# Definition of temperature

Temperature is a physical quantity of matter that quantitatively expresses the common notions of heat and cold. Objects of low temperature are cold, whereas objects of higher temperatures consider them warm or warm. The temperature is measured quantitatively with thermometers. The thermometers can be calibrated for different temperature scales.

## Scales for measurement of temperature

The three most common temperature measurement scales are:

• The Celsius scale (degrees centigrade)
• The Kelvin Scale
• Fahrenheit Scale

Nearly everyone uses the Celsius scale (° C) for the measurement of most temperatures. The temperature variation from one degree to the next on a Celsius scale is the same as on a Kelvin scale. The difference between the Celsius and Kelvin scales is in the setting of their null point: in the Celsius area the 0 ° C corresponds to the freezing point of the water. This temperature expired in the scale Kelvin corresponds to the 273,15 kelvins (273,15 K). The nucleus point of the Kelvin scale, the 0 kelvins, corresponds to the minimum temperature theoretically a body could reach. Colder than 0 kelvins you're impossible.

However, there are a few countries, especially the United States, where the Fahrenheit scale is still used in everyday life. This is a historical temperature scale in which the freezing point of the water is at 32 ° F and the boiling temperature of the water is at 212 ° F.

The unit of measurement of temperature in the International System of Units (SI) is Kelvin. The Kelvin, therefore, is the unit used by scientists.

For practical purposes of measuring temperature within the fields of science, the International System of Units (SI) defines a scale and a unit for thermodynamic temperature based on the triple point of water. The triple point is one in which the solid state, the liquid state and the gaseous state of a substance coexist in equilibrium. It is defined with a temperature and a vapor pressure. The triple point of water is a second easily reproducible reference point.

For historical reasons, the triple point of water has been set at 273.16 units of the measurement range. This range is called kelvin (lower case) represented by the symbol K (capitalized) in honor of the Scottish physicist William Thomson (Lord Kelvin) who first defined the scale.

## Temperature and thermodynamics

One of the main properties studied in the field of thermodynamics is temperature. In thermodynamics, temperature differences between different regions of matter are especially important. These differences are what allow the movement of heat from one region to another. Heat is the transfer of thermal energy.

Spontaneously, heat flows only from regions of higher temperature in regions of lower temperature. As stated in the second law of thermodynamics in the Clausius statement. So if heat is not transferred between two objects it is because both objects have the same temperature.

According to the classical thermodynamics approach, the temperature of an object varies proportionally with the velocity of the particles it contains. It does not depend on the number of particles (of the mass) but on its average speed: at higher temperature higher average speed. Thus, the temperature is directly linked to the average kinetic energy of the particles moving relative to the center of mass of the object.

Temperature is an intensive variable, since it is independent of the amount of particles contained within an object, whether atoms, molecules or electrons. Temperature is a property that depends neither on the amount of substance nor on the type of material.

### How is temperature measured?

In order to determine the temperature of a system, it must be in thermodynamic equilibrium. The temperature can be considered to vary with position only if for each point there is a small area around it that can be treated as a thermodynamic system in equilibrium. In statistical thermodynamics, instead of particles we speak of degrees of freedom. In thermodynamics, a system is said to be in a state of thermodynamic equilibrium, if it is incapable of spontaneously experiencing any change of state or thermodynamic process when subjected to certain boundary conditions.

In a more fundamental approach, the empirical definition of temperature is derived from the conditions of thermal equilibrium, which are expressed at the zero principle of thermodynamics. When two systems are in thermal equilibrium they have the same temperature. The extension of this principle as an equivalence relation between several systems justifies basically the use of the thermometer and establishes the principles of its construction to measure the temperature. Although the zero law of thermodynamics would allow the empirical definition of many temperature scales, the second law of thermodynamics selects a single definition as the preferred one: absolute temperature, known as thermodynamic temperature.

This function corresponds to the variation of the internal energy with respect to the changes in the entropy of a system. Its natural, intrinsic or zero point origin is absolute zero, where the entropy of any system is minimal. Although this is the absolute minimum temperature described by the model, the third law of thermodynamics postulates that absolute zero can not be reached by any physical system.

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Last review: November 9, 2016

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