The magnetic field at the centre of a circular coil of radius $r$,through which a current $I$ flows,is:

Two concentric coils each of radius equal to $4 \pi \text{ cm}$ are placed at right angles to each other. If $10 \text{ A}$ and $24 \text{ A}$ are the currents flowing through the coils,respectively,the magnetic induction at the center of the coils will be

Find the magnetic field due to a semi-infinite length wire at point $P$ as shown in the figure.

$A$ hairpin-like shape as shown in the figure is made by bending a long current-carrying wire. What is the magnitude of the magnetic field at point $P$,which lies at the center of the semicircle?

Three long straight and parallel wires carrying currents are arranged as shown. The wire $C$ which carries a current of $50 \,A$ is so placed that it experiences no force. The distance of wire $C$ from wire $A$ is (in $\,cm$)

Let $B_1$ be the magnitude of the magnetic field at the center of a circular coil of radius $R$ carrying current $I$. Let $B_2$ be the magnitude of the magnetic field at an axial distance $x$ from the center. For $x : R = 3 : 4$,the ratio $\frac{B_2}{B_1}$ is: