$A$ circular coil of wire consisting of $100$ turns,each of radius $8.0 \, cm$,carries a current of $0.40 \, A$. What is the magnitude of the magnetic field $B$ at the centre of the coil?

The magnetic field due to a long straight wire carrying a current $I$ is proportional to:

$A$ long straight metal rod of radius $b$ has a very long hole of radius $a$ drilled parallel to the rod axis as shown in the figure. If the rod carries a total current $i$,find the value of magnetic induction on the axis of the hole,where $OC = c$.

An electric current $I$ enters and leaves a uniform circular wire of radius $r$ through diametrically opposite points. $A$ particle carrying a charge $q$ moves along the axis of the circular wire with speed $v$. What is the magnetic force experienced by the particle when it passes through the centre of the circle?

The magnetic induction at $O$ due to the whole length of the conductor is:

$A$ current loop $ABCD$ is held fixed on the plane of the paper as shown in the figure. The arcs $BC$ (radius $= b$) and $DA$ (radius $= a$) of the loop are joined by two straight wires $AB$ and $CD$. $A$ steady current $I$ is flowing in the loop. The angle made by $AB$ and $CD$ at the origin $O$ is $30^\circ$. Another straight thin wire with steady current $I_1$ flowing out of the plane of the paper is kept at the origin. The magnitude of the magnetic field $(B)$ due to the loop $ABCD$ at the origin $(O)$ is: