Infinite number of straight wires each carrying current $I$ are equally placed as shown in the figure. Adjacent wires have current in opposite direction. Net magnetic field at point $P$ is
$\frac{{{\mu _0}I}}{{4\pi }}\,\,\frac{{\ln \,2}}{{\sqrt 3 \,a}}\hat k$
$\frac{{{\mu _0}I}}{{4\pi }}\,\,\frac{{\ln \,4}}{{\sqrt 3 \,a}}\hat k$
$\frac{{{\mu _0}I}}{{4\pi }}\,\,\frac{{\ln \,4}}{{\sqrt 3 \,a}}( - \hat k)$
Zero
Two concentric circular coils of ten turns each are situated in the same plane. Their radii are $20$ and $40\, cm$ and they carry respectively $0.2$ and $0.3$ $ampere$ current in opposite direction. The magnetic field in $weber/{m^2}$ at the centre is
$A$ and $B$ are two concentric circular conductors of centre $O$ and carrying currents ${i_1}$ and ${i_2}$ as shown in the adjacent figure. If ratio of their radii is $1 : 2$ and ratio of the flux densities at $O$ due to $A$ and $B$ is $1 : 3$, then the value of ${i_1}/{i_2}$ is
Charge $q$ is uniformly spread on a thin ring of radius $R.$ The ring rotates about its axis with a uniform frequency $f\, Hz.$ The magnitude of magnetic induction at the center of the ring is
The magnetic field near a current carrying conductor is given by
$PQRS$ is a square loop made of uniform conducting wire the current enters the loop at $P$ and leaves at $S$. Then the magnetic field will be