Thermal energy
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Thermal energy is the vibrational energy of the disordered motion of microscopic particles such as molecules and atoms.
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Description
Thermal energy is energy in the form of the vibrations and movements of microscopic particles. Because vibrations are as much potential energy as kinetic energy, thermal energy is simply the sum of both. Single particle gas only have kinetic energy (see ideal gas) which thus is the thermal energy of a single particle gas.
Temperature is the measure of thermal energy. Temperature is defined as the average amount of energy per degree of freedom of particle in a system of many particles. Because energy is different in different reference systems, temperature is usually defined versus the system of reference of center of mass. Because kinetic energy of a single particle in proper (=co-moving) reference frame is zero, temperature of a single particle is usually considered as undefined. Therefore, the concept of temperature is only applicable to a system (ensemble) of paticles. If the ensemble is small (few particles), then standard deviation of average energy is large (of the order of energy itself) - therefore the temperature of a small ensemble is very uncertain (by definition). The more particles the ensemble has, the more accurate a temperature of this ansemble is defined.
Heat is the time derivative of thermal energy.
Due to entropy increase law (second law of termodynamics - which is simply a mathematical consequenjce of a statistics), any form of energy can be converted into thermal energy with complete efficiency, but not vice versa. It's impossible to fully convert thermal energy into other form of energy. Because of this, thermal energy is often considered a less usable form of energy than any other. However, most engines first convert some other form of energy into heat and then convert heat into target energy (usually, kinetic or electric energy).
Infrared Radiation
Sometimes infrared (1-100 micrometers wavelength) radiation is linked to thermal energy. This is because objects at room temperature or a few times higher above will emit electromagnetic radiation peaking in the infrared band (see black body and radiant energy). Since hotter objects emit radiation peaking at a higher frequency and at much higher power (proportional to 4th degree of tempaerature, one way to detect the temperature of objects at room temperature is to detect the power of light (in infrared band) emitt.
Utilization
Power stations (or power plants) generate electric power 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 protection from thermal energy. A spacecraft converts the craft's jason thim 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 that the rate of a chemical reaction 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 come to imply anything related to nuclear fusion reactions, which are triggered by "particles" of thermal energy.
See also
es:Energía térmica fr:Énergie thermique he:אנרגיה תרמית pl:Energia termiczna