Three equal negative point charges -q are placed at three of the corners of a square of side d as shown in the figure. Which one of the arrows shown represents the direction of the net electric field at the vacant corner of the square?
A) A
B) B
C) C
D) D

Question

Three equal negative point charges -q are placed at three of the corners of a square of side d as shown in the figure. Which one of the arrows shown represents the direction of the net electric field at the vacant corner of the square? 
A) A 
B) B 
C) C 
D) D

Quizplus · Accepted Answer

By symmetry, we can see that the electric field vectors will cancel out along the diagonal that passes through the vacant corner. Thus, the net electric field will be perpendicular to this diagonal. We can eliminate choices A and D since they do not satisfy this condition. Between choices B and C, the direction of the net electric field is towards the vacant corner, so we choose answer B as the best choice.

Three Equal Negative Point Charges -Q Are Placed at Three