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Sphere Charge Hanging on Thread (Electric Field Question)

  1. Dec 19, 2011 #1
    1. The problem statement, all variables and given/known data
    Charge 1 (0.15 kg) in the following diagram is hanging from a light insulating thread, as shown in diagram (red is what I have added myself) diagram2.png
    Charge 2 is brought close and pulls charge 1 , deflecting the string away from the vertical axis by 40 degrees. The charges have the same magnitude, charge 1 is negative.

    A) Find the tension in the thread
    B) Find the charge on 2.

    2. Relevant equations
    Fg=mg
    Fe=Kq2/r2


    3. The attempt at a solution

    Vertical

    Fnet=0
    0=FtCos40 + (-mg)
    Ft=mg/cos40
    Ft=1.92

    Horizontal

    Fnet=0
    0=Fe+(-Ftsin40)
    0=kq2/r2+(-1.92sin40)
    q=√(1.92sin40)(0.42)/(9x109)
    q=+/- 4.7x10-6C

    How does this look?

    diagram3.png
     
  2. jcsd
  3. Dec 20, 2011 #2

    Simon Bridge

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    Is there anything about that which concerns you?
    How could you work out for yourself if you got the right answer?

    FWIW: without crunching the numbers, your reasoning looks sound.
    Comment the actual paper you hand in.
     
  4. Dec 21, 2011 #3

    ehild

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    the answer q=+/- 4.7x10-6 C is not correct. Read the problem again. It says
    Do the charges attract or repel? What is charge 2 then?

    ehild
     
  5. Dec 21, 2011 #4
    Well I wrote +/- because it was a square root, the charge is actually positive I know because it attracts charge 1

    Thank you for the input however I feel more confident with my physics knowledge everyday !
     
  6. Dec 21, 2011 #5

    ehild

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    Then it was correct but it was not the solution yet.

    It was a nice solution. I especially liked the picture. You can be confident in your knowledge! :smile:

    ehild
     
  7. Dec 21, 2011 #6

    Simon Bridge

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    It is common for the math to give a choice of solutions - you pick the one that makes sense in terms of the physics.
     
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