KEAM 2021 Physics and Chemistry Solved Paper

Inductance of a solenoid coil of length L, total number of turns N and cross-sectional area A, is given as
L=‌

µ0N2A

l

....(i)
where, µ0 is the permeability in vacuum (or air).
If air core is replaced by iron core, inductance will be given as
L′=‌

µN2A

l

....(ii)
On dividing Eq. (ii) by Eq (i). we get
‌

L′

L

=‌

µN2A

l

/‌

µ0N2A

l

=‌

µ

µ0

=µr
where, µr is the relative permeability of iron. Given, L=0.02mH,L′=40mH
∵‌‌µr=‌

40

0.02

=2000