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Homework Help: Static Electricity and particles

  1. Jan 31, 2009 #1
    1. The problem statement, all variables and given/known data

    Particles of charge +70, +48, and -80 [tex]\mu[/tex]C are placed in a line, in the previous order. The center one is .35 m from each of the others. Calculate the net force on each charge due to the other two.

    2. Relevant equations

    F=kq1q2/ d2

    3. The attempt at a solution

    I found the force between each but according to my teacher im not doing this correctly could someone please tell me how i should go about doing this problem.
  2. jcsd
  3. Jan 31, 2009 #2


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    Homework Helper

    Draw a diagram. Consider one charge at a time.

    The ones on the end will be .35 and .7 distant from the other 2 respectively.

    Be careful with the signs. Be careful with the units.

    Be careful that you have taken care to note the direction of the force in each equation. (+,-) attracts and same sign repels.

    Since forces are vectors then you must take that into account when you add them by superposition.
  4. Jan 31, 2009 #3
    I thought thats what I was doing but after trying again i got the same wrong answer, I'm so confused about wht i should do with this problem!
  5. Jan 31, 2009 #4


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    Homework Helper

    Show your calculation then for the middle charge.
  6. Feb 1, 2009 #5
    F1= (9x109)(70 x 10-6)(48x10-6)/ .352

    this equals: 246.86

    F3= (9x109)(-80 x 10-6)(48x10-6)/ .352

    this equals: -282.1

    Then i did
    (246.862)+ (-282.12)= Q2

    Q= 374C

    but according to my teacher this is wrong... can you tell me what it is that im doing wrong?
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