A wire of length $L$ carrying a current $I$ is bent into a circle. The magnitude of the magnetic field at the centre of the circle is

If the current flowing in $ABCD$ is $I$. Then the field at $P$ as shown in the figure will be

A coil of radius $200 \mathrm{mm}$ is to produce a field of $0.4 \mathrm{G}$ in its centre with a current of $0.25 \mathrm{A}$. How many turns must be there in the coil?

A current $I$ ampere flows in a circular arc of wire whose radius is $r$, which subtends an angle $\frac{3\pi }{2}$ radian at its centre. The magnetic induction $B$ at the centre is

A closely wound flat circular coil of $25$ turns of wire has diameter of $10 \mathrm{cm}$ and carries a current of $4$ ampere. Determine the flux density at the centre of a coil.

$A$ and $B$  are two concentric circular conductors of centre $O$ and carrying currents $I_1$ and $I_2$ as shown in the figure. If ratio of their radii is $1:2$ and ratio of the flux densities at $\mathrm{O}$ due to $A$ and $B$ is $1:3$ then the value of $\frac{I_1}{I_2}$ is

Two identical coils carrying equal current have a common centre and their planes are at right angles to each other. What is the ratio of magnitudes of the resultant magnetic field and the field due to one coil along its axis?

The ratio of the magnetic field at the centre of a current carrying circular wire and the magnetic field at the centre of a semi-circular coil made from the same length of wire will be

Two infinite length wires carry currents $8 \mathrm{A}$ and $6 \mathrm{A}$ and are placed along $X$ -axis and Y-axis respectively. Magnetic field at a point $\mathrm{P}(0,0, \mathrm{d}) \mathrm{m}$ will be