Why is the electric field negative in this problem?

cros0 · May 20, 2016

Textbook problem 4: two charged particles on an x axis, particle 1 charge -2.00 × 10^-7 C at x = 6.00 cm, particle 2 charge +2.00 × 10^-7 C at x = 21.0 cm; asks for net electric field midway between them in unit-vector notation.

Typed solution from a textbook: fields E1 and E2 at x = 13.5 cm from charges q1 = -q2 = -2.00 × 10^-7 C, each equal to -(3.196 × 10^5 N/C) i-hat, giving net field E_net = -(6.39 × 10^5 N/C) i-hat.

Hello. The first image is the question, the second image is from the solutions manual. I don't understand why these answers are negative. Anyone have a clue?

berkeman · May 20, 2016

cros0 said:

View attachment 101011
View attachment 101012

Hello. The first image is the question, the second image is from the solutions manual. I don't understand why these answers are negative. Anyone have a clue?

Sure. (and Welcome to the PF)

Electric field points from positive charges to negative charges. So the E-field at the first negative charge is pointing inward to that negative charge in all directions. The E-field at the positive charge is pointing away from it in all directions. They ask for the sum of the two E-fields at a point between them, so both components point to the left, which is the negative direction on the horizontal axis.

Does that help?

cros0 · May 20, 2016

Seems reasonable enough, and I feel much better now. Thanks!

Why is the electric field negative in this problem?

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