The magnetic induction at a point $P$ which is distant $4 \, cm$ from a long current-carrying wire is $10^{-8} \, T$. The magnetic field induction at a distance $12 \, cm$ from the same current would be:

$A$ very long wire carrying a current $I = 4 \sqrt{2} \, A$ is bent at a right angle. The magnitude of the magnetic field $(|B|)$ at a point $P$ lying on a line perpendicular to the bent wire at a distance $d = 20 \, cm$ from the point of the bending will be (Let $\mu_0 = 4 \pi \times 10^{-7} \, H/m$): (in $\, \mu T$)

$A$ current $I$ is flowing in a conductor $PQRST$ as shown in the figure. The radius of the curved path $QRS$ is $r$ and the length of the straight portions $PQ$ and $ST$ is very large. The magnetic field at the centre $O$ of the curved part is $(\mu_0 = \text{permeability of free space})$

$A$ wire of resistance $R$ is bent in the form of a square of side $a$ as shown in the figure. Find the magnetic induction at the center of the square $O$ due to the current flowing through it.

Five long wires $A, B, C, D$ and $E$,each carrying current $I$ are arranged to form edges of a pentagonal prism as shown in the figure. Each carries current out of the plane of the paper.
$(a)$ What will be the magnetic induction at a point on the axis $O$? The axis is at a distance $R$ from each wire.
$(b)$ What will be the field if the current in one of the wires (say $A$) is switched off?
$(c)$ What if the current in one of the wires (say $A$) is reversed?

Two similar coils each of radius $R$ are lying concentrically with their planes at right angles to each other. The currents flowing in them are $I$ and $2I$. The resultant magnetic field of induction at the centre will be ($\mu_0 =$ Permeability of vacuum).