F =

Sol. The system of the ring charge and line charge may be represented as shown in the figure. Here, the electric field intensity due to the ring charge +q at a point distant x on the axis is given by :

E = .............. (along the axis of ring i.e. along wire)

The force due to electric field of ring charge on a small charge element dq concentrated in small length dx of the line charge is given by

dF = E dq

or, dF = × λ dx

Here, λ = linear charge density of the thread.

so, dF = ×

so, F =

let (R 2 + x 2) = t

so, 2x dx = dt

so, x dx =

so, F =

or, =

or, = –

[because, t = R 2 + x 2 ]

so, F = –

or F =