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Homework Help: This one has bothered me for a while (electric field)

  1. Oct 14, 2007 #1
    basically i have a small charged sphere on the left, and at a certain horizontal distance away from it theres a bigger sphere that is made of a thin shell of nonconducting material. the bigger sphere (B) is hanging from a non conducting, uncharged thread, and that thread makes an angle of 20 degrees with the vertical when sphere B is in equilibrium. centers of the sphere are at the same vertical height and are 1.5m away from each other.

    the small sphere(A) has a charge of 120microCoulumns, and the larger sphere has radius of .05m and mass of .025kg

    what is the charge on sphere B?

    now i think if i set up an equation where the potential energy of sphere B ( since the thread hanging it is not vertical, B has a vertical displacement caused by the electric field b/t the two spheres) is equal to the electric field generated by the two spheres, i can figure out the charge on B. but i wasnt given the length of the thread, so that prevent me from getting the vertical displacement of sphere B.

    but i wonder why the mass and radius of the sphere B was given tho..
    any clue on where to tackle this problem?
    Last edited: Oct 14, 2007
  2. jcsd
  3. Oct 14, 2007 #2


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    Homework Helper

    I don't understand your method...

    I'd just use forces... what are all the forces acting on the bigger sphere... The sphere is in equilibrium... so sum of forces in the x direction is 0... and sum of forces in the y-direction is 0. From that you can get the electrical force that the smaller charge exerts on the bigger... and from that you can get the charge.
  4. Oct 14, 2007 #3
    well, the force acting on the bigger sphere is just the repulsion force between the two spheres, but how can i find the electric field between them when i only know the charge of the smaller sphere?
  5. Oct 14, 2007 #4


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    Homework Helper

    First find the tension in the cable... use sum of forces in the vertical direction.
  6. Oct 14, 2007 #5


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    Staff Emeritus
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    Gold Member

    That does not account for ALL forces of sphere B.
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