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Homework Help: Charged mass in electric field

  1. Sep 7, 2009 #1
    1. The problem statement, all variables and given/known data

    a ball with a mass of 0.450 g and positive charge of q=31.1 μC is suspended on a string of negligible mass in a uniform electric field. We observe that the ball hangs at an angle of θ=19.0o from the vertical. What is the magnitude of the electric field?
    2. Relevant equations

    Force of y component=massxGravity
    x component= Force of y component Tan angle

    3. The attempt at a solution
    converted mass to kg.
    F= .00045 X 6.67E-11= 3.00015E-14
    3.00015E-14 Tan 19= 1.034E-14
    convert to Coulombs.

    but answer in incorrect any suggestions what I am doing wrong?
    I tried doing it without converting to kg and got the answer 3.32E-7 and that was wrong as well.
  2. jcsd
  3. Sep 7, 2009 #2
    what's the direction of electric field?
  4. Sep 7, 2009 #3
    I don't think that's relevant here.

    First, draw a free body diagram. Then calculate the force in the vertical direction. Now, that you know an angle and a force vector, you can calculate the other force vector (in the horizontal direction) using trig. Then, use the equation F = Eq; You know F, and you know q, solve for E.
  5. Sep 8, 2009 #4
    There you've assumed that the electric field is HORIZONTAL, so you've given its direction without knowing it as data (i can guess that's what the problem posted by lala78 means but we just can't be sure until lala checks and confirms it).
  6. Sep 8, 2009 #5


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    Homework Helper

    F= .00045 X 6.67E-11= 3.00015E-14
    This step is wrong.
    F = m*g, where g = 9.8 m/s^2
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