Surface charge density on a ring of radius $a$ and width $d$ is $\sigma$ as shown in the figure. It rotates with frequency $f$ about its own axis. Assume that the charge is only on the outer surface. The magnetic field induction at the centre is (Assume that $d \ll a$):
- A$\pi \mu_0 f \sigma d$
- B$\mu_0 f \sigma d$
- C$2 \pi \mu_0 f \sigma d$
- D$\frac{\pi^2}{2 \mu_0} f \sigma d$
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