$A$ long solenoid carrying a current produces a magnetic field $B$ along its axis. If the current is doubled and the number of turns per cm is halved,the new value of the magnetic field is

Inductance per unit length near the middle of a long solenoid is ($\mu_0=$ permeability of free space,$n=$ number of turns per unit length,$d=$ the diameter of the solenoid).

$A$ solenoid of $1200$ turns is wound uniformly in a single layer on a glass tube $2\,m$ long and $0.2\,m$ in diameter. The magnetic intensity at the center of the solenoid when a current of $2\,A$ flows through it is:

The magnetic field intensity $H$ at the centre of a long solenoid having $n$ turns per unit length and carrying a current $I$,when no material is kept in it is ($\mu_0 =$ permeability of free space).

Rank the value of $\oint \vec{B} \cdot d\vec{l}$ for the closed paths shown in the figure from the smallest to largest.

$A$ current $I$ flows along the length of an infinitely long,straight,thin-walled pipe. Then: