A wire $A$, bent in the shape of an arc of a circle, carrying a current of $2\, A$ and having radius $2\, cm$ and another wire $B ,$ also bent in the shape of arc of a circle, carrying a current of $3\, A$ and having radius of $4\, cm ,$ are placed as shown in the figure. The ratio of the magnetic fields due to the wires $A$ and $B$ at the common centre $O$ is

Assertion : A charge, whether stationary or in motion produces a magnetic field around it.

Reason : Moving charges produce only electric field in the surrounding space.

A current $i$ ampere flows in a circular arc of wire whose radius is $R$, which subtend an angle $3\pi /2$ radian at its centre. The magnetic induction at the centre is

A circular coil is in $y-z$ plane with centre at the origin. The coil is carrying a constant current. Assuming direction of magnetic field at $x = -25\, cm$ to be positive direction of magnetic field, which of the following graphs shows variation of magnetic field along $x-$ axis

The magnetic field at the origin due to the current flowing in the wire is

A Helmholtz coil has pair of loops, each with $N$ turns and radius $R$. They are placed coaxially at distance $R$ and the same current $I$ flows through the loops in the same direction. The magnitude of magnetic field at $P$, midway between the centres $A$ and $C$, is given by (Refer to figure)