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Homework Help: Calculate the charge on the ball

  1. Jan 25, 2005 #1
    I have been working on this problem all night. I am not looking for a numberical answer, just help with the equations. A charged cork ball of mass 1.15 g is suspended on a light string in the presence of a uniform electric field.

    When the electric field has an x component of 3.05E+5 N/C, and a y component of 4.81E+5 N/C, the ball is in equilibrium at q = 35.7o. Calculate the charge on the ball.

    I can not figure out how to find the electric field using the x and y components. One I figure that out I can get the problem.

    Any help would be appreciated.
  2. jcsd
  3. Jan 25, 2005 #2
    there are 2 forces acting on the ball
    1. electrostatic force
    2. gravity
    draw the free body diagram, and realize Fx/Fy=tan35.7
  4. Jan 25, 2005 #3
    I still can't figure it out..I have q=(mgtan)/ E What I can't figure out is E. I know how to work the problem in reverse, when Given E and having to find Fx and Fy. Then finding the Magnitude (E= sqrt(fx^2+Fy^2)).
  5. Jan 25, 2005 #4
    I don't know why you're trying to solve E as sqrt(fx^2+Fy^2)... It's actually very useless to solve the problem that way.

    Why don't you just solve the components of the forces seperately since you are given E for each? Keep in mind that you will just have to keep the charge as a variable...
  6. Jan 25, 2005 #5
    do you mean: Ex=(kq/r^2)cos and Ey=-(kq/r^2)sin...if so I don't know how to get the radius without the length of the string.
  7. Jan 25, 2005 #6
    Ex = Fx/q where q is the charge

    Fx = Ex*q <----- you can use this as the substitute for the Force in x component

    I think you can take it from here...
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