A current $i$ ampere flows in a circular arc of wire which subtends an angle $\frac{3\pi }{2}$ radian at its center, whose radius is $R.$ The magnetic field $B$ at its center is

Magnetic field at $P$ due to given structure is

Magnetic field at the centre $O$ due to the given structure is

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 outer surface. The magnetic field induction at centre is (Assume that $d\ll a$ )

The magnetic field intensity at the point $O$ of a loop with current $i,$ whose shape is illustrated below is