$A$ long straight wire carries a current along the $x$-axis. Consider the points $A(0, 1, 0)$,$B(0, 1, 1)$,$C(1, 0, 1)$,and $D(1, 1, 1)$. Which of the following pairs of points will have magnetic fields of the same magnitude?

$A$ long curved conductor carries a current $I$. $A$ small current element of length $dl$ on the wire induces a magnetic field at a point away from the current element. If the position vector between the current element and the point is $\vec{r}$, making an angle $\theta$ with the current element, then the induced magnetic field density $d\vec{B}$ at the point is $(\mu_0 = \text{permeability of free space})$:

$A$ length $L$ of wire carries a steady current $I$. It is bent first to form a circular plane coil of one turn. The same length is now bent more sharply to give a double loop of smaller radius. The magnetic field at the centre caused by the same current is

$A$ long insulated copper wire is closely wound as a spiral of $N$ turns. The spiral has inner radius $a$ and outer radius $b$. The spiral lies in the $X-Y$ plane and a steady current $I$ flows through the wire. The $Z$-component of the magnetic field at the center of the spiral is

$1 \ T = \dots \text{Gauss}$.

Describe and illustrate the magnetic field lines produced by a current-carrying circular loop.