(a) (i) Define mutual inductance. Write its SI unit. (ii) Derive an expression for the mutual inductance of a system of two long coaxial solenoids of same length $l$, having turns $N_1$ and $N_2$ and of radii $r_1$ and $r_2$ ($> r_1$). OR (b) What are ferromagnetic materials ? Explain ferromagnetism with the help of suitable diagrams, using the concept of magnetic domain.