Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot. Quantitatively, temperature is measured withthermometers, which may be calibrated to a variety of temperature scales.
Temperature relates to the thermal energy held by an object or a sample of matter, which is the kinetic energy of the random motion of the particle constituents of matter. While the thermal energy of an object is proportional to the amount of matter it contains, temperature measures thermal energy in a manner that is independent of size; it is an intensive property, while thermal energy is an extensive property.Differences in temperature between regions of matter are the driving force for heat, which is the transfer of thermal energy. Spontaneously, heat flows only from regions of higher temperature to regions of lower temperature. If no heat is transferred between two objects, the objects have the same temperature.Temperature is one of the principal properties studied in the field of thermodynamics. The empirical definition of temperature arises from the conditions of thermodynamic equilibrium, expressed as the zeroth law of thermodynamics. When two systems are in thermal equilibrium, they have the same temperature, which is also a matter of common experience. The extension of this principle as an equivalence relationbetween multiple systems fundamentally justifies the use of a thermometer and prescribes the principles of its construction to measure temperature. While the zeroth law permits the definition of a set of many empirical scales of temperature, the second law of thermodynamics selects the definition of a single preferred, absolute temperature function, whence called the thermodynamic temperature. This function is the variation of the internal energy with respect to changes in the entropy of a system. Its natural, intrinsic origin or null point is absolute zero at which the entropy of any system is at a minimum. Although this is the lowest absolute temperature described by the model, the third law of thermodynamics postulates that absolute zero cannot be attained by any physical system.
For practical purposes of scientific temperature measurement, the International System of Units (SI) defines a scale and unit for the thermodynamic temperature by using the easily reproducible temperature of the triple point of water as a second reference point. For historical reasons, the triple point is fixed at 273.16 units of the measurement increment, which has been named the kelvin in honor of the Scottish physicist who first defined the scale. The unit symbol of the kelvin is K.
While the Kelvin scale is the principal temperature scale for use in science and engineering, much of the world uses the Celsius scale (°C) for most temperature measurements. It has the same incremental scaling as the Kelvin scale, but fixes its null point, at 0°C = 273.15K, the freezing point of water.