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SI units

the units of measurement generally accepted for all scientific and technical uses; together they make up the International System of Units. (See also metric system.) The abbreviation SI, from the French Système International d'Unités, is used in all languages. There are seven base SI units, defined by specified physical measurements, and two supplementary units. Units are derived for any other physical quantities by multiplication and division of the base and supplementary units. The derived units with special names are shown in the accompanying table.

SI is a coherent system. This means that units are always combined without conversion factors. The derived unit of velocity is the meter per second (m/s); the derived unit of volume is the cubic meter (m3). If you know that pressure is force per unit area, then you know that the SI unit of pressure (the pascal) is the unit of force divided by the unit of area and is therefore equal to 1 newton per square meter.

The metric prefixes can be attached to any unit in order to make a unit of a more convenient size. The symbol for the prefix is attached to the symbol for the unit, e.g., nanometer (nm) = 10−9 m. The units of mass are specified in terms of the gram, e.g., microgram (μg) = 10−9 kg.

Only one prefix is used with a unit; the use of units such as the millimicrometer is no longer acceptable. When a unit is raised to a power, the power applies to the prefix as well, e.g., a cubic millimeter (mm3) = 10−9 m3. When a prefix is used with a ratio unit, it should be in the numerator rather than in the denominator, e.g., kilometers/second (km/s) rather than meters/millisecond (m/ms). Only prefixes denoting powers of 103 are normally used. Hecto-, deka-, deci-, and centi- are usually attached only to the metric system units gram, meter, and liter.

Owing to the force of tradition, one noncoherent unit, the liter, equal to 10−3 m3, or 1 dm3, is generally accepted for use with SI. The internationally accepted abbreviation for liter is the letter l; however, this can be confused with the numeral 1, especially in typescript. For this reason, the capital letter L is also used as a symbol for liter. The lower case letter is generally used with prefixes, e.g., dl, ml, fl. The symbols for all other SI units begin with a capital letter if the unit is named after a person and with a lower case letter otherwise. The name of a unit is never capitalized.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


William Thomson, Scottish physicist, 1824-1907. See: kelvin, Kelvin scale.

kel·vin (K),

A unit of thermodynamic temperature equal to 273.16-1 of the thermodynamic temperature of the triple point of water. See: Kelvin scale.
[Lord Kelvin]
Farlex Partner Medical Dictionary © Farlex 2012


(K) (kel'vin)
A unit of thermodynamic temperature equal to 1/273.16 of the thermodynamic temperature of the triple point of water.
See also: Kelvin scale
[Lord Kelvin]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
References in periodicals archive ?
The researchers report that at temperatures as high as 203 kelvins, the samples expelled magnetic fields, exhibiting what's known as the Meissner effect.
Caption: Heating up A new experiment shows that pressurized hydrogen sulfide may be a superconductor at up to 203 kelvins (-70[degrees] Celsius), nearly 40 kelvins above the current record.
The star is the hottest Wolf-Rayet star known, with a surface temperature of 120,O00 kelvins, nearly 20 times that of the sun.
As the material falls onto the dense star, it reaches searing temperatures of up to millions of kelvins, emitting both X rays and ultraviolet light-enough to fully ionize helium atoms.
However, his calculations suggest that a neutron star's surface can't reach temperatures as high as 1 billion kelvins. To ignite carbon at a slightly more modest temperature, the star would require a billion times as much of the material, and it would take a century to stockpile that much carbon, Brown notes.
Earlier studies have shown that external pressure on that highest temperature superconductor raises its maximum from 133 kelvins to about 164 kelvins--but only as long as the pressure remains.
By comparison, the visible surface of the sun has a temperature of 6,000 kelvins.
The Sandia team has proposed building a new machine, called X-1, which would aim for a power of 1,000 terawatts, an energy of 16 megajoules, and temperatures in the range of 2 million to 3 million kelvins, says Jeffrey P.
In this case, the large velocities suggest that at a height of 500,000 kilometers above the visible solar surface-well into the corona-hydrogen reaches a temperature of 6 million kelvins and oxygen an astounding 100 million kelvins.
Forming a dark spot the diameter of Earth, the falling plume apparently heated the atmosphere around it to some 500 kelvins, creating an infrared glow.
The universe has cooled considerably since its birth, and the temperature of the relic radiation in nearby regions of space, 2.73 kelvins, exactly matches the predicted cooling.
But yesterday anglers marked the first-ever opening of the salmon season on the banks of the River Kelvin in central Glasgow in style.