Temperature

Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of energy, when a body is in contact with another that is colder or hotter.

Temperature is measured with a thermometer, which has two important elements: (1) a temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature, plus (2) some means of converting this physical change into a numerical value (e.g. the visible scale that is marked on a mercury thermometer).

Units of temperature

The basic unit of temperature in the International System of Units (SI) is the kelvin (K). One kelvin is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (exactly 0.01 °C or 32.018 °F). The Kelvin scale is an absolute temperature scale, starting at absolute zero, the lowest possible temperature.

Other common temperature scales include the Celsius scale (°C) used in most of the world (except in the US), and the Fahrenheit scale (°F) used in the United States.

Thermodynamic temperature

Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Thermodynamic temperature is defined by the third law of thermodynamics in which the theoretically lowest temperature is the null or zero point. At this point, absolute zero, the particle constituents of matter have minimal motion and can become no colder.

Effects of temperature

Temperature affects the physical state of a substance. It can change a substance from a solid to a liquid (melting), a liquid to a gas (boiling) or the reverse of either of these processes.

Temperature also affects the rate of chemical reactions. Generally, a higher temperature increases the rate of chemical reactions.

See also

• Heat
• Thermodynamics
• Thermal expansion
• Thermal conduction
• Thermal radiation

• Thermally agitated molecule
  Thermally agitated molecule
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  Body temperature variation
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  Thermometer showing Celsius and Fahrenheit
• Temperature
  Temperature
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  Translational motion of particles
• Gas thermometer and absolute zero
  Gas thermometer and absolute zero
• A guide to cosmic temperatures
  A guide to cosmic temperatures