If in a circular coil A of radius R, current I is flowing and in another coil

If in a circular coil A of radius R, current I is flowing and in another coil B of radius 2R a current 2I is flowing, then the ratio of the magnetic fields BA and BB, produced by them will be

  1. 2
  2. 1/2
  3. 1
  4. 1/4

Answer

Magnetic field at the centre of the coil is given by

B = (μoNI)/(2R)

Since both current (numerator) and radius (denominator) are doubled, the magnetic field produced by coil A and B will be same. So, the ratio will be 1.

The correct option is C.