$A$ current $i$ is flowing in a straight conductor of length $L.$ The magnetic induction at a point on its axis at a distance $\frac{L}{4}$ from its centre will be

$A$ straight long wire is carrying current $I$. The ratio of magnetic field due to this wire at perpendicular distance $2 \text{ cm}$ and $5 \text{ cm}$ respectively from the wire is

The adjoining figure shows a conductor carrying a current $i$. The magnetic field at the origin is

$A$ coil having $N$ turns is wound tightly in the form of a spiral with inner and outer radii $a$ and $b$ respectively. Find the magnetic field at the centre,when a current $I$ passes through the coil.

$A$ coil having $N$ turns carries a current $I$ as shown in the figure. The magnetic field intensity at point $P$ is

Two infinitely long wires each carrying the same current $i$ and pointing in $+y$ direction are placed in the $xy$ plane,at $x=-2 \ cm$ and $x=1 \ cm$. An electron is fired with speed $U$ from the origin making an angle of $+45^{\circ}$ from the $x$-axis. The force on the electron at the instant it is fired is [$B_0$ is the magnitude of the field at the origin due to the wire at $x=1 \ cm$ alone].