A circular coil of N turns and radius R produces a magnetic field of magnitude B at its centre when it carries a current I. The same piece of wire is rewound to form another circular coil of half the original radius while the current is kept the same. Assuming no loss of wire length, what is the magnitude of the magnetic field at the centre of the new coil?
a) 8B
b) 4B
c) B
d) 2B