Which Position Has Zero Electric Field?

[Kathi201]

Here is the picture I am given

_____1______Q1____2___3___4___Q2_______5

Charges Q1 = +4q and Q2 = -q are placed as shown. Of the five positions indicated by the number line, which one represents where the electric field E is zero?

 

[LowlyPion]

Here is the picture I am given

_____1______Q1____2___3___4___Q2_______5

Charges Q1 = +4q and Q2 = -q are placed as shown. Of the five positions indicated by the number line, which one represents where the electric field E is zero?

The bigger charge is plus. So its field is pointing away from the point Q1.

The E field for Q2 are all pointing toward the point Q2.

At what point are the vectors pointing in opposite direction so as to be able to net to zero? Once you find the points that satisfy that, then which charge is bigger, then you know to pick the point further away from the larger than the smaller because there is a 1/r² relationship with the field strength and distance from the charge.