A helium nucleus (charge$+ 2e$) completes one round of a circle of radius $0.8 \mathrm{m}$ in $2 \mathrm{s}.$ Find the magnetic field at the centre of the circle.   

A long, straight wire is turned into a loop of radius $\mathrm{R} = 10 \mathrm{cm}$, as shown. If a current of $8 \mathrm{A}$ is passed through the loop, find the magnetic field at the centre $\mathrm{C}$of the loop.

The figure shows two semicircular current-loops of radii $a_1$ and $a_2$. Find the magnitude and direction of the magnetic field at the common centre O.

A current of I ampere is flowing through the adjoining bent wire. Find the magnitude and direction of the magnetic field at point $\mathrm{O} .$ .

Compute the magnitude and direction of the magnetic field at the centre $\mathrm{O}$ of the given current-carrying bent wire.