Physics · JEE | Moving Charges and Magnetism

A current-carrying circular coil of \( N \) turns and radius \( R \) produces magnetic field \( B_1 \) at its center. If the coil is unwound and rewound into a circular coil of radius \( \frac{R}{2} \) with the same number of turns carrying the same current, the new magnetic field \( B_2 \) at the center is:

1. A
   \( 2 B_1 \)
2. B
   \( 4 B_1 \)
3. C
   \( \frac{B_1}{2} \)
4. D
   \( \frac{B_1}{4} \)