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Electric Field Question (Very Difficult)

  1. Jan 24, 2007 #1
    1. The problem statement, all variables and given/known data

    While you read this question look at this diagram http://i3.tinypic.com/3zho01t.jpg...

    A small plastic ball with a mass of 6.50 x 10^-3 kg and with a charge of +0.150 uC is suspended from an insulating thread and hangs between the plates of a capacitor (see the drawing). the ball is in equilibrium, with the thread making an angle of 30.0 degrees with respect to the vertical. the area of each plate is 0.0150 m^2. what is the magnitude of the charge on each plate?

    3. The attempt at a solution

    I tried to find q using Q = mg/E, but it ended up in a mess since I was looking for the magnitude. The area is throwing me off...I don't know how to utilize it. Honestly, this is a very tough question! would love all the help i can get. i give up after 2 days!

    Answer from the textbook is 3.25 x 10^-8 C
    Last edited: Jan 24, 2007
  2. jcsd
  3. Jan 25, 2007 #2
    upping! still waiting for some help.
  4. Jan 25, 2007 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Start by analyzing all the forces acting on the ball. Hint: Three forces act on the ball. Then apply the conditions for equilibrium--net force equals zero--to solve for the electric field. Hint: Write equations for vertical and horizontal force components and then combine them.
    Once you've found the electric field, use what you know about capacitors to figure out the charge on each plate needed to produce that field. That's when the area will come in handy.
  5. Jan 25, 2007 #4
    ^Awesome. i appreciate the help doc.

    i used your advice and it worked out just fine...i got the answer i was lookin for! that hint sparked something in my mind...
    Last edited: Jan 25, 2007
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