Two concentric thin circular rings of radii 50 cm and 40 cm , each carry a current of 3.5 A in opposite directions. If the two rings are coplanar, the net magnetic field due to the rings at their centre is

In hydrogen atom an electron is making $6.6 \times 10^{15} \mathrm{rev} / \mathrm{s}$ around the nucleus of radius $0.47 $$\mathop A\limits^o $. The magnetic field induction produced at the centre of the orbit is nearly.

When two coaxial coils having same current in same direction are brought to each other, then the value of current in both the coils

A wire shaped in a regular hexagon of side 2 cm carries a current of 4 A . The magnetic field at the centre of hexagon is