The magnetic field at a distance $r$ from a long straight wire carrying current $I$ is $0.4 \text{ tesla}$. The magnetic field at a distance $2r$ will be (in $\text{ tesla}$)

The adjoining figure shows a conductor carrying a current $i$. The magnetic field at the origin is

In the following figure,a wire bent in the form of a regular polygon of $n$ sides is inscribed in a circle of radius $a$. The net magnetic field at the centre will be

How can we determine the direction of the magnetic field using the Biot-Savart law?

An electron is revolving around a proton, producing a magnetic field of $16 \, Wb/m^2$ in a circular orbit of radius $1 \, Å$. Its angular velocity will be:

$A$ long wire is bent into a circular coil of one turn and then into a circular coil of smaller radius having $n$ turns. If the same current is passed in both the cases, the ratio of magnetic field produced at the centre for one turn to that of $n$ turns is