Donald Rumsfeld

The temperature of a system is an increasing function of the thermal degree of agitation of the particles, i.e. of thermal sound energy. It is defined by the balance of transfer of heat with other systems.

For example, when agitation is weak, the object is cold with the touch. This feeling is due to a transfer of heat of the fingers towards the object. It is measured by means of a Thermomètre and is studied by the Thermométrie.

The relationship between the thermal quantity of energy and the temperature is called the Specific heat.

Physical origin

The particles which compose a material system (Molécule S or Atome S) are not never at rest. They are in permanent vibration and thus have some kinetic energy. The temperature measurement indirectly by contact the microscopic degree of agitation of the particles.

In addition, an empty matter space but in which Lumière is propagated contains to him also energy. Under good conditions, one can associate a temperature with this radiation which measures the average energy of the particles which constitute it. An important example of thermal radiation is that of the black Corps whose spectacular example observed in nature is the cosmological diffuse Fond.

When one puts two bodies in contact, they exchange thermal energy spontaneously: one of the two bodies has particles which have more kinetic energy, by putting them in contact, the shocks between particles make that this kinetic energy microscopic is transmitted of one body to the other. It is this transfer of energy which, in physical sciences, is called Chaleur. These transfers of energy spontaneously lead to a state of thermal balance where the two involved bodies have the same temperature.

The room temperature is the temperature of the environment, i.e. all the universe except the system considered.

Nevertheless, in practice, in the fields of the Physical and the Chemistry, it is current of speaking room temperature for a current, average temperature. For example, one says “ water is liquid with the room temperature ”. But this denomination is not very formalized and the value of the room temperature is seldom specified (generally evaluated in a way common to 25°C).

Thermodynamic temperature

The fact is that all bodies in contact having different temperatures exchange energy so as to reach a state of balance for which the final temperatures are identical. It is thermal balance. It is thus enough to know and know to quantify perfectly the thermal state of a class of physical body particular to extend the definition of temperature to all the physical objects. It is what is made with the concept of Perfect gas: for a Perfect gas, the temperature (in this kinetic case called ) is proportional to the average kinetic energy of the particles or with the energy gas interns.

One can then define the thermodynamic temperature of a body as being that which it acquires when it is in contact and thermal balance with a perfect gas. It is the principle of thermometry.

In a more mathematical and formal way, the thermodynamic temperature is connected to the concept of Entropie. One can thus define for a thermodynamic Système only subjected to the compressive forces: $T = \ left (\ frac {\ partial U} {\ partial S} \ right) _V$

Unit and scales of measurement

The legal unit of temperature in the international Système is the Kelvin of symbol K (to note the absence of the symbol ° because it is not a scale of measurement). There exist other systems of measurement former and always used: Centigrade scales centigrade, Fahrenheit and Rankine.

; the Kelvin: it is defined starting from the Point triples water: a Kelvin is equal to 1/273,16 times the temperature of the triple point of water. The Absolute zero, would correspond to the limit with an complete absence of microscopic agitation and a temperature of -273,15 °C; but one can never reach it (To think that the physical entity is rather 1/T, and one can never reach the infinite one). This unit makes it possible to define an absolute scale of the temperatures.

Conversion

One can establish correspondences between these three scales:

General formulas of conversion:

Centigrade $\ Longleftrightarrow$ Kelvin $\ Longleftrightarrow$ Fahrenheit:

$\ frac {T_ {Centigrade} - 0} {100} = \ frac {T_ {Kelvin} - 273,15} {100} = \ frac {T_ {Fahrenheit} - 32} {180} \,$

Centigrade $\ Longleftrightarrow$ Réaumur $\ Longleftrightarrow$ Rankine:

$\ frac {T_ {Centigrade} - 0} {100} = \ frac {T_ {Reaumur} - 0} {80} = \ frac {T_ {Rankine} - 32 - 459,67} {180} \,$

kelvin $\ Centigrade Longleftrightarrow$ degré:

$T_ {Centigrade} = T_ {Kelvin} - 273,15 \,$

$T_ {Kelvin} = T_ {Centigrade} + 273,15 \,$

kelvin $\ Longleftrightarrow$ degré Fahrenheit:

$T_ {Fahrenheit} = \ frac {9} {5} \ cdot T_ {Kelvin} - 459,67$

$T_ {Kelvin} = \ frac {5} {9} \ cdot (T_ {Fahrenheit} + 459,67)$

degree celsius $\ Longleftrightarrow$ degré Fahrenheit:

$T_ {Fahrenheit} = 32 + \ frac {9} {5} \ cdot T_ {Centigrade}$

$T_ {Centigrade} = \ frac {5} {9} \ cdot (T_ {Fahrenheit} - 32)$

Comparison of the scales of temperature

Other fields

In the field of the Meteorology, the temperature is written T° and one speaks about wind T° , to express the temperature felt under the effect of the wind, so known under subjective temperature, impression of heat or cold, or temperature with the wind, to see in detail the wind Refroidissement.

The temperature dries corresponds to the traditional temperature given by a thermometer but protected from moisture and radiations. On a diagram of the Humid air, the constant temperature curve is a vertical line.

One measures the wet Temperature with a thermometer on which water evaporates. One generally uses of the wet foam which one ventilates. The wet temperature is always lower than the dry temperature; they are all the more equal as the relative Humidité is close to 100%.L' practical illustration of these measurements are made for example in a psychrometer Pescara

In medicine one measures the body Température .

For the Body (entity) composed of several phase S (Ex : Air humide : Liquide in a Gaz) one speaks about adiabatic Gradient .

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