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Homework Help: Electric Field from Point Charges

  1. May 15, 2014 #1
    1. The problem statement, all variables and given/known data
    Two point charges (q1 = -2.6uC and q2 = 8.6uC) are fixed along the x-axis, separated by a distance d = 9.7cm. Point P is located at (x,y) = (d,d)

    1. What is Ex(P), the x-component of the electric field produced by q1 and q2 at point P?
    2. What is Ey(P), the value of the y-component of the electric field produced by q1 and q2 at point P?
    3.A third charge is now positioned along the y axis at distant d from and above q1 on (x,y) = (0,d). What is the Ex(P) of the field produced by all 3 charges at point P?
    4. Suppose all are doubled, how will the electric field change?
    5. How would you change Q1 in order to make the eletric field at point P equal to zero?

    2. Relevant equations
    E = (kq)/r^2
    Exq1 = ((kq)/r^2)cos(45)
    Exq2 = 0
    Eyq1 = ((kq/r^2)sin(45)
    Eyq2 = (kq)/r^2
    C^2 = A^2 + B^2

    Image for 1 and 2

    Image for 3, 4, and 5

    3. The attempt at a solution
    1. First off, I drew the triangle of q1-q2-P and labelled all d values as 0.097m, used E=(kq)/r^2 to find the magnitude of E at point P a distance r = sqrt(2d^2) and then multiply that value by cos(45) to find that Exq1 is equal to -120608.8429 N/C by having q = 2.6e-6 C, k = 8.987551e9 and r = (which apparently is wrong)

    2. Ey = Eyq1 + Eyq2, where Eyq1 = ((kq1)/r^2) * sin45 and Eyq2 = (kq2)/d^2. For Eyq1 I calculated -120451.7144 N/C and for Eyq2 I calculated 8214787.82 N/C. The sum of the two is 8094336.106 N/C which again, is apparently wrong.

    I have not yet attempted 3,4 or 5 because I recognize after the first two I'm clearly doing something wrong in my steps.
  2. jcsd
  3. May 15, 2014 #2


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    First off, you are using way too many significant figures. The data you are given only have 2 significant figures, so your answers should have no more than that.

    Now, looking at part 1, your procedure seems correct, so you should double check your arithmetic. Make sure whatever you are using to calculate the cosine is expecting degrees and not radians.
  4. May 16, 2014 #3
    I realize that the sig figs aren't accurate. I use smartphysics for my homework assignments and it is notorious for not accepting correct sig figs. My problem was that I did not actually do the arithmetic correctly.

    Thank you
  5. May 16, 2014 #4


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    Even if your program or calculator gives you many digits, you should only report those that are significant. As for arithmetic mistakes, we all make them.
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