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Homework Help: Determine the electric field on the y axis

  1. Jan 13, 2010 #1
    1. The problem statement, all variables and given/known data
    Two 2.04 µC point charges are located on the x axis. One is at x = 1.02 m, and the other is at x = -1.02 m. Determine the electric field on the y axis at y = 0.470 m.

    2. Relevant equations
    E = k*q/r^2

    3. The attempt at a solution

    First I find the distance from one of the charges to the point on the y-axis.
    [tex]\sqrt(1.02^2 + 0.47^2) = 1.123m[/tex]

    I also find the angle between this line and the y-axis.
    [tex] tan^1 \frac{1.02}{0.47} = 65.26[/tex]

    Then I found the component of the electric field at the point on the axis which lies along the y-axis.

    [tex] E = k_e*\frac{q}{r^2} [/tex]
    [tex] E = 8.99e9 * \frac{2.04\mu C}{1.123m}*cos(65.26) [/tex]

    As this only accounts for one charge, and the other charge is equal in magnitude and position, I double this number to get the answer.

    Howerver, this gives me the wrong answer. Any help would be appreciated.
  2. jcsd
  3. Jan 13, 2010 #2
    The E field due to one of the charges (the one at x=1.02) on the y-axis is given by your equation E=k*q/r^2.
    if you draw a line from your charge to the point on the y-axis, this E vector points in this direction. If your other charge (at x=-1.02) has opposite sign, then the y components of the E vectors cancel each other out and the x-components add. Else, the x-components cancel and the y-components add.

    In either case break the E vector that you found into its x and y components using SOH CAH TOA. Then do the simple algebra depending on the signs of your charges.
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