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Homework Help: Electric Field of ping pong ball of mass

  1. Jul 26, 2010 #1
    1. The problem statement, all variables and given/known data
    A ping pong ball of mass 3.0 x 10^-4 kg hangs from a light thread 1.0m long, between 2 vertical parallel plates 10.0cm apart. When the potential difference across the plates is 420 V, the ball comes to equilibrium 1.0 cm to one side of its original position.
    a) Calculate the electric field strength between the plates.
    b) Calculate the tension in the thread
    c) Calculate the magnitude of the electric force deflecting the ball..
    d) Calculate the charge on the ball.

    2. Relevant equations
    Epsilon = (kq)/r^2
    Electrical Force = (kq1q2)/r^2
    V = (kq)/r

    3. The attempt at a solution
    a)Epsilon = V/r = 420V/0.10m = 4200N/C

    b) Fg = mg = (3.0 x 10^-4 kg)(9.8m/s^2) = 2.94 x 10^-3N

    c)I'm not sure how to do the rest.
  2. jcsd
  3. Jul 26, 2010 #2
    Here's what the diagram looks like.

    Attached Files:

  4. Jul 26, 2010 #3
    b) is wrong.
    Draw a free body diagram with the ball 1cm away from one of the plates, show all the forces acting on the ball.
  5. Jul 27, 2010 #4
    Something like this?

    Attached Files:

  6. Jul 27, 2010 #5
    b) alpha is angel between the thread (when V = 420V) and vertical
    alpha~sin(alpha)=0.01/1 =0.01 (rad)
    the tension in the thread (T):
    Because of equilibrium condition:
    Fg/T = cos(alpha) --> T = Fg/cos(alpha)
    c) the magnitude of the electric force deflecting the ball
    Fe/Fg =tan(alpha)~alpha --> Fe = Fg.0.1
    d) Fe=q.E --> q= Fe/E = Fe.r/V
  7. Jul 27, 2010 #6
    the FBD looks alright.

    Notate the angle between the string & the vertical.

    Then balance the forces acting on the ball.
    That should solve b,c & d. :)
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