Eta (letter)

From Wikipedia, the free encyclopedia

For other uses, see Eta.

Greek alphabet
Α α Alpha	Ν ν Nu
Β β Beta	Ξ ξ Xi
Γ γ Gamma	Ο ο Omicron
Δ δ Delta	Π π Pi
Ε ε Epsilon	Ρ ρ Rho
Ζ ζ Zeta	Σ σ ς Sigma
Η η Eta	Τ τ Tau
Θ θ Theta	Υ υ Upsilon
Ι ι Iota	Φ φ Phi
Κ κ Kappa	Χ χ Chi
Λ λ Lambda	Ψ ψ Psi
Μ μ Mu	Ω ω Omega
Obsolete letters
Ϝ ϝ Digamma	Ϻ ϻ San
Ϙ ϙ Qoppa	Ϡ ϡ Sampi
Ϛ ϛ Stigma	Ϸ ϸ Sho

Eta (uppercase Η, lowercase η) is the seventh letter of the Greek alphabet. In the system of Greek numerals it has a value of 8. It was derived from the Phoenician letter Heth . Letters that arose from Eta include the Latin H and the Cyrillic letter И.

In Modern Greek the letter represents a Close front unrounded vowel /i/. In Classical Greek, it was pronounced as a long Open-mid front unrounded vowel: /ɛː/. The symbol was initially used to denote the rough breathing, Voiceless glottal fricative /h/. In the Ionic dialect, where this sound had disappeared by the sixth century BC, the letter was used to represent the long /ɛː/. When the Ionic alphabet was adopted in Athens in 403 BC, /ɛː/ (previously written as Ε) was also represented as Η there too, hence the modern usage.

[edit] Symbol

Eta is sometimes used in place of eng (ŋ) when eng is not available, because of their similar appearance.

The upper-case letter Η is used as a symbol for:

• In textual criticism, the Alexandrian text-type (from Hesychius, its once-supposed editor).
• In chemistry, enthalpy.

The lower-case letter η is used as a symbol for:

• In thermodynamics, the efficiency of a Carnot heat engine.
• In chemistry, the number of electrons shared between a metal center and a ligand in a coordination compound. For example, an allyl group can coordinate to palladium in the η¹ mode or the η³ mode.
• In optics, the refractive index of an optical medium (although the letter n is more common). It may also refer to the electromagnetic impedance of a medium.
• In particle physics, to represent the η mesons.
• In Quantum Field Theory (physics), to represent the metric tensor.
• In statistics, η² is the "partial regression coefficient".
• In economics, η is the elasticity.
• In astronomy, the seventh brightest (usually) star in a constellation. See Bayer designation.
• In experimental particle physics, η stands for pseudorapidity.
• In mathematics, η-conversion, see lambda calculus
• In fluid dynamics, η stands for viscosity.als:Η

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