An Etymological Dictionary of Astronomy and Astrophysics
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A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

→ Newton; → first; → law; → motion.

An approximate empirical relation between the rate of → heat transfer to or from an object and the temperature difference between the object and its surrounding environment. When the temperature difference is not too large: dT/dt = -k(T - T_s), where T is the temperature of the object, T_s is that of its surroundings, t is time, and k is a constant, different for different bodies.

→ Newton; → law; → cooling.

The universal law which states that the force of attraction between any two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G (m₁.m₂)/r², where G is the → gravitational constant.

→ Newton; → law; → gravitation.

The three fundamental laws which are the basis of → Newtonian mechanics. They were stated in Newton's Principia (1687). → Newton's first law, → Newton's second law , → Newton's third law.

→ Newton; → law; → motion.

For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

→ Newton; → second; → law; → motion.

In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

→ Newton; → third; → law; → motion.

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.

A change in the pressure of an enclosed incompressible fluid is conveyed undiminished to every part of the fluid and to the surfaces of its container.

Named after Blaise Pascal (1623-1662), French mathematician, physicist, and religious philosopher for his contribution in the study of hydrodynamics and hydrostatics, in particular establishing the principle of the barometer.

A theoretical principle which is deduced from particular observational facts regarding the behavior of matter. Physical laws are expressed by a general statement that a particular → physical phenomenon always occurs if certain → conditions are present.

→ physical; → law.

An equation that expresses the energy radiated per unit area per unit time per unit wavelength range by a blackbody as a function of temperature. It is expressed by → Planck's blackbody formula.

→ Planck; → radiation; → law.

In fluid dynamics, the law that the rate of flow of a liquid through a horizontal tube of uniform radius is directly proportional to the pressure of the liquid and the fourth power of the radius of the tube and is inversely proportional to the viscosity of the liquid and the length of the tube.

Named after Jean-Louis-Marie Poiseuille (1797-1869), a French physiologist and physician who found the law in 1844; → law.

A mathematical relationship between two quantities expressed by a → power function.

→ power; → law.

For a → random variable X, any → distribution which has the form: P(X ≥ x) = (k/x)^α, where x is a value in the range defined for X, k > 0 is a parameter termed location parameter, and α > 0 is the → slope parameter.

→ power; → law; → distribution.

An → elliptical galaxy whose → surface brightness can be approximated by a single → power law at small radii (r ≤ 10-20''). More modern interpretations have emphasized that these profiles can be better understood as the inward continuation of the galaxy's overall → Sersic profile, usually modified by an additional, nuclear-scale stellar component (S. P. Rusli et al., 2013, AJ 146, 160).

→ power; → law; → galaxy.

A statement concerned with thermodynamic heat exchange, according to which bodies in → thermal equilibrium are simultaneously absorbing and emitting radiant energy. A body radiates in the same way whether other bodies are present or not. Also called Prevost's theory of exchanges.

Named after Pierre Prévost (1751-1839), a Swiss philosopher and physicist, who, in 1791, put forward the statement; → law; → exchange.

The → vapor pressure of an ideal → solution is dependent on the vapor pressure of each chemical component and the → mole fraction of the component present in the solution. This means that the addition of → solute to a liquid lessens the tendency for the liquid to become a → solid or a → gas. For example, the addition of → salt to water causes the water to freeze below its normal → freezing point (0°C) and to boil above its normal → boiling point (100°C).

After François-Marie Raoult (1830-1901), the French chemist who studied the physical properties of chemical solutions; → law.

A classical law approximately describing the intensity of radiation emitted by a → blackbody. It states that this intensity is proportional to the temperature divided by the fourth power of the wavelength (8πkT/λ⁴). The Rayleigh-Jeans law is a good approximation to the experimentally verified Planck radiation formula only at long wavelengths. At short wavelengths it runs into a paradox named the → ultraviolet catastrophe.

→ Rayleigh; → Jeans; → law.

A power-law relation between → star formation rate (SFR) and a corresponding measure of gas density. For external galaxies it is usually expressed in terms of the observable surface density of gas (Σ_gas): SFR ∝ Σ_gasⁿ. The exponent n is determined to be 1.4 ± 0.15 (Kennicutt 1998, ApJ 498, 541). The validity of the Schmidt law has been tested in dozens of empirical studies. The Schmidt law provides a tight parametrization of the global star formation law, extending over several orders of magnitude in SFR and gas density.

Named after Maarten Schmidt (1929-), a dutch-born American astronomer, who also discovered the first → quasar (3C 273) in 1963.

The surface area of a black hole's horizon can never decrease.

→ second; → law; → black hole; → mechanics.

1) Heat cannot be transferred from a colder to a hotter body without some other effect, i.e. without → work being done. Expressed in terms of → entropy: the entropy of an → isolated system tends toward a maximum and its available energy tends toward a minimum.
2) In language of → statistical physics, an isolated physical system will tend toward an equilibrium → macrostate with as large a total → entropy as possible, because then the number of → microstates is the largest. See also → Kelvin's postulate, → Clausius's postulate.

→ second; → law; → thermodynamics.