A closely wound flat circular coil of 25 turns of wire has diameter of 10 cm which carries current of 4 A. The flux density at the centre of a coil will be

The magnetic field at the centre of the coil,B=μ04π  2πNiawhere N is number of turns, i is current and a is radius of coil.Given,  N=25, D=10  cm =10  ×  10−2  mRadius of the coil,a=D2  = 10  ×  10−22  m=0.05  mAnd current i = 4 A.  Putting these values in the expression for B , we get B=1.256 mT