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Attempting to find tension force with electric fields

  1. Feb 18, 2013 #1
    1. The problem statement, all variables and given/known data
    A small conducting sphere of mass 5.0x10^-3 kg attached to a string of length 20 cm, is at rest in a uniform electric field E. There is a charge of -5.0x10^-6 coulomb on the sphere. The string makes an angle of 30° with the vertical. Calculate the tension in the string. Calculate the magnitude of the electric field.


    2. Relevant equations
    Probably have to use Fm = BIlsin° Not sure what else, though.


    3. The attempt at a solution
    Honestly, I have no idea where to start. We haven't really covered this in class yet, and I don't know what to Google to figure it out that way... Help, please?
     
  2. jcsd
  3. Feb 18, 2013 #2

    haruspex

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    Can you draw the free body diagram? What are all the forces on the mass? What do you know about the magnitude and direction of each?
     
  4. Feb 18, 2013 #3
    Okay, I think this is the right free-body diagram. I could be wrong, though. But this is the apparatus set-up we're given.
     

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  5. Feb 19, 2013 #4

    haruspex

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    The OP doesn't say the field is horizontal (so doesn't provide enough info for a solution), but maybe you left that out. So your diagram looks ok. Can you write down the equilibrium equations?
     
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