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Electric Field & Coulomb's Law Problem

  1. Feb 10, 2007 #1
    1. The problem statement, all variables and given/known data
    A small 2g plastic ball is suspended by a 20 cm long string in a uniform electric field. If the ball is in equilibrium when the string makes a 15 degree angle with the vertical as indicated, what is the net charge on the ball?

    Here's the diagram:

    2. Relevant equations
    Coulumb's Law of electric force equation. F=kq1q2/r^2
    Electric force equations. F=qE

    3. The attempt at a solution

    First I drew a FBD of the ball and the forces acting on it.


    Did I draw it correctly? I just want to confirm this first. There are only 2 x direction forces acting on it correct? The F_e is equal to q*E?? E being the electric force?

    So do I just make T_x=qE and solve for the q? I don't get what they mean by net charge on the ball. Thanks! =D
    Last edited: Feb 11, 2007
  2. jcsd
  3. Feb 11, 2007 #2
    Can someone tell me if I'm going in the right direction? I think I know how to do the problem, but I just need to know if I'm missing anything. I also don't get what they want when they say "net charge on the ball." Thanks!:smile: Btw, this is an even problem and has no answer. Therefore, I'm being more cautious and careful on this problem.
  4. Feb 11, 2007 #3
    So are the only (x) direction forces acting on the charge Tx going leftwards and the Electric force? The net charge would be the charge of the ball correct? which would be (q)? The equation I'm using is: F_e+T_x=0

    F_e=qE so, qE+T_x=0 Solving for q=T_x/E Would (q) be the net charge that the problem is asking for? Can someone please confirm my work? thanks!
  5. Feb 11, 2007 #4
    you are on the right track.--now you need to figure out how to determine the force of T_x.
  6. Feb 11, 2007 #5
    Ok, thanks for the reply! I just solve for (q) right? is that the net charge that they want?
  7. Jan 3, 2009 #6
    sir will u please explain how to find electric potential between two charged spherical shells of radius a and b such that b>a
  8. Jan 4, 2009 #7


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    Homework Helper

    Hi, if you're asking a question which is unrelated to the one above it's best to do so in a new thread to avoid "hijacking" it. The above one is a little old, anyway. And in your question you need to make clear if the 2 shells are concentric; ie. sharing the same centre.

    If so, what can you conclude from Gauss law?
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