1. Not finding help here? Sign up for a free 30min 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!

Balls in a magnetic field

  1. Feb 25, 2009 #1
    2 identical small balls hang in a 100, 000 N/C electric field as shown. They have charges of 100 and -100 nC respectively. The string length is 50cm and the angle each makes with the vertical is 10 degrees. Find the mass of the balls.

    http://img514.imageshack.us/img514/633/phees.th.jpg [Broken]


    2. Relevant equations

    F = Eq , F = k|q1||q2|/r^2, F = mg
    k = 9 x 10^9 Nm^2/C^2

    3. The attempt at a solution

    For the left ball :

    Fx = Tsina - Eq - kq^2/(sina^2) = 0
    T = (Eq + kq^2/sina^2)/ sina

    Fy = Tcosa - mg = 0
    m = Tcosa / g

    m =(cosa/g) * (Eq + kq^2/sina^2) / sina

    but when i evaluate this with a = 10 , g = 9.8, E = 100 000, k = 9x10^9, q = 1 x 10^7
    i get m = 7.5 grams

    the answer in the back of the book is 4.1 grams

    where am i going wrong?
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 25, 2009 #2

    LowlyPion

    User Avatar
    Homework Helper

    First of all, its not a magnetic field. Looks like a uniform E-field of 100,000 V/m.

    And it makes a difference.

    Left to themselves the + and - charges on the balls would be attractive, not repulsive. Hence the m*g AND the attractive forces between the balls are being balanced by the attraction of the balls toward their respective potentials.
     
  4. Feb 25, 2009 #3
    ohhhhhhhh thats right , thank you!!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Balls in a magnetic field
Loading...