$A$ long straight wire of radius $a$ carries a steady current $I$. The current is uniformly distributed across its cross-section. The ratio of the magnetic field at $\frac{a}{2}$ and $2a$ from the axis of the wire is:

The fractional change in the magnetic field intensity at a distance $r$ from the centre on the axis of a current-carrying coil of radius $a$ to the magnetic field intensity at the centre of the same coil is: (Take $r << a$)

$A$ current of $i$ ampere is flowing through each of the bent wires as shown. Find the magnitude of the magnetic field at $O$.

$A$ charge $q$ $C$ moves in a circle at $n$ revolutions per second and the radius of the circle is $r$ $m$. The magnetic field at the centre of the circle is:

The resistances of three parts of a circular loop are as shown in the figure. The magnetic field at the centre $O$ is:

$A$ long solenoid has $100 \, \text{turns/m}$ and carries current $i$. An electron moves within the solenoid in a circle of radius $2.30 \, \text{cm}$ perpendicular to the solenoid axis. The speed of the electron is $0.046 \, c$ ($c = 3 \times 10^8 \, \text{m/s}$ is the speed of light). Find the current $i$ in the solenoid (approximate). (in $ \text{A}$)