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Homework Help: Electric Field Vector Resulting from Two Point Charges (not getting right answer)

  1. Apr 19, 2015 #1
    1. The problem statement, all variables and given/known data
    A +20nC point charge is placed on the x axis at x = 2.0 m, and a -25 nC point charge is placed on the y axis at y= -3.0 m. What is the angle that the direction of the electric field at the origin makes with the x-axis.

    (a) 209 (deg)
    (b) 61 (deg)
    (c) 29 (deg)
    (d) 241(deg)
    (e) 119 (deg)

    2. Relevant equations

    E = ke(q/r^2)

    3. The attempt at a solution

    E= (8.99x10^9) [ (20x10^-9)/4 (i) - (25x10^-9)/9 (j)]
    = (8.99x10^9) [ 5.0x10^-9 (i) - 2.78x10^-9 (j)]
    = 44.95i - 24.97j
    = 51.42

    The angle between the x-axis is arccos(44.95/51.42) = 29 (deg)

    But the correct answer is 209 (deg).
  2. jcsd
  3. Apr 19, 2015 #2


    User Avatar

    Staff: Mentor

    Check the sign of your first term. Which way does the E field at the origin point when you have a positive charge at x=2m? :smile:
  4. Apr 19, 2015 #3
    Oh makes sense, thank you !:)
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