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Homework Help: Find the net electric field these charges produce

  1. Sep 8, 2013 #1
    1. The problem statement, all variables and given/known data
    Two point charges q1 = -6.15 nC, and q2 = -10.5 nC are separated by 25.0 cm (see figure below).

    (a) Find the net electric field these charges produce at point A

    (b) Find the net electric field these charges produce at point B

    (c) What would be the magnitude and direction of the electric force this combination of charges would produce on a proton at A?

    2. Relevant equations

    3. The attempt at a solution

    I got the correct answer for (b), but got an incorrect answer for (a) using the same method.

    E = (ke*Q)/(r2)
    ƩE = 9*109 * [(6.15*10-9)/(0.152) + (10.5*10-9)/(0.102)] = 11910 N/C to the right
  2. jcsd
  3. Sep 8, 2013 #2


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    The electric field is a vector quantity, it has direction. What are the directions of the electric fields due to the individual charges at point (a)?

  4. Sep 8, 2013 #3
    Ah, q1 is pointing to the right and q2 to the left, so I subtract the q2 field from the q1 field.

    One question: Do I include the +/- charges in my equation? ie. would my values for q1 and q2 in the equation be negative since they are both negative charges?
    I got the write answer by just writing the absolute value of my (negative) answer.

    edit: completed part (c) with no problems.
  5. Sep 9, 2013 #4


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    Yes, you need to include the sign of the charge. In case of a negative charge, the electric field points toward the charge. If it is positive, the electric field points away from it. So the electric field due to q1 points to the left, that of q2, points to the right. As the field due to q2 is of greater magnitude, the net electric field points to the right, it is positive.

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