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Homework Help: Two charges and electric field?

  1. Apr 14, 2015 #1
    1. The problem statement, all variables and given/known data
    Two charges are placed on the x axis. One charge (q1= +8.5 micro coulombs) is at x1 = +3.0cm and the other (q2 = -21 micro coulombs) is at x1 = +9.0cm. Find the net electric field (magnitude and direction) at (a) x = 0cm and (b) x = +6.0cm

    2. Relevant equations
    E = kq1/r^2

    3. The attempt at a solution
    (9x10^9 Nm^2/c^2)(8.5x10^-6 c) / (0.03m)^2 + (9x10^9 Nm^2/c^2)((-21)x10^-6 c) / (0.09m)^2 =

    8.5x10^7 N/c + -2.3x10^7 N/c =

    6.2x10^7 N/c

    Answer key says -6.2x10^7 N/c, I can't figure out why the electric field is negative.
  2. jcsd
  3. Apr 14, 2015 #2


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    Staff Emeritus
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    Please state coulomb's Law as it was given to you in class, or in your textbook?
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