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A sphere problem I can't figure out

  1. Feb 2, 2012 #1
    Hello everyone!

    I'm trying to solve some physic problems for my school and I'm very bad at it. I've only 1 problem left and I would appreciate it if you guys could point me in the right direction. But I wanna say that I'm not from an English speaking country so I'll have to translate the problem, it could be hard but I will try my best.

    So here it is:

    There is a little sphere with the mass 25g and 50nC, it hangs vertically on a thread between two plates and there is 10cm between the plates. Then 131 V are put on the plates so that the sphere pulls(?) to one of the plates. What angle does the thread of the ball (or and the ball) make? (The last sentence has wrong grammar in my own language so it could be wrong but I hope you can understand it).

    -Thanks in advance!
     
  2. jcsd
  3. Feb 2, 2012 #2

    ehild

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    Hi hrappur,

    See attached picture. Is that the problem?

    Show what you think about the problem. What forces act on the ball?

    ehild
     

    Attached Files:

  4. Feb 2, 2012 #3
    Thanks for your reply ehild!
    I'm trying to figure this out but I just can't, if stuff doesn't fit perfectly into formulas I just get so lost :(

    I got some ideas but they are probably stupid. Should I find what E is and than find s to find how long the ball has moved?
     
  5. Feb 2, 2012 #4

    ehild

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    Show what you think.
    Yes, find E first.
    The resultant of the electric force and gravity is opposite to the force of tension in the string. The yellow angles are equal. How is the angle related to the forces?

    ehild
     
  6. Feb 3, 2012 #5
    I can't figure this out :(
     
  7. Feb 3, 2012 #6

    ehild

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    Do you know the electric field?

    ehild
     
  8. Feb 3, 2012 #7
    is it E = k(Q/r^2) = (9*10^9)*((50*10^-9)/(10*10^-2)^2) = 4,5*10^4 N/C?
     
  9. Feb 3, 2012 #8

    ehild

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    It is the electric field around a point charge. Here you need the electric field between the parallel plates. This electric field acts on the charge of little sphere.

    Given the voltage (131 V) between the plates which are 10 cm apart. The field is homogeneous. How are the voltage and electric field related?

    ehild
     
  10. Feb 3, 2012 #9
    So E = V/d and than F = qE? Well I don't know
     
  11. Feb 3, 2012 #10

    NascentOxygen

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    That looks right.
     
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