In the figure,the magnetic induction at the centre $O$ due to the current in the straight portion $AB$ will be:

The magnetic field at a distance of $10 \ cm$ from a long straight thin wire carrying a current of $4 \ A$ is (in $\mu T$)

$A$ thin ring of $10 \, cm$ radius carries a uniformly distributed charge. The ring rotates at a constant angular speed of $40 \pi \, rad \, s^{-1}$ about its axis,perpendicular to its plane. If the magnetic field at its centre is $3.8 \times 10^{-9} \, T$,then the charge carried by the ring is close to $\left( \mu_0 = 4 \pi \times 10^{-7} \, N/A^2 \right)$.

The dimensions of the ratio of magnetic flux $(\phi)$ and permeability $(\mu)$ are

$A$ current of $10\,A$ is passing through a long wire which has a semicircular loop of radius $20\,cm$ as shown in the figure. The magnetic field produced at the centre $P$ of the loop is:

The unit of permeability of vacuum $(\mu_{0})$ is . . . . . . .