Define self-inductance of a coil. Show that magnetic energy required to build up the current I in a coil of self-inductance L is given by 
Self-inductance of a coil is numerically equal to the magnetic flux linked with the coil when the current through coil is one Ampere.
Mathematically, it is given by,
where, L is the constant of proportionality and is called the self-inductance.
Energy stored in an inductor:
Consider a source of emf connected to an inductor L.
With increase in current, the opposing induced emf is given by, 
If the source of emf sends a current i through the inductor for a small time dt, then the amount of work done by the source, 
Hence, the total amount of work done by source of emf when the current increases from its initial values (i = 0) to its final value (I) is given by,
This work done gets stored in the inductor in the form of energy.
