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Homework Help: Charge on object between two charged plates Problem

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


    The mass of the ball is 40 milligrams, the applied potential difference is 480 V and the plate separation is 6 cm. If the string hangs at an angle of 20deg to the vertical, determine the charge on the pith ball.

    2. Relevant equations
    E = V / d

    E = F / q

    3. The attempt at a solution

    I figured first i should use E = v/d so i did

    E = 480/0.06 = 8000

    Then i found F using trig and vectors

    Cos20 = (40*10^-6) / F

    Thus F = 4.26*10^-4 N

    Then i plugged in my F and E into the equation q = F / E

    q = 4.26*10^-4 N / 8000 = 5.3*10^-9 C

    Which is 5.3 nC

    But the correct answer is supposed to be 18nC

    Forever grateful for assistance.
  2. jcsd
  3. May 1, 2007 #2


    User Avatar
    Homework Helper

    You must consider all forces, including the tension in the string. That will affect where the ball hangs. Draw a free body diagram. Resolve the tension into its components, and sum up the forces in each direction.
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