$A$ closely wound solenoid of $1200$ turns and area of cross-section $5 \,cm^2$ carries a current. If the magnetic moment of the solenoid is $1.2 \,J \,T^{-1}$, then the current through the solenoid is (in $\,A$)

$A$ thick uniformly charged hollow cylinder of inner radius $a$ and outer radius $b$ rotates with constant angular speed $\omega$ about its axis $APB$,with charge density $\rho$. Given that $L \gg a$ and $L \gg b$,and $P$ is the midpoint of $AB$. Choose the incorrect option.

$A$ direct current $I$ flows along the length of an infinitely long straight thin-walled pipe. Then the magnetic field is

$A$ 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

The magnetic flux near the axis and inside the air core solenoid of length $60$ cm carrying current $I$ is $\frac{\pi}{2} \times 10^{-6} \text{ Wb}$. Its magnetic moment will be (cross-sectional area is very small as compared to length of solenoid, $\mu_0 = 4\pi \times 10^{-7} \text{ SI unit}$) (in $\text{ Am}^2$)

$A$ long straight wire with a circular cross-section having radius $R$ is carrying a steady current $I$. The current $I$ is uniformly distributed across this cross-section. Then the variation of magnetic field due to current $I$ with distance $r$ $(r < R)$ from its centre will be: