Two fixed point charges `+4e` and `+e` units are separated by a distance a. Where should a third point charge be placed for it to be in equilibrium?

Answer

`2/3a`

Explanation

The third charge (q) is in equilibrium only when
`(k(4e)q)/(x^2)=(k(e )q)/((a-x)^(2))`

On solving, we get `x=(2)/(3)a`.