In: Physics
Three particles are fixed on an x axis. Particle 1 of charge q1 is at x = -a and particle 2 of charge q2 is at x = +a. If their net electrostatic force on particle 3 of charge Q is to be zero, what must be the ratio q1/q2 when particle 3 is at (a) x = +0.754a and (b) x = +1.65a?
To start with we are considering that the charge Q is at location x from the origin. The force on Q due to charge q1 located at x=-a is F1 and can be calculated as

Both are positively charges so they will repel each other so the direction of the force is towards the +x axis.
The force on Q due to charge q2 located at x=+a is F2 and can be calculated as

Again both are positively charged so they will repel each other so force will be along -x axis or in the backward direction.
these two forces are in opposite direction so the resultant force on Q charge is

It has to be zero so


(a) If x=+0.754 a then

The ratio of the charges q1/q2=50.84
(b) When x=+1.65 a then the force on Q due to q1 and q2 will be in the same directions and will be directed forward so in this case the net force FR will be

The resultant force has to be zero so


Putting x=+1.65 a in the above we get

The ratio of the charges in this case q1/q2=-16.62