There are three circuit elements that exist and can be used to model all
electronic devices. Previously, we met resistance the first circuit element and
the only one responsible for energy dissipation (as heat). **Inductors** are
the second circuit element. They have the property of storing energy in magnetic
fields.

Physically inductors are nothing more than coils. Cylindrical (solonoidal) and doughnut shaped (toroidal) geometries are most common.

Symbol :

- Self Inductance, L
- The self inductance of a coil may be defined as the ratio of the magnetic flux linking the coil (due to current flowing in the coil) to the current flowing in the coil.

Mathematically, L = φ/I

where L = inductance of coil measured in Henries (H)

I = current flowing
through coil in Amperes (A)

φ = magnetic flux linking the coil in Webers (Wb)

The self inductance of a coil may also be
defined as the ratio of its __back-emf__ to the current flowing in the coil.

The current flowing through a coil sustains a magnetic field around the coil. The energy stored in this field is independent of time and is given by the following equation.

W = ½LI^{2}

where L = inductance in Henries (H)

I = current in Amperes
(A)

W = energy in Joules (J)

Example 2.6.1

Calculate the energy stored in the B field of an inductor of value 4 H when 3 A flows through it.

Solution: W = ½LI^{2}

W = 0.5 × 4 ×
3^{2} = 18 J

Example 2.6.2

Calculate the inductance of a coil that stores 27 J in its B field when 3 A flows through it.

Solution: W = ½LI^{2}

Transposing, L = 2W/I^{2}

L = 2×27/3^{2} = 6 H