A point charge `q` is situated at a distance `r` from one end of a thin conduction rod of length `L` having a charge `Q` (uniformly distributed a long its length).find the magnitudes of electric force between the two.

Options

A. `pifQl^(2)`
B. `(pifQl^(2))/3`
C. `(2pifQl^(2))/3`
D. `2piRQ^2`

Answer

B

Explanation

Charge on the differential element `dx,dq=Q/l.dx`
equivalent current `di=f dq`
`:'` magnetic moment of this element `dmu=(di)Na , (N=1)`
`dmu=(pix^(2))fQ/ldx`
`rArr (pifQ)/lunderset(0)overset(l)int x^(2)dx, mu=1/3pifQl^(2)`