$A$ 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 $x = a$ on the $X$-axis will be:

The resistances of three parts of a circular loop are as shown in the figure. The magnetic field at the centre $O$ is:

In the hydrogen atom,the electron is making $6.6 \times 10^{15} \, r.p.s.$ If the radius of the orbit is $0.53 \times 10^{-10} \, m,$ then the magnetic field produced at the centre of the orbit is (in $Tesla$):

The figure below shows three circuits consisting of concentric circular arcs and straight radial lines. The center of the circle is shown by the dot. The same current flows through each of the circuits. If $B_1, B_2, B_3$ are the magnitudes of the magnetic field at the center, which of the following is true?

$A$ vertical straight conductor carries a current vertically upwards. $A$ point $P$ lies to the east of it at a small distance and another point $Q$ lies to the west at the same distance. The magnetic field at $P$ is

$A$ coil of $n$ number of turns is wound tightly in the form of a spiral with inner and outer radii $a$ and $b$ respectively. When a current of strength $I$ is passed through the coil,the magnetic field at its centre is