At which of the labeled points on the x–axis is the electric field zero?

Question

Particles of charge Q and –4Q are located on the x–axis as shown in the figure above. Assume the particles are isolated from all other charges.

At which of the labeled points on the x–axis is the electric field zero?

(A) A 

(B) B 

(C) C 

(D) D 

(E) E

Answer 1

The correct option is: (A) A 

Explanation:

Where E is zero must be closer to the smaller charge to make up for the weaker field. The vectors point in opposite directions outside the two opposite charges. These two criteria eliminate 3 of the choices. For the magnitudes of the electric fields to be zero the ratios Q/r² must be equal giving (in units along the x axis) Q/r² = 4Q/(r + 4 units)² giving r = 4 units