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Homework Help: Forces with electricity

  1. Jan 19, 2008 #1
    1. The problem statement, all variables and given/known data

    An electric field 3.00×10^5 N/C causes the ball in the figure to hang at an angle. What is the angle


    2. Relevant equations

    F_g = mg

    3. The attempt at a solution

    Here's a free body diagram that I drew in paint:

    http://img339.imageshack.us/img339/6721/fbdnc7.jpg [Broken]

    I got f_g = mg = (0.002)(9.8) = 0.0196N = T_y

    so would T_x be equal to E?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 19, 2008 #2


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    Homework Helper

    The force exerted by the electric field on the 25nC ball is equal to T_x. Use F=EQ to find the electric force exerted on the ball.
  4. Jan 19, 2008 #3

    F = EQ = (3x10^5)(25x10^-9) = 0.0075 N

    then I just use tangent

    tan[tex]\vartheta[/tex] = [tex]\frac{T_y}{T_x}[/tex] = [tex]\frac{0.0196}{0.0075 }[/tex] = 2.61

    Does anyone see a mistake somewhere I made?
  5. Jan 19, 2008 #4


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    Homework Helper

    Check your trig. You've got Tx and Ty flipped.

    Don't forget to actually calculate the angle.
  6. Jan 19, 2008 #5
    the 2.61 is after I used arctangent.

    So let's try this one more time

    tan[tex]\vartheta[/tex] = [tex]\frac{T_x}{T_y}[/tex] = [tex]\frac{0.0075}{0.0196}[/tex] = 20.9 (after arctangent was used).

    This sounds more reasonable.
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