1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electric Potential of 3 point charges

  1. Feb 5, 2010 #1
    29.P68.jpg
    1. The arrangement of charges shown in the figure is called a linear electric quadrupole. The positive charges are located at +-s. Notice that the net charge is zero.
    Find an expression for the electric potential on the x-axis at distances y>>s.

    2. Relevant equations
    V= [tex]\sum[/tex]1/4pi [tex]\epsilon[/tex]0 * qi/ri


    3. The attempt at a solution
    Im just not sure how to go about modeling this equation.. otherwise I don't know were to start.
    I have tried: 1/4pi[tex]\epsilon[/tex]0 *Q/y thinking that it was a simple question, and I now understand that I should pick a s-value and use that to come up with the radius. However, I am stuck on how.
    Thanks
     
    Last edited: Feb 5, 2010
  2. jcsd
  3. Feb 5, 2010 #2

    CompuChip

    User Avatar
    Science Advisor
    Homework Helper

    Let's start by finding expressions for the ri.

    Can you express them in terms of y and/or s?
     
  4. Feb 5, 2010 #3


    You have the definition of the potential. Say I sit at x=0, y=10. Ignore the constants but not the charge and distance. You should be able to write down the potential at this point, (remember the distance from x=0, y=10 to each charge are 10 + s, 10, 10-s. Your general formula must take into account the fact that the charges are not at the same place.

    V(y=10) = 1/(10 + s) -2/10 + 1/(10 - s)

    In general V(y) = 1/(y + s) - 2/y + 1/(y - s).
     
  5. Feb 6, 2010 #4
    so I change the constants to -s or s depending on which direction on the direction on the y axis? that makes sense.
    What I tried was V = K [tex]\sum[/tex]Q/(y+s) -2Q/(y) + Q/(y-s) , and it said that the answer does not depend on the variable s. Would this have something to do with Q= 2qs^2, being the electric quadrupole moment.
     
    Last edited: Feb 6, 2010
  6. Feb 6, 2010 #5

    CompuChip

    User Avatar
    Science Advisor
    Homework Helper

    You solved the problem for a point on the y-axis, like Spinnor said in his example.

    However, you want to take a point on the x-axis.
    Did you read my post (#2) ?
     
  7. Feb 7, 2010 #6
    ahhhh, okay so now I see so I get:
    K* Q/(y^3)
    which is the answer masteringphysics took!
    Thanks So much!!!
     
  8. Feb 7, 2010 #7

    CompuChip

    User Avatar
    Science Advisor
    Homework Helper

    I assume that you worked this out using the Pythagorean theorem, and found that s cancels out
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Electric Potential of 3 point charges
Loading...