Temperature

Temperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. Temperature is a measure of the average kinetic energy of the particles in a sample of matter. In other words, temperature is a measure of activity and the frequency of collisions of molecules.

Temperature measurement

Main article: Temperature measurement

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Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Rømer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

Related Topics:
Thermometer - Gabriel Fahrenheit - Mercury - Ole Christensen Rømer - Celsius - Kelvin

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Units of temperature

The basic unit of temperature (symbol: T) in the International System of Units (SI) is the kelvin (K). One kelvin is formally defined as 1/273.16 of the temperature of the triple point of water (the point at which water, ice and water vapor exist in equilibrium). The temperature 0 K is called absolute zero and corresponds to the point at which the molecules and atoms have the least possible thermal energy. An important unit of temperature in theoretical physics is the Planck temperature (1.4 × 1032 K).

Related Topics:
International System of Units (SI) - Kelvin - Triple point - Water - Ice - Vapor - Equilibrium - Absolute zero - Molecule - Atom - Thermal energy - Planck temperature

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In the field of plasma physics, because of the high temperatures encountered and the electromagnetic nature of the phenomena involved, it is customary to express temperature in electron volts (eV) or kilo electron volts (keV), where 1 eV = 11,605 K. In the study of QCD matter one routinely meets temperatures of the order of a few hundred MeV, equivalent to about 10 ^{12} K.

Related Topics:
Plasma physics - Electromagnetic - Electron volt - QCD matter - MeV

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For everyday applications, it is often convenient to use the Celsius scale, in which 0 °C corresponds to the temperature at which water freezes and 100 °C corresponds to the boiling point of water at sea level. In this scale a temperature difference of 1 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the kelvin scale, but offset by the temperature at which water freezes (273.15 K). Thus the following equation can be used to convert from Celsius to kelvin.

Related Topics:
Celsius - Freezes - Boiling point

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:

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mathrm{K = left(rac{1 , K}{1, ^circ C} ight) + 273.15, K}

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In the United States, the Fahrenheit scale is widely used. On this scale the freezing point of water corresponds to 32 °F and the boiling point to 212 °F. The following formula can be used to convert from Fahrenheit to Celsius:

Related Topics:
United States - Fahrenheit

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:

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mathrm{ !^circ C = rac{5, ^circ C}{9, ^circ F}( - 32, ^circ F)}

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See temperature conversion formulas for conversions between most temperature scales.

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¹ Only the kelvin, Celsius, Fahrenheit, and Rankine scales are in use today.

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² Some numbers in this table have been rounded off.

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³ Normal human body temperature is 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F.

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Negative temperatures

:See main article: Negative temperature.

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For some systems and specific definitions of temperature, it is possible to obtain a negative temperature. A system with a negative temperature is not colder than absolute zero, but rather it is, in a sense, hotter than infinite temperature (sic).

Related Topics:
Negative temperature - Absolute zero - Infinite

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Articles about temperature ranges:

• 10⁻¹² K = 1 picokelvin (pK)
• 10⁻⁹ K = 1 nanokelvin (nK)
• 10⁻⁶ K = 1 microkelvin (µK)
• 10⁻³ K = 1 millikelvin (mK)
• 10⁰ K = 1 kelvin
• 10¹ K = 10 kelvins
• 10² K = 100 kelvins
• 10³ K = 1,000 kelvin = 1 kilokelvin (kK)
• 10⁴ K = 10,000 kelvins = 10 kK
• 10⁵ K = 100,000 kelvins = 100 kK
• 10⁶ K = 1 megakelvin (MK)
• 10⁹ K = 1 gigakelvin (GK)
• 10¹² K = 1 terakelvin (TK)