Question

An electric charge Q is placed at the origin. A charge q is placed at point B and the force on charge q due to charge Q is F. What is the force on charge q if charge q is moved to point A? (A) 4F (B) 2F (C) F/4 (D) F/2

Answer 1

O, A, B, C, D points are equally spaced

Let the spacings be say x,

We know that electric force between two charges say Q and q separated by a distance of r is,

`F_(electric)=(kQq)/(r^2)`

So when q is at B, the separation will be equal to 2x,

So when q is at B, the force is say F_B,

`F_B=(kQq)/((2x)^2)=(kQq)/(4x^2)`

When q is at A, the force is say F_A,

`F_A=(kQq)/(x^2)`

Comparing the two we get,

`F_A=4F_B`

Given

`F_B=F`

So

`F_A=4F`

Result: (A) 4F