# An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 8 Search : ohm
 Hohmann transfer   تراوژ ِ هوهمن   tarâvaž-e HohmannFr.: transfert de Hohmann   An → orbital maneuver using two timed engine impulses to move a spacecraft between two coplanar circular orbits. It is performed through an elliptic orbit which is tangent to both circles at their periapses (→ periapsis). Hohmann transfer orbit   مدار ِ تراوژ ِ هوهمن   madâr-e tarâvaž-e HohmannFr.: orbite de trandfer   An elliptical orbit that is the most economical path for a spacecraft to take from one planet to another. In the case of Earth-Mars travel, the desired orbit's → perihelion will be at the distance of Earth's orbit, and the → aphelion will be at the distance of Mars' orbit. The portion of the solar orbit that takes the spacecraft from Earth to Mars is called its trajectory. Earth and Mars align properly for a Hohmann transfer once every 26 months. → Hohmann transfer.Named after Walter Hohmann (1880-1945), German engineer, who developed basic principles and created advanced tools necessary for the conquest of space. In 1925 he published The Attainability of the Heavenly Bodies in which he described the mathematical principles that govern space vehicle motion, in particular spacecraft transfer between two orbits. ohm   اُهم   ohm (#)Fr.: ohm   A unit of electrical resistance equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals.Named after Georg Simon Ohm (1789-1854), the German physicist who discovered the law which bears his name. Ohm's law   قانونِ اُهم   qânun-e Ohm (#)Fr.: loi d'Ohm   1) For a → conductor at rest, the → voltage across the conductor is equal to the product of the current flowing through it and its → resistance. In other words, when such a conductor is subjected to an electric field E, the resulting → current density, J, is proportional to the electric field E: J = σE, where σ is the → conductivity, i.e. the reciprocal of → resistivity, ρ = 1/σ. 2) Ohm's law for a moving conductor is expressed by: J = σ(E + v x B), where v is the velocity and B the → magnetic induction.→ ohm; → law. Ohmic   اُهمی   Ohmi (#)Fr.: ohmique   Of or relating to a system which obeys Ohm's law.→ ohm + → -ic. Ohmic decay time   زمان ِ تباهی ِ اُهمی   zamân-e tabâhi-ye OhmiFr.: temps de dissipation ohmique   An upper bound on the time scale on which the magnetic field of a system would decay in the absence of any other agent. It is expressed as: τμ = R2 / μ, where R is the scale size of the system, η the magnetic diffusivity (η = 1 / μσ, where μ is the magnetic permeability and σ the electrical conductivity). For a star like the Sun, τμ  ≅ 1010 years, so a fossil magnetic field could survive for the star's lifetime on the main sequence. For the Earth, τμ  ≅ 104 years, so a → dynamo is required to explain the persistence of the geomagnetic field.→ Ohmic; → decay; → time. Ohmic dissipation   افتالِ اُهمی   eftâl-e ohmiFr.: dissipation ohmique   1) A loss of electric energy due to conversion into heat when a current flows through a resistance. Same as Ohmic loss. 2) In plasma physics, the energy released by charged particles as they make collisions with other particles.→ Ohmic; → dissipation. Ohmic loss   دسترفت ِ اهمی   dastraft-e OhmiFr.: perte ohmique   Same as → Ohmic dissipation.→ Ohmic; → loss.