Two identical long parallel wires carry currents $I_1$ and $I_2$ such that $I_1 > I_2$. When the currents are in the same direction,the magnetic field at a point midway between the wires is $8 \times 10^{-6} \ T$. If the direction of $I_2$ is reversed,the field becomes $3.2 \times 10^{-5} \ T$. The ratio of $I_2$ to $I_1$ is

$A$ long straight wire carries a current of $18 \,A$. The magnitude of the magnetic field at a point $12 \,cm$ from it is

Find the magnetic field due to a semi-infinite length wire at point $P$ as shown in the figure.

The figure shows a square loop $ABCD$ with edge length $a$. The resistance of the wire $ABC$ is $r$ and that of $ADC$ is $2r$. The value of the magnetic field at the centre of the loop is:

In the following figure,the magnitude of the magnetic field at the point $P$ is:

$A$ circular coil of radius $R$ is carrying a current $I_{1}$ in an anticlockwise sense. $A$ long straight wire is carrying a current $I_{2}$ in the negative direction of the $x$-axis. Both are placed in the same plane and the distance between the centre of the coil and the straight wire is $d$. The magnetic field at the centre of the coil will be zero for the value of $d$ equal to: