**Self Inductance **

Whenever you change the current in a coil of wire, the magnetic field
it produces will change. That will change the magnetic flux through
the coil and hence produce a voltage across the coil. This phenomenon
is called *self-inductance*, and the coil is referred to as an
*inductor*.

The quantitative measure of this effect is the inductance
** L**
defined by

An inductor can store energy. The power (= energy / time) being stored in an inductor is

This implies (by a little calculus) that the energy stored in an inductor is

As an example, we can derive the inductance of a solenoid. The magnetic
field inside a solenoid of length
*l* is

Therefore, the inductance * L*
can be given in terms of the geometry of the solenoid.

Longer solenoids and solenoids
with more cross-sectional area have more inductance. The strongest dependence
is on the number of turns per unit length. One can also increase the
inductance by using iron instead of a vacuum; in that case, the parameter
**m**_{0}
would be replaced by a larger value
**m**
.

*
Examples
Induction index
Lecture index*