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A particle of charge $-q$ and mass $m$ moves in a circle of radius $r$ around an infinitely long line charge of linear density $+\lambda$. Then time period will be given as
(Consider $k$ as Coulomb's constant)
$\mathrm{T}^2=\frac{4 \pi^2 \mathrm{~m}}{2 \mathrm{k} \lambda \mathrm{q}} \mathrm{r}^3$
$T=2 \pi r \sqrt{\frac{m}{2 k \lambda q}}$
$\mathrm{T}=\frac{1}{2 \pi \mathrm{r}} \sqrt{\frac{\mathrm{m}}{2 \mathrm{k} \lambda \mathrm{q}}}$
$\mathrm{T}=\frac{1}{2 \pi} \sqrt{\frac{2 \mathrm{k} \lambda \mathrm{q}}{\mathrm{m}}}$
Solution
$\frac{2 \mathrm{k} \lambda \mathrm{q}}{\mathrm{r}}=\mathrm{m} \omega^2 \mathrm{r}$
$\omega^2=\frac{2 \mathrm{k} \lambda \mathrm{q}}{\mathrm{mr}^2}$
$\left(\frac{2 \pi}{\mathrm{T}}\right)^2=\frac{2 \mathrm{k} \lambda \mathrm{q}}{\mathrm{mr}^2}$
$\mathrm{~T}=2 \pi \mathrm{r} \sqrt{\frac{\mathrm{m}}{2 \mathrm{k} \lambda \mathrm{q}}}$
