Question

Where a charge q is places so that resultant force is 0 betweem charge q and 4q?

Answer 1

let us consider the distance between the two charges is 'L'

let x be the distance from charge q where the resultant force is zero.

so distance from charge 4q= (L-x)

Using coulomb's law,

force between charge q and q,

F(q)= k q.q/x²

force between charge q and 4q

F(4q)= k q.4q/(L-x)²

equating them we get x=L/3

so the charge q should be kept at L/3 distance from charge q