(a) Define self inductance. Write its S.I. units
(b) Derive and expression for self inductance of a long solenoid of length l cross-sectional area A having N number of turns.

Explanation

### Solution:

**(a) Definition of Self Inductance:**

Self-inductance is defined as the property of a coil (or inductor) that quantifies the induced electromotive force (emf) in itself due to a change in current flowing through it. It is the ratio of the magnetic flux linked with the coil to the current flowing through it. Mathematically, it can be expressed as:

\[
L = \frac{\Phi}{I}
...