 2015
  9. Sep 7, 2015 #8
    im having trouble solving this. this is my first step from the starting equations; q1+(3.5605E-10/q1)=2.3E-4. and then i factor out q1 from the left side of the equation and solve from there.
  10. Sep 7, 2015 #9
    Now you have quadratic equation there, q12+3.56E-10 = 2.3E-4*q1, which you can easily solve.
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