The magnetic field near a current-carrying conductor is given by

$A$ length $L$ of wire carries a steady current $I$. It is bent first to form a circular plane coil of one turn. The same length is now bent more sharply to give a double loop of smaller radius. The magnetic field at the centre caused by the same current is

$A$ vertical wire kept in the $Z-X$ plane carries a current from $Q$ to $P$ (see figure). The magnetic field due to the current will have the direction at the origin $O$ along

Two protons $A$ and $B$ move parallel to the $x$-axis in opposite directions with equal speeds $v$. At the instant shown,the ratio of magnetic force and electric force acting on the proton $A$ is ($c=$ speed of light in vacuum).

The ratio of the magnetic field at the centre of a current-carrying coil of radius $a$ to the magnetic field at a distance $a$ from the centre of the coil along its axis is

Two very long straight conductors (wires) are set parallel to each other. Each carries a current $I$ in the same direction and the separation between them is $2r$. The intensity of the magnetic field at point $P$ (as shown in the figure) ($\mu_0=$ permeability of free space) is