# Thermal energy

Thermal energy is kinetic energy due to disordered motions and vibrations of microscopic particles such as molecules and atoms.

 Contents

## Description

Thermal energy is quantified by temperature (e.g., the physical property of a system which underlies the common notions of "hot" and "cold"; material with the higher temperatures are said to be "hot"). Heat (also heat change) is the transfer of thermal energy between two bodies which are at different temperatures. The SI unit for heat is the joule.

Infrared radiation is often linked to thermal energy, since objects at room temperature or above will emit radiation mostly concentrated in the mid-infrared band (see black body and radiant energy). In real-world oscillator systems, the second law of thermodynamics dictates that there is some continual and inevitable conversion of energy into the thermal energy of the environment. Thus, damped oscillations tend to decay with time unless there is some net source of energy in the system. The simplest description of this decay process can be illustrated by the harmonic oscillator.

The first law of thermodynamics states that energy (such as thermal) can be converted into others without loss. The second law of thermodynamics states that thermal energy is special among the types of energies: all the forms of energy can be converted into heat, but in a way that is not reversible; it is not possible to convert the heat back fully in its original form. In other words, heat is a form of energy of lower quality.

## Utilization

Power stations (or power plants) generate electric power which usually involves some manner of heat engine. Usually, this transforms thermal energy, often from combustion of a fuel, into rotational energy. These power stations are often easily identified by cooling towers, huge cylindrical chimney-like structures that release the heat in the form of steam. A radioisotope thermoelectric generator is a very simple electrical generator which obtains its power from passive radioactive decay. The thermal energy is converted into electricity by an array of thermocouples which then provides power.

A power transfer (or energy transfer) is the process of passing working energy from one system or device to another (such as potential energy in coal transferred into thermal energy, or, thermal energy converted to kinetic energy in steam). Such transfers can be between energy types, as from electricity to mechanical motion, or within a type, from one mechanical motion to another. The amount of energy transfer depends on efficiency.

## Space programs

In a Nuclear electric rocket, nuclear thermal energy is changed into electrical energy that is used to power one of the electrical propulsion technologies. So technically the powerplant is nuclear, not the propulsion system, but the terminology is standard. A number of heat-to-electricity schemes have been proposed.

In the Voyager program, the bi-metallic thermocouples are used to convert thermal energy into electrical energy; working at about 80%. The power level represent better performance than the pre-launch predictions, which included a conservative degradation model for the thermocouples.

During atmospheric reentry, crafts rely mainly on the heat shield for protect for thermal energy. A spacecraft converts the craft's high kinetic energy into thermal energy (heat) by atmospheric friction. Any errors in this portion of the flight profile are difficult to recover from and will probably have serious impact upon the mission. Death and/or mission failures have occurred during re-entry. Nevertheless, the use of strong heat shields has so far been regarded as the only practical approach and all orbital returning spacecraft have been equipped with such. This unavoidably rapid conversion of a large amount of kinetic energy to heat results in extremely high temperatures, so the heat shield needs to be extremely strong and reliable.

Asteroseismology is the study of the internal structure of pulsating stars by the interpretation of their frequency spectra. The oscillations studied by asterioseismologists are driven by thermal energy converted into kinetic energy of pulsation. This process is similar to what goes on with any heat engine, in which heat is absorbed in the high temperature phase of oscillation and emitted when the temperature is low.

## Chemicals

The Arrhenius equation predicts the rate of a chemical reaction at a certain temperature and average amount of thermal energy that molecules possess at a certain temperature is equal to RT, where R is the molar gas constant. The fraction of molecules that have enough energy to overcome the energy barrier—those with energy over the activation energy, EA (joule)— depends exponentially on the ratio of the activation to thermal energy.

## Particles

Electrically charged particles, such as those in Charged particle beams, usually have the higher kinetic energies than the thermal energies of particles at ordinary temperatures. Thermonuclear has came to imply anything which has to do with fusion nuclear reactions which are triggered by "particles" of thermal energy.es:Energía térmica fr:énergie thermique

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