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Homework Help: I keep getting this wrong but i think im doing it right can some1help

  1. Feb 8, 2010 #1
    i keep getting this question wrong can some1 help


    two point charges located on the x axis: one charge, q1 = -16.0 , is located at x1= -1.650m ; the second charge, q2= 38.5nC , is at the origin x=0. A
    3rd charge is at x=-1.080m and its charge is 45.5nC
    wat is the netforce of the charges acting on charge3?
    (E_o=8.854*10^-12 therefore K=8.99*10^9)

    i know net force on 3 would be to the left and all you have to do is use faradays law twice( Force 1 on 3 and Force 2 on 3) and add those up to get the net force. I keep getting -6.64*10^-6 but its wrong.. can some1help plz

    Fnet= Force 1 on 3+ Force 2 on 3
    =(Kq_1q_3)/D^2(fromx1 to x3) + (Kq_2q_3)D^2(from x2 to x3)

    and yes i converted nC int C (factor of 1*10^-9 rii?)
  2. jcsd
  3. Feb 8, 2010 #2


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    Faraday's Law relates time varying magnetic fields to the electric fields they induce. You are thinking of Coulomb's Law.

    Careful, force is a vector. It has both a magnitude and a direction. What are the directions of F1on3 and F2on3?
  4. Feb 8, 2010 #3
    well, since the charge1 is negative and is on the leftside of charge3, and charge2 is positive and on the rightside of charge 3, charge 1 will exert a pull force and charge 2 will exert a push force.. so the force directions should both be to the left if im not mistaken..ps ty for the reply
  5. Feb 8, 2010 #4


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    Correct, so if you say that the positive direction is to the right (positive x-direction), then you would expect both F1on3 and F2on3 to be negative, correct?

    Is that what you are getting when you calculate them?
  6. Feb 8, 2010 #5
    in terms of magnitude i get -0.000020143 and postive 0.000013501 if u consider the vecetors u would get postive 6.64*10^-6....im doing this for an online test at uft and ive treid the negative value for that and its wrong..i have only one chance left to answer it and the question says your answer can be either negative or postive so im guessing my maginitude is wrong. soo... do u think im correct?
  7. Feb 8, 2010 #6


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    How are you getting a negative magnitude for the first one? Magnitudes of vectors are always positive by definition.
  8. Feb 8, 2010 #7
    check my numbers plz:

    i beleieve it comes out to be negative becuase charge1 is given as -16nC
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