Inductance is a fundamental property of an electrical conductor, which quantifies its ability to store energy in a magnetic field when an electric current is flowing through it. Inductance is typically represented by the symbol “L” and is measured in units called henrys (H).

When a current flows through a conductor, it generates a magnetic field around it. If the current changes, the magnetic field also changes, inducing an electromotive force (EMF) or voltage across the conductor, which opposes the change in current. This phenomenon is known as electromagnetic induction and is the basis for the concept of inductance.

## Two types of inductance

- Self-inductance: Self-inductance refers to the inductance of a single conductor or coil, where the changing magnetic field generated by the current flowing through the conductor induces a voltage across the conductor itself. This voltage, known as self-induced EMF, opposes any change in the current.

The self-inductance of a coil is primarily determined by its shape, size, the number of turns in the coil, and the core material (if any) around which the coil is wound.

- Mutual inductance: Mutual inductance occurs when two or more conductors or coils are placed in proximity, and the changing magnetic field generated by the current flowing through one conductor induces a voltage across the other conductor(s). This voltage, known as mutually induced EMF, depends on the relative orientation and distance between the conductors and their individual inductance.

## Self-inductance

Self-induction is a phenomenon that occurs within a single coil or inductor when a change in the current flowing through it induces an electromotive force (EMF) within the coil itself. This happens because the magnetic field generated by the coil changes as the current through it changes, and this varying magnetic field induces a voltage in the coil according to Faraday’s law of electromagnetic induction. The induced EMF opposes the change in current, as described by Lenz’s law.

Self-inductance, also known simply as inductance (L), is a measure of the coil’s ability to oppose changes in current due to self-induction. It is defined as the ratio of the induced EMF to the rate of change of current through the coil:

EMF_induced = -L * (dI / dt)

where: EMF_induced = Induced EMF in the coil (V) L = Self-inductance (H) dI/dt = Rate of change of current through the coil (A/s)

The self-inductance of a coil depends on its geometry, the number of turns, the spacing between the turns, and the core material (if any). For a solenoid-shaped air-core coil, the self-inductance can be calculated using the following formula:

L = (μ₀ * N^2 * A) / l

where:

L = Self-inductance (H)

μ₀ = Permeability of free space, approximately 4π x 10^-7 H/m

N = Number of turns in the coil

A = Cross-sectional area of the coil (m^2)

l = Length of the coil (m)

This formula assumes a uniform coil with a consistent cross-sectional area and evenly spaced turns. For other coil geometries or when a magnetic core is present, the calculation may be more complex.

Self-inductance is an important property in various electrical and electronic circuits, such as inductors, transformers, and inductive loads like motors and solenoids. It influences the transient and steady-state response of circuits, causing time delays and phase shifts in AC circuits. Designers need to consider the effects of self-inductance to ensure proper circuit operation and to prevent undesirable effects like voltage spikes and oscillations.