If self induction of an air core solenoid increases from 0.04 mH to 16 mH on introducing an iron core into it, what is relative magnetic permeability of the core ?

Explanation

As `L_(0) = (mu_(0) N^(2) A)/(l)` and `L = (mu N^(2) A)/(l)`
`:. mu_(r) = (mu)/(mu_(0)) = (L)/(L_(0)) = (16)/(0.04) = 400`