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Homework Help: 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!!!
     
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