$A$ long straight wire of circular cross-section of radius $a$ is carrying a steady current. The current is distributed uniformly across the cross-section of the wire. The ratio of the magnetic fields at points $0.5a$ and $1.5a$ from the centre of the wire is

The magnetic induction at the centre $O$ of the current-carrying bent wire shown in the adjoining figure is

Let $B_1$ be the magnitude of the magnetic field at the center of a circular coil of radius $R$ carrying current $I$. Let $B_2$ be the magnitude of the magnetic field at an axial distance $x$ from the center. For $x : R = 3 : 4$,the ratio $\frac{B_2}{B_1}$ 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].

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

In the figure shown,there are two semicircles of radii $r_1$ and $r_2$ in which a current $i$ is flowing. The magnetic induction at the centre $O$ will be