A conducting wire is in the shape of a regular hexagon which is inscribed inside an imaginary circle of radius $R$. If a current $I$ flows the wire, the magnitude of magnetic field at the centre of the circle is

A coil having 100 turns is wound tightly in the form of a spiral with inner and outer radii 1 cm and 2 cm , respectively. When a current 1 A passes through the coil, the magnetic field at the centre of the coil is
Let $m$ and $r$ are the dipole moment and radius of earth respectively. Then, the earth's magnetic field at the equator is
A circular coil consists 70 closely wound turns and has a radius of 10 cm . An externally produced magnetic field of magnitude $2 \times 10^{-3} \mathrm{~T}$ is applied perpendicular to the coil. The net flux through the coil is found to vanish when the current in the coil is 2.2 A . The inductance of the coil is
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