Two identical blocks of charge q each are connected by a massless spring of force constant `k`. They are placed over a smooth horizontal surface. They are released when the separation between them is r and spring is unstretched. If maximum extension of the spring is `r`, the value of `k` is (neglect gravitational effect)

Options

A. `(q^(2))/(4piepsilon_(0)r^(2))`
B. `(q^(2))/(2piepsilon_(0)r^(3))`
C. `(q^(2))/(piepsilon_(0)r^(3))`
D. none of these

Explanation

Conservation of energy gives
`(q^(2))/(4piepsilon_(0)r)=(1)/(2)kr^(2)+(q^(2))/(4piepsilon_(0).2r)rArrk=(q^(2))/(4piepsilon_(0)r^(3))`