The current $I$ is flowing through a loop $A B C D A$ as shown in the figure. The magnitude of the magnetic field at the centre ' $O$ ' is $x$ times

$$ \left(\frac{\mu_0 \mathrm{I}}{\mathrm{R}}\right)\left(\mathrm{OD}=\mathrm{R}, \mathrm{DC}=\mathrm{R}, \angle \mathrm{AOD}=90^{\circ}\right) $$

The value of ' $x$ ' is ( $\mu_0=$ permeability of free space)

A rod of circular cross-sectional area A and length $L$ is wound uniformly with $n$ turns of an insulated wire. If current flowing through the windings is I, the total magnetic flux produced inside windings is $\phi$. The relative permeability of the rod is ( $\mathrm{N}=$ number of turns per unit length) $\left(\mu_0=\right.$ permeability of vacuum $)$

Magnetic field at the centre of a circular loop of area ' $A$ ' is ' $B$ '. The magnetic moment of the loop is ' $x B$ '. The value of ' $x$ ' is ( $\mu_0=$ permeability of vacuum or free space)

A straight wire of mass ' M ' and length 2 m is placed in a magnetic field of 2 T which is acting perpendicular to the length of the wire. When a current of 1 A flows through the wire, the wire experiences an upthrust and leviates in a magnetic field. The mass ' $M$ ' of the wire is (acceleration due to gravity $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^2$ )