magnetism Questions

A current $I$ ampere flows along an infinitely long straight thin walled tube, then the magnetic induction at any point inside the tube is:

A long wire carries a steady current. It is bent into a circle of one turn and the magnetic field at the centre of the coil is $B$. It is then bent into a circular loop of $n$ turns. The magnetic field at the centre of the coil will be:

The magnetic field due to a current carrying circular loop of radius $3\text{cm}$ at a point on the axis at a distance of $4\text{cm}$ from the centre is $54\mu \text{T}$. What will be its value at the centre of the loop?

Two long conductors, separated by a distance $d$ carry current $I_1$ and $I_2$ in the same direction. They exert a force $F$ on each other. Now the current in one of them increased to two times and its direction reversed. The distance is also increased to $3d$. The new value of the force between them is:

The length of a magnet is large compared to its width and breadth. The time period of its oscillation in a vibration magnetometer is $2\text{s}$. The magnet is cut along its length into three equal parts and three parts are then placed on each other with their like poles together. The time period of this combination will be:

The materials suitable for making electromagnets should have: