What is the magnetic field at a distance $R$ from a coil of radius $r$ carrying current $I$ along its axis?

Two concentric circular coils $A$ and $B$ have radii $20 \text{ cm}$ and $10 \text{ cm}$ respectively and lie in the same plane. The current in coil $A$ is $0.5 \text{ A}$ in the anticlockwise direction. What is the current in coil $B$ such that the net magnetic field at the common centre is zero?

An equilateral triangle is made by uniform wires $AB, BC, CA$. A current $I$ enters at $A$ and leaves from the midpoint of $BC$. If the length of each side of the triangle is $L$, the magnetic field $B$ at the centroid $O$ of the triangle is:

The magnetic field at the centre of a coil of $n$ turns,bent in the form of a square of side $2l$,carrying current $i$,is

The magnetic induction at the centre of a current-carrying circular coil of radius $10 \, cm$ is $5\sqrt{5}$ times the magnetic induction at a point on its axis. The distance of the point from the centre of the coil (in $cm$) is

Describe and illustrate the magnetic field lines produced by a current-carrying circular loop.