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Electric Force physics question

  1. Aug 28, 2009 #1
    1. The problem statement, all variables and given/known data

    Three positive charges are located in the x-y plane (see plot below), with Q1=3.10 μC, Q2=5.20 μC and Q3=6.30 μC. Note that the charges are located at grid intersections, and that the x and y coordinates are in cm.
    dynamically generated plot
    A. Calculate the magnitude of the electric force on Q1 due to Q2.


    2. Relevant equations
    F=kq1q2/r^2



    3. The attempt at a solution
    Fx=k(3.1*10^-6)(5.2*10^-6)/((4/100)^2)
    Fy=k(3.1*10^-6)(5.2*10^-6)/((3/100)^2)
    Then for the magnitude: sqrt((160.977)^2+(90.54)^2))
    Thank you in advance because I really cannot figure out what I'm doing wrong.
     

    Attached Files:

  2. jcsd
  3. Aug 28, 2009 #2
    Dear GreyGus;
    The solution is just as simple as two steps:
    1- Calculate the distance between the two charges using Pythagoras theory (i.e. r = sqrt((the x component of distance)^2 + (the y component of distance)^2)).
    2- Calculate the electeric force exerted on Q1 by Q2 using coloumb's law
    (F= kQ1Q2/r^2). and hen proceed.

    I hope this answers your question.
     
  4. Aug 28, 2009 #3
    Thank you very much. I appreciate it.
     
  5. Aug 28, 2009 #4
    ok so I did
    x=.04 m
    y=.03m
    3.10 uC=.000003 C
    5.20 uC=.0000052 C
    r=sqrt((.04^2)+(.03^2))=.05
    F=(9*10^29)(.000003)(.000005)/(.05^2)=5.803e21 N but that's wrong. What am I doing wrong now?
     
  6. Aug 29, 2009 #5
    The error would be with the proportionality constant k, it's (8.9*10^9) not (8.9*10^29).
    I hope this makes sense
     
  7. Aug 29, 2009 #6
    Oh I see, Thank you very much for your help.
     
  8. Aug 29, 2009 #7
  9. Aug 30, 2009 #8
    How would you find the x-component of force q1 to q3? I have tried:
    r=.1
    f=k(3.1*10^-6)(5.2*10^-6)/(.1^2)
    tantheta=(.06/.08)=36.9
    x-component=11.61 N. But that's not right. What did I do wrong on this one?
     
  10. Aug 30, 2009 #9

    Doc Al

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    Staff: Mentor

    Make sure you're using the right charge.
     
  11. Aug 30, 2009 #10
    Oh my bad. Ok so I got the answer, but I want to know why is the answer negative?
     
  12. Aug 30, 2009 #11

    Doc Al

    User Avatar

    Staff: Mentor

    The x-component of the force on q3 should not be negative.
     
  13. Sep 1, 2009 #12
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