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

The length of wire will be same in two cases as the same coil is unwound and rewound.

Length of wire of coil 1 = Length of wire of coil 2