Comparison of the Fahrenheit, Celsius, and Kelvin scales.
Temperature is the quantity that tells how warm or cold an object is with respect to some standard. It is a measure of the average kinetic energy of the molecules or atoms in an object. The temperature of matter is expressed by a number that corresponds to the degree of hotness on a chosen scale: Celsius (°C), Fahrenheit (°F), or kelvin (K) and is measured quantitatively by thermometers.
In most laboratory work, temperature is expressed in degrees Celsius, named after the eighteenth century Swedish astronomer Anders Celsius. On the Celsius scale, zero is assigned to the temperature at which water freezes, and 100 is given to the boiling point of water. The Celsius scale is the same as the centigrade scale.
The United States uses the Fahrenheit scale to express temperatures. In this scale, the value 32 is assigned to the freezing point of water and 212 to its boiling point. Conversion to the Fahrenheit scale from Celsius is through the formula: degrees Celsius = 5/9(degrees Fahrenheit - 32).
Scientists also used the kelvin scale, named after the British scientist Lord Kelvin (William Thomson). On this scale, zero is assigned to absolute zero – the lowest possible temperature. At absolute zero, a substance has no kinetic energy. Absolute zero corresponds to -273° on the Celsius scale. The absolute temperature scale, unlike the other temperature scales, is not arbitrary; it is defined with an ideal reversible heat engine working on a Carnot cycle between two temperatures T1 and T2. If Q1 is the heat received at the higher temperature T1, and Q2 the heat lost at the lower temperature T2, the T1/T2 is defined equal to Q1/Q2. Such absolute temperature is independent of the properties of particular substances, and is a basic thermodynamic function, arising out of the zeroth law.
Temperature is an intensive property, unlike heat, which is an extensive property – that is, the temperature of a body is independent of its mass or nature; it is thus only indirectly related to the heat content (internal energy) of the body. Heat flows always from a higher temperature to a lower. On the molecular scale, temperature may be defined in terms of the statistical distribution of the kinetic energy of the molecules.