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Temperature (symbolized T ) is an expression of heat energy. Temperature can mean different things in different situations.

Thermodynamic temperature is a measure of the kinetic energy in molecules or atom s of a substance. The greater this energy, the faster the particles are moving, and the higher the reading an instrument will render. This is the method lay people most often use.

Spectral temperature is defined according to the wavelength at which the electromagnetic (EM) energy that an object emits is greatest. The shorter the wavelength, the higher the frequency of maximum EM energy, and the higher the spectral temperature. This is the temperature scheme that astronomers use to measure the heat in distant objects such as the sun's corona or the gas and dust between stars.

There are three temperature scales in common use today: kelvin (K), centigrade or Celsius (C), and Fahrenheit (F). A fourth scale, known as the Rankine (R) temperature scale, is less often used. The kelvin is the unit of temperature in the International System of Units ( SI ).

One kelvin is formally defined as 1/273.16 (3.6609 x 10 -3 ) of the thermodynamic temperature of the triple point of pure water (H 2 O). A temperature of 0 K represents absolute zero, the absence of all heat.

In the centigrade or Celsius temperature scale, the freezing point of pure water at one atmosphere is assigned the value zero; the boiling point is +100 C. One-degree increments in the centigrade scale are the same size as those in the kelvin scale. At standard Earth-atmospheric sea-level pressure, water freezes at 0 C or +273.15 K, and boils at +100 C or +373.15 K. A temperature of 0 K thus corresponds to -273.15 C. To convert a kelvin temperature figure to Celsius, subtract 273.15. To convert a Celsius temperature figure to kelvin, add 273.15.

The Fahrenheit scale is used mostly by non-scientists. Pure water at one atmosphere (the average sea-level pressure) freezes at +32 degrees Fahrenheit (F); pure water at one atmosphere boils at +212 degrees F. Absolute zero is -459.67 degrees F. One Fahrenheit degree increment is 5/9 (0.55555) times the size of a kelvin or centigrade degree. If C is the centigrade temperature, the Fahrenheit temperature F is given by F = (1.8 x C) + 32. The Fahrenheit and centigrade scales agree at -40.

The Rankine scale is not often used. The degree increments in this temperature scheme are the same size as those of the Fahrenheit scale, but 0 R corresponds to absolute zero or 0 K. Degrees Rankine can be obtained from degrees kelvin by multiplying the kelvin temperature by 1.8. Degrees Fahrenheit are obtained from Rankine readings by subtracting 459.67.

Also see kelvin , and International System of Units ( SI ).

This was last updated in March 2011
Posted by: Margaret Rouse

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