Solve this :


12. Alpha particle is revolving around an infinitely long line charge of linear charge density $\mathrm{\lambda }$ at perpendicular distance r from it. The centripetal force on the particle is :

$1. \frac{\mathrm{\lambda e}}{2{\mathrm{\pi \epsilon }}_{\circ }\mathrm{r}} 2. \frac{\mathrm{\lambda e}}{{\mathrm{\pi \epsilon }}_{\circ }\mathrm{r}} 3. \frac{\mathrm{\lambda e}}{4{\mathrm{\pi \epsilon }}_{\circ }\mathrm{r}} 4. \frac{\mathrm{\lambda e}}{{\mathrm{\pi \epsilon }}_{\circ }{\mathrm{r}}^2}$