A circular coil of N turns and diameter D carries current I. It is unwound and rewound to make another coil of diameter 2D, current I remaining the same. The ratio of magnetic moments of the original coil to the new coil is

A wire AB is carrying steady current $I_1$ and is kept on the table. Another wire CD carrying current $I_2$ is held directly above as shown in figure. When the wire CD is left free and it remains suspended at its position, its mass per unit length is (g=acceleration due to gravity, $\mu_0$ = permeability of free space)

Two parallel wires of equal lengths are separated by a distance of $3$ m from each other. The currents flowing through first and second wire is $3$ A and $4.5$ A respectively in opposite directions. The resultant magnetic field at mid point of both the wires is ($\mu_0$=permeability of free space)

A coil having 'n' turns and resistance 'R' is connected with a galvanometer of resistance '2R'. This combination is moved in time 't' from flux $\phi_1$ to $\phi_2$ Wb. The induced current in the circuit is