An iron core coil with 25 turns has an inductance of 0.625

An iron core coil with 25 turns has an inductance of 0.625 mH. In order to obtain an inductance of 0.1 mH, the number of turns will have to be changed to—

As we know that

e = −N	dφ
	dt

where e = induced voltage
N = number of turns

dφ	= rate of a change of flux with time from the relation (i)
dt

N₁	1
	φ₁

so,

N₁	=	φ₁	⇒ N₂ =	N₁φ₁	=	25 × .625
N₂	=	φ₂	⇒ N₂ =	f2	=	.1

or N₂ = 157