1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

HELP on electric field problem

  1. May 6, 2009 #1
    1. The problem statement, all variables and given/known data
    A 2g plastic ball is suspended by a 2cm string in a uniform electric field of 1.0 x 10^4 N/C (the arrows point to the right)

    a) is the balls charge positive of negative?
    (B) If the ball is in equilibrium when the string makes a 15 degree angle with the vertical, what is the net charge. (15 degree angle going towards where the E arrows point)


    2. Relevant equations
    E=Ck((q1*q2)/r^2)

    3. The attempt at a solution
    a) i know it is positive
    b) i did work but i can never get the right answer which is 5.3*10^-7C
     
  2. jcsd
  3. May 6, 2009 #2

    LowlyPion

    User Avatar
    Homework Helper

    I'm not sure which direction your suspended object is going, but for b) consider the angle of the string. Won't it be the sum of m*g down and the electrostatic force horizontally? So isn't tan15 then the ratio of the 2 forces?
     
  4. May 6, 2009 #3
    its going right
     
  5. May 6, 2009 #4
    i dont understand. help, pleaseee
     
  6. May 6, 2009 #5

    LowlyPion

    User Avatar
    Homework Helper

    F = q*E

    and

    q*E/(m*g) = tan15

    q = m*g*tan15/E

    Isn't that all there is to it?
     
  7. May 6, 2009 #6

    rl.bhat

    User Avatar
    Homework Helper

    The force on the charged ball due to electric field is Eq, and weight of the ball is mg. The ratio of the forces is Eq/mg = tanθ. Solve for q.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: HELP on electric field problem
Loading...