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Homework Help: Electric Charge and Electric Field Problem

  1. Sep 5, 2008 #1
    1. The problem statement, all variables and given/known data

    Two small spheres, each of mass 8.00 g , are suspended by light strings 15.0 cm in length (see figure). A uniform electric field is applied in the x direction. The spheres have charges equal to -6*10^-8 and 6*10^-8 Determine the electric field that enables the spheres to be in equilibrium at an angle of 11 degree.


    2. Relevant equations

    ke = 8.99*10^9
    gravity= 9.8 m/s^2

    ke(q1*q2)/r^2 ?

    3. The attempt at a solution

    I don't know how to approach the problem.
    I don't know how gravity is affecting the problem.
    Can someone guide me through the problem???
    What should I do in order to get the answer?

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  3. Sep 5, 2008 #2


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    Homework Helper

    If you examine the diagram, you'll see that there are 3 forces acting on the system. Two of them, electrostatic attraction between the positive and negative charges and gravity cause the spheres to move closer together, and downwards respectively. The 3rd force, that due to the applied E-field, repels the positive charge and attracts the negative one. Those 3 forces in conjunction create a static equilibrium. Your job is to find the magnitude of the E-field applied.

    Note that you must take into account the E-field due to either the positive or negative charge (unless it's small enough to be negligible).
  4. Nov 5, 2010 #3
    Can someone please help with this question :) I am confused as to what to do as well :(

  5. Nov 6, 2010 #4


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Have you drawn free-body diagrams showing the forces on each sphere? You'll need to show an attempt at solving the problem before we can help you out more.
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