The magnetic field at the centre of a circular coil of radius $r$,through which a current $I$ flows,is:

Two long straight wires are placed along the $x$-axis and $y$-axis. They carry currents $I_1$ and $I_2$ respectively. The equation of the locus of points with zero magnetic induction in the magnetic field produced by them is:

Two identical wires $A$ and $B$,each of length $l$,carry the same current $I$. Wire $A$ is bent into a circle of radius $R$ and wire $B$ is bent to form a square of side $a$. If $B_A$ and $B_B$ are the values of magnetic field at the centres of the circle and square respectively,then the ratio $\frac{B_A}{B_B}$ is

In the figure,two parallel infinitely long current-carrying wires are shown. If the resultant magnetic field at point $A$ is zero,then determine the current $I$ (in $A$).

Magnetic lines of force due to a current in a circular conductor are

$A$ current $I$ is flowing along an infinite,straight wire in the positive $Z$-direction,and the same current $I$ is flowing along a similar parallel wire $5 \ m$ apart in the negative $Z$-direction. $A$ point $P$ is at a perpendicular distance of $3 \ m$ from the first wire and $4 \ m$ from the second. What is the magnitude of the magnetic field $B$ at point $P$?