In physics, two systems are coupled if they are interacting with each other. Of special interest is the coupling of two (or more) vibratory systems (e.g. pendula or resonant circuits) by means of springs or magnetic fields, etc. Characteristic for a coupled oscillation is the effect of beat.

The concept of coupling is particularly important in physical cosmology, in which various forms of matter gradually decouple and recouple between each other.

Coupling is also important in physics for the generation of plasmas. In electrical discharges, the coupling of an exciting field and a medium creates plasmas. The quality of the coupling of an exciting field of given frequency to a charged particle depends on resonance.

See also

A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.

The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. By careful selection, installation and maintenance of couplings, substantial savings can be made in reduced maintenance costs and downtime.

Uses

Shaft couplings are used in machinery for several purposes. The most common of which are the following.

In electronics and telecommunication, coupling is the desirable or undesirable transfer of energy from one medium, such as a metallic wire or an optical fiber, to another medium.

Coupling is also the transfer of electrical energy from one circuit segment to another. For example, energy is transferred from a power source to an electrical load by means of conductive coupling, which may be either resistive or hard-wire.  An AC potential may be transferred from one circuit segment to another having a DC potential by use of a capacitor.  Electrical energy may be transferred from one circuit segment to another segment with different impedance by use of a transformer. This is known as impedance matching. These are examples of electrostatic and electrodynamic inductive coupling.

Types of coupling

Electrical conduction:

Electromagnetic induction:

An azo coupling is an organic reaction between a diazonium compound and another aromatic compound that produces an azo compound. In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile and the activated arene is a nucleophile. In most cases, including the examples below, the diazonium compound is also aromatic.

Diazotization

The treatment of aniline with nitrous acid, produces a diazonium salt, in a reaction called diazotization. Diazonium salts are important synthetic intermediates that can undergo coupling reactions to form azo dyes and substitution reaction to effect the functional group present on aromatic rings.

Uses of the reaction

The product will absorb longer wavelengths of light (specifically they absorb in the visible region) than the reactants because of increased conjugation. Consequently, aromatic azo compounds tend to be brightly colored due to the extended conjugated systems. Many are used as dyes (see azo dye). Important azo dyes include methyl red and pigment red 170.