$A$ 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?

Write the formula for the force per unit length between two parallel,current-carrying conducting wires.

$A$ 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 $(\mu_0 = \text{permeability of free space})$

Do magnetic forces obey Newton's third law? Verify for two current elements $\overrightarrow{dl_1} = dl(\hat{i})$ located at the origin and $\overrightarrow{dl_2} = dl(\hat{j})$ located at $(0, R, 0)$. Both carry current $I$.

The figure shows the cross-section of a long cylindrical conductor through which an axial hole of radius $r$ is drilled with its centre at point $A$. $O$ is the centre of the conductor. If an identical hole were to be drilled centred at point $B$ while maintaining the same current density,the magnitude of the magnetic field at $O$:

Two identical conducting wires $AOB$ and $COD$ are placed at right angles to each other. The wire $AOB$ carries an electric current $I_1$ and $COD$ carries a current $I_2$. The magnetic field at a point lying at a distance $d$ from $O$,in a direction perpendicular to the plane of the wires $AOB$ and $COD$,will be given by