The thermal energy is the kinetic energy of an object, which is due to a disordered agitation of its Molécule S and of its Atome S. the transfers of thermal energy between body are called transfers of heat and play a crucial role in Thermodynamique. They reach a balance when the Température of the bodies in contact is equal.

A dissipative energy

Thermal energy tends to be distributed uniformly in the space. This natural mechanism of balance is one of the mechanisms stated in principle zero of the Thermodynamique, which includes/understands also mechanical balance (pressures balance), and chemical balance (the fluids mix).

The mechanisms of distribution of thermal energy are:

• By direct contact

• the thermal Conduction (transfer of the kinetic energy of the molecules without exchange of matter)
• the Convection (exchange of matter between fluids or gas)

The thermal dissipation of energy per contact occurs in a dissymmetrical way compared to the Temps, and is always made the hottest body (that whose Température is highest) towards the coldest body. This phenomenon is formalized in the second principle of the Thermodynamique and limit the possibilities of transforming energy into work. He was discovered at the time of the realization of the first Moteur S.

• Through the vacuum

• Emission of photons, by the phenomenon called radiation of the black body

A quantum phenomenon of origin on a macroscopic scale

Of share its nature, being the reflection of an activity at the molecular level, thermal energy is deeply quantum. In particular, this capacity to be transformed into luminous energy by the radiation of the black body is a direct testimony of the changes of state of the molecules and atoms composing the object. It is a phenomenon which we can observe daily, while observing of embers in a wood fire, or quite simply the light of an electric bulb.

A phenomenon at the origin of discovered laws of quantum mechanics

The precise study of the radiation of the black Body, and the laws determining the nature of the light emitted with conduit with paradoxes when it was made with the equations of the traditional Mécanique. It is while seeking to solve this paradox which Planck introduces the idea that the light was not emitted in a continuous way, but by packages. This assumption appeared true and is one of the bases of the quantum Mécanique.

This property of the matter to radiate its thermal energy has a considerable influence on our environment. It is by this phenomenon that the sun radiates part of its energy to the ground, there bringing an energy source essential with the conditions of existence of the life. Moreover, the living organisms evolved/moved to benefit from the information made available by these luminous rays, by working out the eye. The visible wavelengths luminous for the man, for example, are around the wavelength of maximum energy emitted by the radiation of the black Corps of the sun, which is located at 500 Nm.

Thermal energy and temperature

At the time of the setting in contact between two bodies, a thermal energy exchange occurs. The point of balance is reached when the two bodies reached same the Température. The concept of balance is a transitive concept. If a body has is in balance with B, and that this body B is in balance with a body C, then has and C are also in balance. With, B and C have the same temperature. At the beginning of the XXe century, it was judged that this law, which seems to hold of the simple good sense, deserved to be formulated like principle 0 of thermodynamics.

Although difficult to define formally, the temperature is a concept used in the everyday life, because easy to observe. To measure the temperature of a body, it is enough to put it in contact with a Thermomètre (for example a mercury thermometer), and to measure the graduation with thermal balance of it.

The mass heat capacity of a body is a value defined to express the quantity of thermal energy necessary to increase the temperature of this body of a Degree Celsius. The mass heat capacity depends in general on the composition of the body and its temperature.

Thermal energy and state of the matter

By providing thermal energy to a body, one can change the state it, making it pass for example from the solid state to the state Liquide.

In condition of pressure given, these changes of state are always done at the same temperature, for a body of a given composition. It is this property which was used, as well as considerations of origin Thermodynamique S to define the scale of temperature Kelvin, while being based on the changes of state of the Eau.

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