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Calculating the magnitude of the electric field homework

  1. Sep 14, 2009 #1
    1. The problem statement, all variables and given/known data
    Consider three charges arranged as shown:

    Q1---------Q2----------Q3

    Q1 has a value of 6.8 uC
    and is 3.5 cm away from O2
    It is also a positive charge

    Q2 has a value of 1.2 uC
    and is 5cm away from O3
    It is also positive

    Q3 has a value of -4.4 uC
    and is negative.

    What is the magnitude of the electric field strength at a point 1.1 cm to the left of the middle charge? The value of the Coulomb constant is K = 8.98755 x 10^9 N*m^2/C^2. Answer in units of N/C.



    2. Relevant equations
    I think that I am going to have to use E = k*Q/r^2 (electric field)


    3. The attempt at a solution

    Well using E = k*Q/r^2 this is what I did. I

    I did K*(6.8 x 10^-6)/(2.4 x ^-2) = 106103020.8
    K*(1.2 x 10^-6)/(1.1 x 10^-2) = 89132727.27
    K*(4.4 x 10^-6)/(6.1 x 10^-2) = 10627578.61
    Final summation = 205863327.4 N/C
    I used the fact that the question was asking for the point charge to be 1.1cm away from the middle charge, so I used that distance to find the other distances in the upper equations. I think that this has to do with superpositioning. The thing is, I am not too sure that if that concept deals with this question. I basically used the electric field formula and applied it to each charge along the x axis. I tried to submit it into my online home and it was spit back out as wrong. Still trying to work on it.
     

    Attached Files:

  2. jcsd
  3. Sep 14, 2009 #2

    ideasrule

    User Avatar
    Homework Helper

    The problem is that you can't add the electric fields like that. Q2, being positive, counteracts the effect of Q1 (they push charges in opposite directions); Q3, being negative, helps Q1 and counteracts Q2. You want to add the fields vectorially, not arithmetically.
     
  4. Sep 16, 2009 #3
    Thanks, solved a little bit ago. Forgot to include the direction.
     
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