Current $I$ is flowing in a conductor shaped as shown in the figure. The radius of the curved part is $r$ and the length of straight portion is very large. The value of the magnetic field at the centre $o$ will be

 

The magnetic induction at the centre $O$ in the figure shown is 

Two long parallel wires $P$ and $Q$ are both perpendicular to the plane of the paper with distance $5 \mathrm{m}$ between them. If $P$ and $Q$ carry current of $2.5 \mathrm{A}$ and $5 \mathrm{A}$ respectively in the same direction, then the magnetic field at a point half way between the wires is 

Which of the following graphs shows the variation of magnetic induction $B$ with distance $r$ from a long wire carrying current

Magnetic field induction at the centre of a circular coil shown in the figure is

A long wire carrying a steady current is bent into a single coil such that magnetic induction at centre is $B$. Then same wire is bent to form a coil of smaller radius of $4$ turns when magnetic induction at centre is $B\text{'}$. Then

An infinite straight conductor carrying current $2i$ is split into a loop of radius $r$ as shown in the figure. The magnetic field at the centre of the coil is

Magnetic field produced by electrons in atoms and molecules is due to their