Two concentric circular coils of five turns each are situated in the same plane. Their radii are $20 \mathrm{cm}$ and $30 \mathrm{cm}$ and they carry respectively $0.2 \mathrm{A}$ and $0.3 \mathrm{A}$ current in same direction. The magnetic field in $\mathrm{Tesla}$ at the centre is

A circular coil $A$ has a radius $R$ and the current flowing through it is $I$. Another circular coil $B$ has radius $2R$ and current flowing through it is $2I$. The magnetic field at the centre of the circular coil is in the ratio of (i.e., $B_{\mathrm{A}}$to $B_{\mathrm{B}}$ )

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?