An iron-cored solenoid has self- inductance. 2.8 H. When the core is removed, the self inductance becomes 2 mH. What is the relative permeability of the core used?

Explanation

If Li initial inductance and La is inductance after removing the core :
`(Li = cancel (u_0)u_(r)cancel n^ cancel (2) cancel (Al))/(La =cancel(u_(0)) cancel n ^cancel(2) cancel (Al))`
`2.8/(2 xx 10^(-3))=(Li)/(La)=u_(r)`
`u_(r) = 1.4 xx 10^(3)=1400`