Question

As shown in Figure 3, Q1 and Q2 point charges are placed at (-1, 0) and (1, 0) points on the x-axis.
a) Find the total force on the Q3(y,0) point charge placed on the y-axis.
b) Find the value of y coordinate of the Q3 charge where the total force on it is the maximum.

Answer 1

all charges are positive ?

Answer 2

arent stated in the figure. see my attach picture above. let's assume it as positive?

Answer 3

Okay whatever, If Q1 and Q2 are of same sign then the force is maximum at Y-coordinate = +infinity or - infinity
If they are of opposite sign then the force is maximum at orgin

Answer 4

To answer part "a" we should know about the sign of charges

Answer 5

thanks for the reply. Can you explain more about part b? Why would same-sign/opposite sign effect the force? Should we consider the +/- sign of the third charge, Q3 as well?

Answer 6

Do you know that Like charges repel and Unlike charges attract ?

So if they are of same sign and say the middle charge is negetive, so both try to attract the cetre charge. Their horizontal components are in opposite directions and vertical components are in same direction. So the vertical components will increase as we move to infinity.
So if they are of opposite sign and say the middle charge is negetive, one try to attract the cetre charge and the other try to repel. Their horizontal components are in same directions and vertical components are in opposite direction. So the vertical horizontal will increase as we move to origin.