# Formulas for Inductance

In this section we will cover the formulas for inductance .

Inductance is the characteristic of an electrical conductor that opposes a change in electrical current. The symbol for inductance is L and the basic unit of measurement is the HENRY(H).

All practical circuits have some inductance, which may provide either beneficial or detrimental effects. In a tuned circuit inductance is used to provide a frequency selective circuit. Practical inductors may be used to provide filtering or energy storage in a system.

The inductance of a transmission line is one of the properties that determines its characteristic impedance; balancing the inductance and capacitance of cables is important for distortion-free telegraphy and telephony.

The inductance of long power transmission lines limits the AC power that can be sent over them. Sensitive circuits such as microphone and computer network cables may use special cable constructions to limit the mutual inductance between signal circuits.

The formulas for inductance will provide you with the necessary calculations required to compute the various aspects of inductance in different circuits.

An inductor has an inductance of 1 henry if an electromotive force (emf) of 1 volt is induced in the inductor when the current through the inductor is changing at the rate of one ampere per second.

Mathematically:

Mutual inductance:

Series inductors without magnetic coupling:

Series inductors with magnetic coupling:

Parallel inductors without coupling:

Provided the coefficient of coupling between inductors is zero.

Resistive/inductive circuit:

The time required for the current in an inductor to increase to 63.2 percent of its final value or decrease to 36.8 percent is known as the time constant.

Mathematically expressed:

The picture below shows an L/R time constant chart.

Inductive reactance:

Ohm's law for reactive circuit:

You can find more information about inductance in NEETS, Module 2, Introduction to Alternating Current and Transformers.