A non conducting ring (of mass $m,$ radius $r,$ having charge $Q$) is placed on a rough horizontal surface (in a region with transverse magnetic field). The field is increasing with time at the rate $R$ and coefficient of friction between the surface and the ring is $\mu .$ For ring to remain in equilibrium $\mu$ should be greater than
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For ring to be in equilibrium,
$\mu \mathrm{mgr} \geq \mathrm{Q} \frac{\pi \mathrm{r}^{2} \mathrm{R}}{2 \pi \mathrm{r}} \mathrm{r}$
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