Magnetic field $dB$ due to a current element at any point on its axis is . . . . . . .

Which of the following graphs represents the magnetic field $(B)$ versus distance $(r)$ from the centre of a long straight conducting wire of uniform cross-sectional area carrying a steady current $I$ and having radius $a$?

Consider a tightly wound $100$ turn coil of radius $10 \,cm$ carrying a current of $2 \,A$. The magnitude of the magnetic field at the centre of the coil is

If an electron revolves around a nucleus in a circular orbit of radius $R$ with frequency $n$,then the magnetic field produced at the centre of the nucleus will be

Two long parallel wires carrying currents $I_1 = 4 \ A$ and $I_2 = 3 \ A$ in opposite directions are placed at a distance of $d = 5 \ cm$ from each other. $A$ point $P$ is at equidistance from both the wires such that the lines joining the point $P$ to the wires are perpendicular to each other. The magnitude of the magnetic field at point $P$ is ( $\mu_0 = 4 \pi \times 10^{-7} \ T \cdot m/A$ ).

Give the characteristics of magnetic field lines.