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# Temperature, Heat, Expansion

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(The Sun, the main source of heat on Earth.)

## Temperature

Temperature is a physical quantity measured using a thermometer and studied in thermometry. In everyday life, it is connected to the sensations of cold and heat, coming from the heat transfer between the human body and its environment. In physics, it is defined in several ways: as an increasing function of the degree of thermal agitation of the particles (in the kinetic theory of gases), by the equilibrium of thermal transfers between several systems or from entropy (in thermodynamics and in statistical physics). Temperature is an important variable in other disciplines: meteorology and climatology, medicine, and chemistry.

The most common temperature scale is the degree Celsius, in which the ice (formed of water) melts at 0 °C and the water boils at about +100 °C under standard pressure conditions. In countries using the imperial (Anglo-Saxon) system of units, the degree Fahrenheit is used where the ice melts at +32 °F and the water boils at +212 °F. The unit of the International System of Units (SI), of scientific use and defined from absolute zero, is kelvin, whose graduation is almost identical to that of the degrees centigrade.

The particles that make up matter (molecules or atoms) are never at rest. They are in permanent vibration and therefore have a certain kinetic energy. Temperature is an indirect measure of the degree of microscopic agitation of the particles. On the other hand, an empty space of matter but in which light propagates itself also contains energy. In good conditions, a temperature can be associated with this radiation, which measures the average energy of the particles that constitute it. An important example of thermal radiation is that of the black body, an example of which is given by stars whose radiation reveals the temperature of the atoms on its surface.

When two bodies come into contact, they spontaneously exchange thermal energy: one of the two bodies has particles that have more kinetic energy, putting them in contact, the shocks between particles make this microscopic kinetic energy transmits from one body to another. It is this transfer of energy that in the physical sciences is called heat. Thus, the greater the kinetic energy difference between two particles, the more they exchange energy. More precisely, the more the bodies of a system have velocity values ​​that are far from the average velocity of the considered system (large deviation), the higher the temperature of the system will be. necessary.

## Heat

Heat is the amount of energy that passes from a warmer object to a cooler one. Generally, heat comes from many microscopic scale modifications of objects and can be defined as the amount of energy transferred, excluding both macroscopic mechanical work and the transfer of a part of the object itself. Heat transfer may occur through contact or a common wall that is impermeable to the material between the source and the body of destination as well as the conduction; or by radiation between bodies at a distance; or through an intermediate fluid body as well as in the convective circulation; or by a combination thereof. In thermodynamics, heat is often contrary to mechanical work: heat is applied to individual particles (such as atoms or molecules), mechanical work applies to objects (or a system as a whole). Heat involves the stochastic (or random) movement distributed evenly between all degrees of freedom, while the mechanical work is directional, limited to one or more specific degrees of freedom.

Since the heat (as well as mechanical work) represents an amount of energy transferred between two bodies through certain processes, no body “has” a certain amount of heat (just as a body itself does not have “mechanical work”); Instead, a body really has properties (state functions), such as temperature and internal energy. Thus, energy exchanged in the form of heat during a given process changes the (internal) energy of each body with equal and opposite values. The heat quantity sign may indicate the direction of the transfer, for example from system A to system B; the minus sign indicates that the energy flows in the opposite direction.

Although the heat spontaneously flows from a hot to a cooler body, it is possible to build a heat pump or cooling system that works to increase the temperature difference between two systems. Instead, a thermal engine reduces an existing temperature difference to work on another system.

Heat is a consequence of the microscopic movement of particles (the kinetic energy of atoms and molecules). When heat is transferred between two objects or systems, the energy of the object or system particles increases. While this occurs, the particle arrangement becomes increasingly messy. In other words, the heat is related to the concept of entropy.

From a historical point of view, many energy units have been used to measure heat. The unit based on standards in the International System of Units (SI) is Joule (J). Heat is measured by its effect on the states of the bodies in interaction, for example by the amount of ice melted or by changing the temperature. Quantification of heat by changing the temperature of a body is called calorimetry and is widely used in practice. In calorimetry, sensitive heat is defined in relation to a certain system state variable such as pressure or volume. Sensitive heat causes a change in the temperature of the system, leaving unchanged the variable of the chosen state. The heat transfer that takes place at a constant temperature of the system but changes the state variable is called latent heat in relation to the variable. For infinitesimal changes, the total heat transfer is then the sum of the latent and sensitive heat.

## Expansion

Thermal expansion is the expansion at constant pressure of the volume of a body caused by its warming, usually imperceptible. In the case of a gas, there is expansion at constant pressure or maintenance of the volume and increase of the pressure as the temperature increases.

(Interatomic potential.)

In a solid, atoms have a thermal energy and vibrate around their average position. This vibration depends on the temperature but also on the vicinity of the atoms, more precisely on the interatomic potential created by the surrounding atoms.

At low temperatures, the interatomic potentials can be described in a harmonic way: for temperatures close to T = 0 K, the atoms remain centered on their average position r0. This is no longer the case for high temperatures: the anharmonicity of the interatomic potentials introduces a dependence of the average position of the atoms with the temperature, which causes the phenomenon of thermal expansion.

When a gas is subjected to a warming, the amount of movement of the particles that compose it increases. At constant volume, this results in an increase in pressure because the number of shocks between particles per unit area increases. If the pressure must remain constant, the volume of the gas must then increase, according to the law of perfect gases. For non-perfect gases, the attractive forces between the gas particles can reduce thermal expansion.

The thermal expansion of liquids in principle has the same causes as that of gases, but the effect of the attraction forces between the particles on the expansion is significantly increased because they are closer to each other.