A straight section $PQ$ of a circuit lies along the $X$-axis from $x = - \frac{a}{2}$ to $x = \frac{a}{2}$ and carries a steady current $i$. The magnetic field due to the section $PQ$ at a point $X = + a$ will be

As shown in the figure, two infinitely long, identical wires are bent by $90^o$ and placed in such a way that the segments $LP$ and $QM$ are along the $x-$ axis, while segments $PS$ and $QN$ are parallel to the $y-$ axis. If $OP = OQ = 4\, cm$, and the magnitude of the magnetic field at $O$ is $10^{-4}\, T$, and the two wires carry equal current (see figure), the magnitude of the current in each wire and the direction of the magnetic field at $O$ will be $(\mu_ 0 = 4\pi \times10^{-7}\, NA^{-2})$

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

Apply Biot-Savart law to find the magnetic field due to a circular current carrying loop at a point on the axis of the loop.

The magnetic field at the origin due to the current flowing in the wire is -

When equal current is passed through two coils, equal magnetic field is produced at their centres. If the ratio of number of turns in the coils is $8: 15$, then the ratio of their radii will be