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!

How to find a charge on a sphere in equilibrium?

  1. Jun 16, 2011 #1
    1. The problem statement, all variables and given/known data

    A small, plastic sphere of mass m = 126 g is attached to a string as shown in the figure.

    There is an electric field of 151 N/C directed along the + x axis. If the string makes an angle 30 with the y axis when the sphere is in equilibrium, what is the charge on the sphere?

    2. Relevant equations

    kakz21.jpg

    3. The attempt at a solution

    mg = 0.126kg * 9.81m/s^2 = 1.24 N

    To make an angle of 30 degrees, the horizontal force must be 1.24 N * sin30 = 0.62 N

    So then i did .62 N/151N/C = .0041 C or 4.1 mC which is the units they want, i entered it but this isnt correct.

    Can someone tell me where i went wrong?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 16, 2011 #2

    vela

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    That's not correct.
    Start by drawing the free-body diagram for the sphere.
     
  4. Jun 16, 2011 #3
    Don't forget about the string! :wink:
     
  5. Jun 16, 2011 #4
    How do i incorporate the string in?!
     
  6. Jun 16, 2011 #5

    vela

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    The tension in the string exerts a force on the sphere, so it's another force in the free-body diagram.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How to find a charge on a sphere in equilibrium?
Loading...