166-445: Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter . All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electronic, molecular, and lattice oscillations in a material. Kinetic energy is converted to electromagnetism due to charge-acceleration or dipole oscillation. At room temperature , most of
332-435: A magnetic-dipole –type that dies out with distance from the current. In a similar manner, moving charges pushed apart in a conductor by a changing electrical potential (such as in an antenna) produce an electric-dipole –type electrical field, but this also declines with distance. These fields make up the near field. Neither of these behaviours is responsible for EM radiation. Instead, they only efficiently transfer energy to
498-422: A microwave oven . These interactions produce either electric currents or heat, or both. Like radio and microwave, infrared (IR) also is reflected by metals (and also most EMR, well into the ultraviolet range). However, unlike lower-frequency radio and microwave radiation, Infrared EMR commonly interacts with dipoles present in single molecules, which change as atoms vibrate at the ends of a single chemical bond. It
664-405: A thermodynamic operation be isolated, and upon the event of isolation, no change occurs in it. A system in a relation of contact equilibrium with another system may thus also be regarded as being in its own state of internal thermodynamic equilibrium. The thermodynamic formalism allows that a system may have contact with several other systems at once, which may or may not also have mutual contact,
830-503: A thermodynamic operation . In a macroscopic equilibrium, perfectly or almost perfectly balanced microscopic exchanges occur; this is the physical explanation of the notion of macroscopic equilibrium. A thermodynamic system in a state of internal thermodynamic equilibrium has a spatially uniform temperature. Its intensive properties , other than temperature, may be driven to spatial inhomogeneity by an unchanging long-range force field imposed on it by its surroundings. In systems that are at
996-461: A transverse wave , where the electric field E and the magnetic field B are both perpendicular to the direction of wave propagation. The electric and magnetic parts of the field in an electromagnetic wave stand in a fixed ratio of strengths to satisfy the two Maxwell equations that specify how one is produced from the other. In dissipation-less (lossless) media, these E and B fields are also in phase, with both reaching maxima and minima at
1162-611: A wave form of the electric and magnetic equations , thus uncovering the wave-like nature of electric and magnetic fields and their symmetry . Because the speed of EM waves predicted by the wave equation coincided with the measured speed of light , Maxwell concluded that light itself is an EM wave. Maxwell's equations were confirmed by Heinrich Hertz through experiments with radio waves. Maxwell's equations established that some charges and currents ( sources ) produce local electromagnetic fields near them that do not radiate. Currents directly produce magnetic fields, but such fields of
1328-903: A "scholarly and rigorous treatment", and cited by Adkins as having written a "classic text", A.B. Pippard writes in that text: "Given long enough a supercooled vapour will eventually condense, ... . The time involved may be so enormous, however, perhaps 10 years or more, ... . For most purposes, provided the rapid change is not artificially stimulated, the systems may be regarded as being in equilibrium." Another author, A. Münster, writes in this context. He observes that thermonuclear processes often occur so slowly that they can be ignored in thermodynamics. He comments: "The concept 'absolute equilibrium' or 'equilibrium with respect to all imaginable processes', has therefore, no physical significance." He therefore states that: "... we can consider an equilibrium only with respect to specified processes and defined experimental conditions." According to L. Tisza : "... in
1494-413: A black body is the body's emissivity , so a black body has an emissivity of one. Absorptivity, reflectivity , and emissivity of all bodies are dependent on the wavelength of the radiation. Due to reciprocity , absorptivity and emissivity for any particular wavelength are equal at equilibrium – a good absorber is necessarily a good emitter, and a poor absorber is a poor emitter. The temperature determines
1660-727: A blackbody, I λ , b {\displaystyle I_{\lambda ,b}} was first determined by Max Planck. It is given by Planck's law per unit wavelength as: I λ , b ( λ , T ) = 2 h c 2 λ 5 ⋅ 1 e h c / k B T λ − 1 {\displaystyle I_{\lambda ,b}(\lambda ,T)={\frac {2hc^{2}}{\lambda ^{5}}}\cdot {\frac {1}{e^{hc/k_{\rm {B}}T\lambda }-1}}} This formula mathematically follows from calculation of spectral distribution of energy in quantized electromagnetic field which
1826-581: A bulk collection of charges which are spread out over large numbers of affected atoms. In electrical conductors , such induced bulk movement of charges ( electric currents ) results in absorption of the EMR, or else separations of charges that cause generation of new EMR (effective reflection of the EMR). An example is absorption or emission of radio waves by antennas, or absorption of microwaves by water or other molecules with an electric dipole moment, as for example inside
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#17327722210141992-411: A certain minimum frequency, which depended on the particular metal, no current would flow regardless of the intensity. These observations appeared to contradict the wave theory, and for years physicists tried in vain to find an explanation. In 1905, Einstein explained this puzzle by resurrecting the particle theory of light to explain the observed effect. Because of the preponderance of evidence in favor of
2158-409: A common temperature, a total internal energy, and a total entropy. Amongst intensive variables, this is a unique property of temperature. It holds even in the presence of long-range forces. (That is, there is no "force" that can maintain temperature discrepancies.) For example, in a system in thermodynamic equilibrium in a vertical gravitational field, the pressure on the top wall is less than that on
2324-605: A component wave is said to be monochromatic . A monochromatic electromagnetic wave can be characterized by its frequency or wavelength, its peak amplitude, its phase relative to some reference phase, its direction of propagation, and its polarization. Interference is the superposition of two or more waves resulting in a new wave pattern. If the fields have components in the same direction, they constructively interfere, while opposite directions cause destructive interference. Additionally, multiple polarization signals can be combined (i.e. interfered) to form new states of polarization, which
2490-476: A constant temperature. However, it does require that each small locality change slowly enough to practically sustain its local Maxwell–Boltzmann distribution of molecular velocities. A global non-equilibrium state can be stably stationary only if it is maintained by exchanges between the system and the outside. For example, a globally-stable stationary state could be maintained inside the glass of water by continuously adding finely powdered ice into it to compensate for
2656-409: A continuous spectrum of photon energies, its characteristic spectrum. If the radiating body and its surface are in thermodynamic equilibrium and the surface has perfect absorptivity at all wavelengths, it is characterized as a black body . A black body is also a perfect emitter. The radiation of such perfect emitters is called black-body radiation . The ratio of any body's emission relative to that of
2822-499: A fluorescence on a nearby plate of coated glass. In one month, he discovered X-rays' main properties. The last portion of the EM spectrum to be discovered was associated with radioactivity . Henri Becquerel found that uranium salts caused fogging of an unexposed photographic plate through a covering paper in a manner similar to X-rays, and Marie Curie discovered that only certain elements gave off these rays of energy, soon discovering
2988-576: A higher energy (and hence shorter wavelength) than gamma rays and vice versa. The origin of the ray differentiates them, gamma rays tend to be natural phenomena originating from the unstable nucleus of an atom and X-rays are electrically generated (and hence man-made) unless they are as a result of bremsstrahlung X-radiation caused by the interaction of fast moving particles (such as beta particles) colliding with certain materials, usually of higher atomic numbers. EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields )
3154-406: A letter describing his experiments on the relationship between color and heat absorption. He found that darker color clothes got hotter when exposed to sunlight than lighter color clothes. One experiment he performed consisted of placing square pieces of cloth of various colors out in the snow on a sunny day. He waited some time and then measured that the black pieces sank furthest into the snow of all
3320-528: A linear medium such as a vacuum. However, in nonlinear media, such as some crystals , interactions can occur between light and static electric and magnetic fields—these interactions include the Faraday effect and the Kerr effect . In refraction , a wave crossing from one medium to another of different density alters its speed and direction upon entering the new medium. The ratio of the refractive indices of
3486-539: A lower energy level, it emits a photon of light at a frequency corresponding to the energy difference. Since the energy levels of electrons in atoms are discrete, each element and each molecule emits and absorbs its own characteristic frequencies. Immediate photon emission is called fluorescence , a type of photoluminescence . An example is visible light emitted from fluorescent paints, in response to ultraviolet ( blacklight ). Many other fluorescent emissions are known in spectral bands other than visible light. Delayed emission
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#17327722210143652-410: A mathematical description of thermal equilibrium (i.e. Kirchhoff's law of thermal radiation ). By 1884 the emissive power of a perfect blackbody was inferred by Josef Stefan using John Tyndall 's experimental measurements, and derived by Ludwig Boltzmann from fundamental statistical principles. This relation is known as Stefan–Boltzmann law . The microscopic theory of radiation is best known as
3818-454: A mildly dull red color, whether or not a chemical reaction takes place that produces light as a result of an exothermic process. This limit is called the Draper point . The incandescence does not vanish below that temperature, but it is too weak in the visible spectrum to be perceptible. The rate of electromagnetic radiation emitted by a body at a given frequency is proportional to the rate that
3984-424: A natural thermodynamic process . It is allowed in equilibrium thermodynamics just because the initial and final states are of thermodynamic equilibrium, even though during the process there is transient departure from thermodynamic equilibrium, when neither the system nor its surroundings are in well defined states of internal equilibrium. A natural process proceeds at a finite rate for the main part of its course. It
4150-630: A non-uniform force field but is held stationary there by local forces, such as mechanical pressures, on its surface. Thermodynamic equilibrium is a primitive notion of the theory of thermodynamics. According to P.M. Morse : "It should be emphasized that the fact that there are thermodynamic states, ..., and the fact that there are thermodynamic variables which are uniquely specified by the equilibrium state ... are not conclusions deduced logically from some philosophical first principles. They are conclusions ineluctably drawn from more than two centuries of experiments." This means that thermodynamic equilibrium
4316-421: A particular kind of permeability, they have common values of the intensive variable that belongs to that particular kind of permeability. Examples of such intensive variables are temperature, pressure, chemical potential. A contact equilibrium may be regarded also as an exchange equilibrium. There is a zero balance of rate of transfer of some quantity between the two systems in contact equilibrium. For example, for
4482-417: A particular star. Spectroscopy is also used in the determination of the distance of a star, using the red shift . When any wire (or other conducting object such as an antenna ) conducts alternating current , electromagnetic radiation is propagated at the same frequency as the current. As a wave, light is characterized by a velocity (the speed of light ), wavelength , and frequency . As particles, light
4648-414: A perfectly specular or a diffuse manner. In a specular reflection , the angles of reflection and incidence are equal. In diffuse reflection , radiation is reflected equally in all directions. Reflection from smooth and polished surfaces can be assumed to be specular reflection, whereas reflection from rough surfaces approximates diffuse reflection. In radiation analysis a surface is defined as smooth if
4814-490: A point of contention for the theory as a whole. In his first memoir, Augustin-Jean Fresnel responded to a view he extracted from a French translation of Isaac Newton 's Optics . He says that Newton imagined particles of light traversing space uninhibited by the caloric medium filling it, and refutes this view (never actually held by Newton) by saying that a body under illumination would increase indefinitely in heat. In Marc-Auguste Pictet 's famous experiment of 1790 , it
4980-466: A radiation object meets the physical characteristics of a black body in thermodynamic equilibrium . Planck's law describes the spectrum of blackbody radiation, and relates the radiative heat flux from a body to its temperature. Wien's displacement law determines the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the radiant intensity. Where blackbody radiation
5146-417: A receiver very close to the source, such as inside a transformer . The near field has strong effects its source, with any energy withdrawn by a receiver causing increased load (decreased electrical reactance ) on the source. The near field does not propagate freely into space, carrying energy away without a distance limit, but rather oscillates, returning its energy to the transmitter if it is not absorbed by
Thermal radiation - Misplaced Pages Continue
5312-467: A receiver. By contrast, the far field is composed of radiation that is free of the transmitter, in the sense that the transmitter requires the same power to send changes in the field out regardless of whether anything absorbs the signal, e.g. a radio station does not need to increase its power when more receivers use the signal. This far part of the electromagnetic field is electromagnetic radiation. The far fields propagate (radiate) without allowing
5478-558: A semi-sphere region. The energy, E = h ν {\displaystyle E=h\nu } , of each photon is multiplied by the number of states available at that frequency, and the probability that each of those states will be occupied. The Planck distribution can be used to find the spectral emissive power of a blackbody, E λ , b {\displaystyle E_{\lambda ,b}} as follows, Electromagnetic radiation In physics , electromagnetic radiation ( EMR ) consists of waves of
5644-404: A single phase in the absence of external forces, in its own internal thermodynamic equilibrium, is homogeneous. This means that the material in any small volume element of the system can be interchanged with the material of any other geometrically congruent volume element of the system, and the effect is to leave the system thermodynamically unchanged. In general, a strong external force field makes
5810-578: A solid ice block. Della Porta's experiment would be replicated many times with increasing accuracy. It was replicated by astronomers Giovanni Antonio Magini and Christopher Heydon in 1603, and supplied instructions for Rudolf II, Holy Roman Emperor who performed it in 1611. In 1660, della Porta's experiment was updated by the Accademia del Cimento using a thermometer invented by Ferdinand II, Grand Duke of Tuscany . In 1761, Benjamin Franklin wrote
5976-574: A special term 'thermal equilibrium'. J.R. Waldram writes of "a definite thermodynamic state". He defines the term "thermal equilibrium" for a system "when its observables have ceased to change over time". But shortly below that definition he writes of a piece of glass that has not yet reached its " full thermodynamic equilibrium state". Considering equilibrium states, M. Bailyn writes: "Each intensive variable has its own type of equilibrium." He then defines thermal equilibrium, mechanical equilibrium, and material equilibrium. Accordingly, he writes: "If all
6142-586: A state of non-equilibrium there are, by contrast, net flows of matter or energy. If such changes can be triggered to occur in a system in which they are not already occurring, the system is said to be in a "meta-stable equilibrium". Though not a widely named "law," it is an axiom of thermodynamics that there exist states of thermodynamic equilibrium. The second law of thermodynamics states that when an isolated body of material starts from an equilibrium state, in which portions of it are held at different states by more or less permeable or impermeable partitions, and
6308-436: A surface can propagate in any direction from the surface. Irradiation can also be incident upon a surface from any direction. The amount of irradiation on a surface is therefore dependent on the relative orientation of both the emitter and the receiver. The parameter radiation intensity, I {\displaystyle I} is used to quantify how much radiation makes it from one surface to another. Radiation intensity
6474-400: A surface layer of caloric fluid which insulated the release of the rest within. He described a good radiator to be a substance with a rough surface as only a small proportion of molecules held caloric in within a given plane, allowing for greater escape from within. Count Rumford would later cite this explanation of caloric movement as insufficient to explain the radiation of cold, which became
6640-418: A thermodynamic operation removes or makes the partitions more permeable, then it spontaneously reaches its own new state of internal thermodynamic equilibrium and this is accompanied by an increase in the sum of the entropies of the portions. Classical thermodynamics deals with states of dynamic equilibrium . The state of a system at thermodynamic equilibrium is the one for which some thermodynamic potential
6806-426: A thermodynamic system may also be regarded as another thermodynamic system. In this view, one may consider the system and its surroundings as two systems in mutual contact, with long-range forces also linking them. The enclosure of the system is the surface of contiguity or boundary between the two systems. In the thermodynamic formalism, that surface is regarded as having specific properties of permeability. For example,
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6972-445: A third type of radiation, which in 1903 Rutherford named gamma rays . In 1910 British physicist William Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914 Rutherford and Edward Andrade measured their wavelengths, finding that they were similar to X-rays but with shorter wavelengths and higher frequency, although a 'cross-over' between X and gamma rays makes it possible to have X-rays with
7138-431: A very large (ideally infinite) distance from the source. Both types of waves can have a waveform which is an arbitrary time function (so long as it is sufficiently differentiable to conform to the wave equation). As with any time function, this can be decomposed by means of Fourier analysis into its frequency spectrum , or individual sinusoidal components, each of which contains a single frequency, amplitude and phase. Such
7304-653: A wall permeable only to heat defines an empirical temperature. A contact equilibrium can exist for each chemical constituent of the system of interest. In a contact equilibrium, despite the possible exchange through the selectively permeable wall, the system of interest is changeless, as if it were in isolated thermodynamic equilibrium. This scheme follows the general rule that "... we can consider an equilibrium only with respect to specified processes and defined experimental conditions." Thermodynamic equilibrium for an open system means that, with respect to every relevant kind of selectively permeable wall, contact equilibrium exists when
7470-434: A wall permeable only to heat, the rates of diffusion of internal energy as heat between the two systems are equal and opposite. An adiabatic wall between the two systems is 'permeable' only to energy transferred as work; at mechanical equilibrium the rates of transfer of energy as work between them are equal and opposite. If the wall is a simple wall, then the rates of transfer of volume across it are also equal and opposite; and
7636-467: A wave is its rate of oscillation and is measured in hertz , the SI unit of frequency, where one hertz is equal to one oscillation per second. Light usually has multiple frequencies that sum to form the resultant wave. Different frequencies undergo different angles of refraction, a phenomenon known as dispersion . A monochromatic wave (a wave of a single frequency) consists of successive troughs and crests, and
7802-420: A wide range of frequencies. The frequency distribution is given by Planck's law of black-body radiation for an idealized emitter as shown in the diagram at top. The dominant frequency (or color) range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases. For example, a red hot object radiates mainly in the long wavelengths (red and orange) of the visible band. If it
7968-522: Is quantized and proportional to frequency according to Planck's equation E = hf , where E is the energy per photon, f is the frequency of the photon, and h is the Planck constant . Thus, higher frequency photons have more energy. For example, a 10 Hz gamma ray photon has 10 times the energy of a 10 Hz extremely low frequency radio wave photon. The effects of EMR upon chemical compounds and biological organisms depend both upon
8134-403: Is a body which has the property of allowing all incident rays to enter without surface reflection and not allowing them to leave again. Blackbodies are idealized surfaces that act as the perfect absorber and emitter. They serve as the standard against which real surfaces are compared when characterizing thermal radiation. A blackbody is defined by three characteristics: The spectral intensity of
8300-472: Is a more subtle affair. Some experiments display both the wave and particle natures of electromagnetic waves, such as the self-interference of a single photon . When a single photon is sent through an interferometer , it passes through both paths, interfering with itself, as waves do, yet is detected by a photomultiplier or other sensitive detector only once. A quantum theory of the interaction between electromagnetic radiation and matter such as electrons
8466-404: Is a stream of photons . Each has an energy related to the frequency of the wave given by Planck's relation E = hf , where E is the energy of the photon, h is the Planck constant , 6.626 × 10 J·s, and f is the frequency of the wave. In a medium (other than vacuum), velocity factor or refractive index are considered, depending on frequency and application. Both of these are ratios of
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#17327722210148632-474: Is a type of electromagnetic radiation which is often modeled by the propagation of waves. These waves have the standard wave properties of frequency, ν {\displaystyle \nu } and wavelength , λ {\displaystyle \lambda } which are related by the equation λ = c ν {\displaystyle \lambda ={\frac {c}{\nu }}} where c {\displaystyle c}
8798-436: Is an axiomatic concept of thermodynamics . It is an internal state of a single thermodynamic system , or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls . In thermodynamic equilibrium, there are no net macroscopic flows of matter nor of energy within a system or between systems. In a system that is in its own state of internal thermodynamic equilibrium, not only
8964-537: Is any other state of a given system. This is partly, but not entirely, because all flows within and through the system are zero. R. Haase's presentation of thermodynamics does not start with a restriction to thermodynamic equilibrium because he intends to allow for non-equilibrium thermodynamics. He considers an arbitrary system with time invariant properties. He tests it for thermodynamic equilibrium by cutting it off from all external influences, except external force fields. If after insulation, nothing changes, he says that
9130-489: Is ascribed to astronomer William Herschel . Herschel published his results in 1800 before the Royal Society of London . Herschel used a prism to refract light from the sun and detected the calorific rays, beyond the red part of the spectrum, by an increase in the temperature recorded on a thermometer in that region. At the end of the 19th century it was shown that the transmission of light or of radiant heat
9296-523: Is associated with those EM waves that are free to propagate themselves ("radiate") without the continuing influence of the moving charges that produced them, because they have achieved sufficient distance from those charges. Thus, EMR is sometimes referred to as the far field , while the near field refers to EM fields near the charges and current that directly produced them, specifically electromagnetic induction and electrostatic induction phenomena. In quantum mechanics , an alternate way of viewing EMR
9462-541: Is between the system of interest and a system in the surroundings, brought into contact with the system of interest, the contact being through a special kind of wall; for the rest, the whole joint system is isolated. Walls of this special kind were also considered by C. Carathéodory , and are mentioned by other writers also. They are selectively permeable. They may be permeable only to mechanical work, or only to heat, or only to some particular chemical substance. Each contact equilibrium defines an intensive parameter; for example,
9628-422: Is called phosphorescence . The modern theory that explains the nature of light includes the notion of wave–particle duality. Together, wave and particle effects fully explain the emission and absorption spectra of EM radiation. The matter-composition of the medium through which the light travels determines the nature of the absorption and emission spectrum. These bands correspond to the allowed energy levels in
9794-563: Is classified by wavelength into radio , microwave , infrared , visible , ultraviolet , X-rays and gamma rays . Arbitrary electromagnetic waves can be expressed by Fourier analysis in terms of sinusoidal waves ( monochromatic radiation ), which in turn can each be classified into these regions of the EMR spectrum. For certain classes of EM waves, the waveform is most usefully treated as random , and then spectral analysis must be done by slightly different mathematical techniques appropriate to random or stochastic processes . In such cases,
9960-669: Is consequently absorbed by a wide range of substances, causing them to increase in temperature as the vibrations dissipate as heat. The same process, run in reverse, causes bulk substances to radiate in the infrared spontaneously (see thermal radiation section below). Infrared radiation is divided into spectral subregions. While different subdivision schemes exist, the spectrum is commonly divided as near-infrared (0.75–1.4 μm), short-wavelength infrared (1.4–3 μm), mid-wavelength infrared (3–8 μm), long-wavelength infrared (8–15 μm) and far infrared (15–1000 μm). Thermodynamic equilibrium Thermodynamic equilibrium
10126-418: Is defined by a rigid volume in space. It may lie within external fields of force, determined by external factors of far greater extent than the system itself, so that events within the system cannot in an appreciable amount affect the external fields of force. The system can be in thermodynamic equilibrium only if the external force fields are uniform, and are determining its uniform acceleration, or if it lies in
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#173277222101410292-729: Is described by the theory of quantum electrodynamics . Electromagnetic waves can be polarized , reflected, refracted, or diffracted , and can interfere with each other. In homogeneous, isotropic media, electromagnetic radiation is a transverse wave , meaning that its oscillations are perpendicular to the direction of energy transfer and travel. It comes from the following equations : ∇ ⋅ E = 0 ∇ ⋅ B = 0 {\displaystyle {\begin{aligned}\nabla \cdot \mathbf {E} &=0\\\nabla \cdot \mathbf {B} &=0\end{aligned}}} These equations predicate that any electromagnetic wave must be
10458-483: Is essential for the strict meaning of thermodynamic equilibrium. A student textbook by F.H. Crawford has a section headed "Thermodynamic Equilibrium". It distinguishes several drivers of flows, and then says: "These are examples of the apparently universal tendency of isolated systems toward a state of complete mechanical, thermal, chemical, and electrical—or, in a single word, thermodynamic—equilibrium. " A monograph on classical thermodynamics by H.A. Buchdahl considers
10624-413: Is heated further, it also begins to emit discernible amounts of green and blue light, and the spread of frequencies in the entire visible range cause it to appear white to the human eye; it is white hot . Even at a white-hot temperature of 2000 K, 99% of the energy of the radiation is still in the infrared. This is determined by Wien's displacement law . In the diagram the peak value for each curve moves to
10790-451: Is in complete thermal equilibrium with the radiating object. Planck's law shows that radiative energy increases with temperature, and explains why the peak of an emission spectrum shifts to shorter wavelengths at higher temperatures. It can also be found that energy emitted at shorter wavelengths increases more rapidly with temperature relative to longer wavelengths. The equation is derived as an infinite sum over all possible frequencies in
10956-603: Is in units of steradians and I {\displaystyle I} is the total intensity. The total emissive power can also be found by integrating the spectral emissive power over all possible wavelengths. This is calculated as, E = ∫ 0 ∞ E λ ( λ ) d λ {\displaystyle E=\int _{0}^{\infty }E_{\lambda }(\lambda )d\lambda } where λ {\displaystyle \lambda } represents wavelength. The spectral emissive power can also be determined from
11122-422: Is its frequency. Bodies at higher temperatures emit radiation at higher frequencies with an increasing energy per quantum. While the propagation of electromagnetic waves of all wavelengths is often referred as "radiation", thermal radiation is often constrained to the visible and infrared regions. For engineering purposes, it may be stated that thermal radiation is a form of electromagnetic radiation which varies on
11288-762: Is known as Kirchhoff's law of thermal radiation . An object is called a black body if this holds for all frequencies, and the following formula applies: If objects appear white (reflective in the visual spectrum ), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared – see the diagram at the left. Most household radiators are painted white, which is sensible given that they are not hot enough to radiate any significant amount of heat, and are not designed as thermal radiators at all – instead, they are actually convectors , and painting them matt black would make little difference to their efficacy. Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature (meaning
11454-453: Is known as parallel polarization state generation . The energy in electromagnetic waves is sometimes called radiant energy . An anomaly arose in the late 19th century involving a contradiction between the wave theory of light and measurements of the electromagnetic spectra that were being emitted by thermal radiators known as black bodies . Physicists struggled with this problem unsuccessfully for many years, and it later became known as
11620-544: Is minimized (in the absence of an applied voltage), or for which the entropy ( S ) is maximized, for specified conditions. One such potential is the Helmholtz free energy ( A ), for a closed system at constant volume and temperature (controlled by a heat bath): Another potential, the Gibbs free energy ( G ), is minimized at thermodynamic equilibrium in a closed system at constant temperature and pressure, both controlled by
11786-412: Is no equilibrated neighborhood, the concept of temperature doesn't hold, and the temperature becomes undefined. This local equilibrium may apply only to a certain subset of particles in the system. For example, LTE is usually applied only to massive particles . In a radiating gas, the photons being emitted and absorbed by the gas do not need to be in a thermodynamic equilibrium with each other or with
11952-436: Is not an accurate approximation, emission and absorption can be modeled using quantum electrodynamics (QED). Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero . Thermal radiation reflects the conversion of thermal energy into electromagnetic energy . Thermal energy is the kinetic energy of random movements of atoms and molecules in matter. It
12118-430: Is not to be defined solely in terms of other theoretical concepts of thermodynamics. M. Bailyn proposes a fundamental law of thermodynamics that defines and postulates the existence of states of thermodynamic equilibrium. Textbook definitions of thermodynamic equilibrium are often stated carefully, with some reservation or other. For example, A. Münster writes: "An isolated system is in thermodynamic equilibrium when, in
12284-417: Is often modeled using a spherical coordinate system . Emissive power is the rate at which radiation is emitted per unit area. It is a measure of heat flux . The total emissive power from a surface is denoted as E {\displaystyle E} and can be determined by, E = π I {\displaystyle E=\pi I} where π {\displaystyle \pi }
12450-409: Is one of the three principal mechanisms of heat transfer . It entails the emission of a spectrum of electromagnetic radiation due to an object's temperature. Other mechanisms are convection and conduction . Thermal radiation is characteristically different from conduction and convection in that it does not require a medium and, in fact it reaches maximum efficiency in a vacuum . Thermal radiation
12616-486: Is partially absorbed and scattered in the atmosphere , the latter process being the reason why the sky is visibly blue. Much of the Sun's radiation transmits through the atmosphere to the surface where it is either absorbed or reflected. Thermal radiation can be used to detect objects or phenomena normally invisible to the human eye. Thermographic cameras create an image by sensing infrared radiation. These images can represent
12782-400: Is present in all matter of nonzero temperature. These atoms and molecules are composed of charged particles, i.e., protons and electrons . The kinetic interactions among matter particles result in charge acceleration and dipole oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons , radiating energy away from
12948-413: Is proportional to the absolute temperature T of the black body. The photosphere of the sun, at a temperature of approximately 6000 K, emits radiation principally in the (human-)visible portion of the electromagnetic spectrum. Earth's atmosphere is partly transparent to visible light, and the light reaching the surface is absorbed or reflected. Earth's surface emits the absorbed radiation, approximating
13114-477: Is that it consists of photons , uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic field , responsible for all electromagnetic interactions. Quantum electrodynamics is the theory of how EMR interacts with matter on an atomic level. Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation . The energy of an individual photon
13280-486: Is the emission spectrum of nebulae . Rapidly moving electrons are most sharply accelerated when they encounter a region of force, so they are responsible for producing much of the highest frequency electromagnetic radiation observed in nature. These phenomena can aid various chemical determinations for the composition of gases lit from behind (absorption spectra) and for glowing gases (emission spectra). Spectroscopy (for example) determines what chemical elements comprise
13446-402: Is the speed of light in the medium. Thermal irradiation is the rate at which radiation is incident upon a surface per unit area. It is measured in watts per square meter. Irradiation can either be reflected , absorbed , or transmitted . The components of irradiation can then be characterized by the equation where, α {\displaystyle \alpha \,} represents
13612-410: Is the unique stable stationary state that is approached or eventually reached as the system interacts with its surroundings over a long time. The above-mentioned potentials are mathematically constructed to be the thermodynamic quantities that are minimized under the particular conditions in the specified surroundings. The various types of equilibriums are achieved as follows: Often the surroundings of
13778-443: Is there an absence of macroscopic change, but there is an “absence of any tendency toward change on a macroscopic scale.” Systems in mutual thermodynamic equilibrium are simultaneously in mutual thermal , mechanical , chemical , and radiative equilibria. Systems can be in one kind of mutual equilibrium, while not in others. In thermodynamic equilibrium, all kinds of equilibrium hold at once and indefinitely, until disturbed by
13944-531: Is thereby radically different from a fictive quasi-static 'process' that proceeds infinitely slowly throughout its course, and is fictively 'reversible'. Classical thermodynamics allows that even though a process may take a very long time to settle to thermodynamic equilibrium, if the main part of its course is at a finite rate, then it is considered to be natural, and to be subject to the second law of thermodynamics, and thereby irreversible. Engineered machines and artificial devices and manipulations are permitted within
14110-516: The Archimedes' heat ray anecdote, Archimedes is purported to have developed mirrors to concentrate heat rays in order to burn attacking Roman ships during the Siege of Syracuse ( c. 213–212 BC), but no sources from the time have been confirmed. Catoptrics is a book attributed to Euclid on how to focus light in order to produce heat, but the book might have been written in 300 AD. During
14276-648: The Planck energy or exceeding it (far too high to have ever been observed) will require new physical theories to describe. When radio waves impinge upon a conductor , they couple to the conductor, travel along it and induce an electric current on the conductor surface by moving the electrons of the conducting material in correlated bunches of charge. Electromagnetic radiation phenomena with wavelengths ranging from as long as one meter to as short as one millimeter are called microwaves; with frequencies between 300 MHz (0.3 GHz) and 300 GHz. At radio and microwave frequencies, EMR interacts with matter largely as
14442-473: The Planck–Einstein equation . In quantum theory (see first quantization ) the energy of the photons is thus directly proportional to the frequency of the EMR wave. Likewise, the momentum p of a photon is also proportional to its frequency and inversely proportional to its wavelength: The source of Einstein's proposal that light was composed of particles (or could act as particles in some circumstances)
14608-440: The absorptivity , ρ {\displaystyle \rho \,} reflectivity and τ {\displaystyle \tau \,} transmissivity . These components are a function of the wavelength of the electromagnetic wave as well as the material properties of the medium. The spectral absorption is equal to the emissivity ϵ {\displaystyle \epsilon } ; this relation
14774-523: The electromagnetic (EM) field , which propagate through space and carry momentum and electromagnetic radiant energy . Classically , electromagnetic radiation consists of electromagnetic waves , which are synchronized oscillations of electric and magnetic fields . In a vacuum , electromagnetic waves travel at the speed of light , commonly denoted c . There, depending on the frequency of oscillation, different wavelengths of electromagnetic spectrum are produced. In homogeneous, isotropic media,
14940-405: The quantum theory and was first offered by Max Planck in 1900. According to this theory, energy emitted by a radiator is not continuous but is in the form of quanta. Planck noted that energy was emitted in quantas of frequency of vibration similarly to the wave theory. The energy E an electromagnetic wave in vacuum is found by the expression E = hf , where h is the Planck constant and f
15106-418: The ultraviolet catastrophe . In 1900, Max Planck developed a new theory of black-body radiation that explained the observed spectrum. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy. These packets were called quanta . In 1905, Albert Einstein proposed that light quanta be regarded as real particles. Later
15272-413: The "equilibrium of a thermodynamic system", without actually writing the phrase "thermodynamic equilibrium". Referring to systems closed to exchange of matter, Buchdahl writes: "If a system is in a terminal condition which is properly static, it will be said to be in equilibrium ." Buchdahl's monograph also discusses amorphous glass, for the purposes of thermodynamic description. It states: "More precisely,
15438-481: The "minus first" law of thermodynamics. One textbook calls it the "zeroth law", remarking that the authors think this more befitting that title than its more customary definition , which apparently was suggested by Fowler .) Such states are a principal concern in what is known as classical or equilibrium thermodynamics, for they are the only states of the system that are regarded as well defined in that subject. A system in contact equilibrium with another system can by
15604-513: The Renaissance, Santorio Santorio came up with one of the earliest thermoscopes . In 1612 he published his results on the heating effects from the Sun, and his attempts to measure heat from the Moon. Earlier, in 1589, Giambattista della Porta reported on the heat felt on his face, emitted by a remote candle and facilitated by a concave metallic mirror. He also reported the cooling felt from
15770-549: The Stefan-Boltzmann law. Encountering this "ideally calculable" situation is almost impossible (although common engineering procedures surrender the dependency of these unknown variables and "assume" this to be the case). Optimistically, these "gray" approximations will get close to real solutions, as most divergence from Stefan-Boltzmann solutions is very small (especially in most standard temperature and pressure lab controlled environments). Reflectivity deviates from
15936-432: The atoms in the star's atmosphere. A similar phenomenon occurs for emission , which is seen when an emitting gas glows due to excitation of the atoms from any mechanism, including heat. As electrons descend to lower energy levels, a spectrum is emitted that represents the jumps between the energy levels of the electrons, but lines are seen because again emission happens only at particular energies after excitation. An example
16102-413: The atoms. Dark bands in the absorption spectrum are due to the atoms in an intervening medium between source and observer. The atoms absorb certain frequencies of the light between emitter and detector/eye, then emit them in all directions. A dark band appears to the detector, due to the radiation scattered out of the light beam . For instance, dark bands in the light emitted by a distant star are due to
16268-403: The average number of photons in the cube of the relevant wavelength is much smaller than 1. It is not so difficult to experimentally observe non-uniform deposition of energy when light is absorbed, however this alone is not evidence of "particulate" behavior. Rather, it reflects the quantum nature of matter . Demonstrating that the light itself is quantized, not merely its interaction with matter,
16434-404: The behavior of a black body at 300 K with spectral peak at f max . At these lower frequencies, the atmosphere is largely opaque and radiation from Earth's surface is absorbed or scattered by the atmosphere. Though about 10% of this radiation escapes into space, most is absorbed and then re-emitted by atmospheric gases. It is this spectral selectivity of the atmosphere that is responsible for
16600-477: The body absorbs radiation at that frequency, a property known as reciprocity . Thus, a surface that absorbs more red light thermally radiates more red light. This principle applies to all properties of the wave, including wavelength (color), direction, polarization , and even coherence . It is therefore possible to have thermal radiation which is polarized, coherent, and directional; though polarized and coherent sources are fairly rare in nature. Thermal radiation
16766-477: The body remains sufficiently nearly in thermodynamic equilibrium during the process. A. Münster carefully extends his definition of thermodynamic equilibrium for isolated systems by introducing a concept of contact equilibrium . This specifies particular processes that are allowed when considering thermodynamic equilibrium for non-isolated systems, with special concern for open systems, which may gain or lose matter from or to their surroundings. A contact equilibrium
16932-413: The body. Electromagnetic radiation, including visible light, will propagate indefinitely in vacuum . The characteristics of thermal radiation depend on various properties of the surface from which it is emanating, including its temperature and its spectral emissivity , as expressed by Kirchhoff's law . The radiation is not monochromatic, i.e., it does not consist of only a single frequency, but comprises
17098-404: The bottom wall, but the temperature is the same everywhere. A thermodynamic operation may occur as an event restricted to the walls that are within the surroundings, directly affecting neither the walls of contact of the system of interest with its surroundings, nor its interior, and occurring within a definitely limited time. For example, an immovable adiabatic wall may be placed or removed within
17264-420: The colors, indicating that they got the hottest and melted the most snow. Antoine Lavoisier considered that radiation of heat was concerned with the condition of the surface of a physical body rather than the material of which it was composed. Lavoisier described a poor radiator to be a substance with a polished or smooth surface as it possessed its molecules lying in a plane closely bound together thus creating
17430-408: The combined energy transfer of many photons. In contrast, high frequency ultraviolet, X-rays and gamma rays are ionizing – individual photons of such high frequency have enough energy to ionize molecules or break chemical bonds . Ionizing radiation can cause chemical reactions and damage living cells beyond simply heating, and can be a health hazard and dangerous. James Clerk Maxwell derived
17596-411: The conditions for all three types of equilibrium are satisfied, the system is said to be in a state of thermodynamic equilibrium". P.M. Morse writes that thermodynamics is concerned with " states of thermodynamic equilibrium ". He also uses the phrase "thermal equilibrium" while discussing transfer of energy as heat between a body and a heat reservoir in its surroundings, though not explicitly defining
17762-413: The contacts having respectively different permeabilities. If these systems are all jointly isolated from the rest of the world those of them that are in contact then reach respective contact equilibria with one another. If several systems are free of adiabatic walls between each other, but are jointly isolated from the rest of the world, then they reach a state of multiple contact equilibrium, and they have
17928-416: The criterion for equilibrium is circular. Operationally, a system is in an equilibrium state if its properties are consistently described by thermodynamic theory! " J.A. Beattie and I. Oppenheim write: "Insistence on a strict interpretation of the definition of equilibrium would rule out the application of thermodynamics to practically all states of real systems." Another author, cited by Callen as giving
18094-540: The discussion of phenomena near absolute zero. The absolute predictions of the classical theory become particularly vague because the occurrence of frozen-in nonequilibrium states is very common." The most general kind of thermodynamic equilibrium of a system is through contact with the surroundings that allows simultaneous passages of all chemical substances and all kinds of energy. A system in thermodynamic equilibrium may move with uniform acceleration through space but must not change its shape or size while doing so; thus it
18260-400: The distance between two adjacent crests or troughs is called the wavelength . Waves of the electromagnetic spectrum vary in size, from very long radio waves longer than a continent to very short gamma rays smaller than atom nuclei. Frequency is inversely proportional to wavelength, according to the equation: where v is the speed of the wave ( c in a vacuum or less in other media), f is
18426-524: The electromagnetic spectrum includes: radio waves , microwaves , infrared , visible light , ultraviolet , X-rays , and gamma rays . Electromagnetic waves are emitted by electrically charged particles undergoing acceleration , and these waves can subsequently interact with other charged particles, exerting force on them. EM waves carry energy, momentum , and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Electromagnetic radiation
18592-447: The electromagnetic vacuum. The behavior of EM radiation and its interaction with matter depends on its frequency, and changes qualitatively as the frequency changes. Lower frequencies have longer wavelengths, and higher frequencies have shorter wavelengths, and are associated with photons of higher energy. There is no fundamental limit known to these wavelengths or energies, at either end of the spectrum, although photons with energies near
18758-522: The emission is in the infrared (IR) spectrum, though above around 525 °C (977 °F) enough of it becomes visible for the matter to visibly glow. This visible glow is called incandescence . Thermal radiation is one of the fundamental mechanisms of heat transfer , along with conduction and convection . The primary method by which the Sun transfers heat to the Earth is thermal radiation. This energy
18924-401: The equilibrium refers to an isolated system. Like Münster, Partington also refers to the mixture of oxygen and hydrogen. He adds a proviso that "In a true equilibrium state, the smallest change of any external condition which influences the state will produce a small change of state ..." This proviso means that thermodynamic equilibrium must be stable against small perturbations; this requirement
19090-527: The fields present in the same space due to other causes. Further, as they are vector fields, all magnetic and electric field vectors add together according to vector addition . For example, in optics two or more coherent light waves may interact and by constructive or destructive interference yield a resultant irradiance deviating from the sum of the component irradiances of the individual light waves. The electromagnetic fields of light are not affected by traveling through static electric or magnetic fields in
19256-406: The frequency and λ is the wavelength. As waves cross boundaries between different media, their speeds change but their frequencies remain constant. Electromagnetic waves in free space must be solutions of Maxwell's electromagnetic wave equation . Two main classes of solutions are known, namely plane waves and spherical waves. The plane waves may be viewed as the limiting case of spherical waves at
19422-429: The glass may be regarded as being in equilibrium so long as experimental tests show that 'slow' transitions are in effect reversible." It is not customary to make this proviso part of the definition of thermodynamic equilibrium, but the converse is usually assumed: that if a body in thermodynamic equilibrium is subject to a sufficiently slow process, that process may be considered to be sufficiently nearly reversible, and
19588-414: The height of the surface roughness is much smaller relative to the wavelength of the incident radiation. A medium that experiences no transmission ( τ = 0 {\displaystyle \tau =0} ) is opaque, in which case absorptivity and reflectivity sum to unity: ρ + α = 1. {\displaystyle \rho +\alpha =1.} Radiation emitted from
19754-477: The individual frequency components are represented in terms of their power content, and the phase information is not preserved. Such a representation is called the power spectral density of the random process. Random electromagnetic radiation requiring this kind of analysis is, for example, encountered in the interior of stars, and in certain other very wideband forms of radiation such as the Zero point wave field of
19920-544: The intense radiation of radium . The radiation from pitchblende was differentiated into alpha rays ( alpha particles ) and beta rays ( beta particles ) by Ernest Rutherford through simple experimentation in 1899, but these proved to be charged particulate types of radiation. However, in 1900 the French scientist Paul Villard discovered a third neutrally charged and especially penetrating type of radiation from radium, and after he described it, Rutherford realized it must be yet
20086-488: The intensive variables become uniform, thermodynamic equilibrium is said to exist." He is not here considering the presence of an external force field. J.G. Kirkwood and I. Oppenheim define thermodynamic equilibrium as follows: "A system is in a state of thermodynamic equilibrium if, during the time period allotted for experimentation, (a) its intensive properties are independent of time and (b) no current of matter or energy exists in its interior or at its boundaries with
20252-800: The known speed of light. Maxwell therefore suggested that visible light (as well as invisible infrared and ultraviolet rays by inference) all consisted of propagating disturbances (or radiation) in the electromagnetic field. Radio waves were first produced deliberately by Heinrich Hertz in 1887, using electrical circuits calculated to produce oscillations at a much lower frequency than that of visible light, following recipes for producing oscillating charges and currents suggested by Maxwell's equations. Hertz also developed ways to detect these waves, and produced and characterized what were later termed radio waves and microwaves . Wilhelm Röntgen discovered and named X-rays . After experimenting with high voltages applied to an evacuated tube on 8 November 1895, he noticed
20418-609: The left as the temperature increases. The total radiation intensity of a black body rises as the fourth power of the absolute temperature, as expressed by the Stefan–Boltzmann law . A kitchen oven, at a temperature about double room temperature on the absolute temperature scale (600 K vs. 300 K) radiates 16 times as much power per unit area. An object at the temperature of the filament in an incandescent light bulb —roughly 3000 K, or 10 times room temperature—radiates 10,000 times as much energy per unit area. As for photon statistics , thermal light obeys Super-Poissonian statistics . When
20584-423: The massive particles of the gas for LTE to exist. In some cases, it is not considered necessary for free electrons to be in equilibrium with the much more massive atoms or molecules for LTE to exist. As an example, LTE will exist in a glass of water that contains a melting ice cube . The temperature inside the glass can be defined at any point, but it is colder near the ice cube than far away from it. If energies of
20750-447: The media determines the degree of refraction, and is summarized by Snell's law . Light of composite wavelengths (natural sunlight) disperses into a visible spectrum passing through a prism, because of the wavelength-dependent refractive index of the prism material ( dispersion ); that is, each component wave within the composite light is bent a different amount. EM radiation exhibits both wave properties and particle properties at
20916-889: The melting, and continuously draining off the meltwater. Natural transport phenomena may lead a system from local to global thermodynamic equilibrium. Going back to our example, the diffusion of heat will lead our glass of water toward global thermodynamic equilibrium, a state in which the temperature of the glass is completely homogeneous. Careful and well informed writers about thermodynamics, in their accounts of thermodynamic equilibrium, often enough make provisos or reservations to their statements. Some writers leave such reservations merely implied or more or less unstated. For example, one widely cited writer, H. B. Callen writes in this context: "In actuality, few systems are in absolute and true equilibrium." He refers to radioactive processes and remarks that they may take "cosmic times to complete, [and] generally can be ignored". He adds "In practice,
21082-585: The molecules located near a given point are observed, they will be distributed according to the Maxwell–Boltzmann distribution for a certain temperature. If the energies of the molecules located near another point are observed, they will be distributed according to the Maxwell–Boltzmann distribution for another temperature. Local thermodynamic equilibrium does not require either local or global stationarity. In other words, each small locality need not have
21248-478: The nature of a surface and its temperature. Radiation waves may travel in unusual patterns compared to conduction heat flow . Radiation allows waves to travel from a heated body through a cold non-absorbing or partially absorbing medium and reach a warmer body again. An example is the case of the radiation waves that travel from the Sun to the Earth. Thermal radiation emitted by a body at any temperature consists of
21414-407: The nearby violet light. Ritter's experiments were an early precursor to what would become photography. Ritter noted that the ultraviolet rays (which at first were called "chemical rays") were capable of causing chemical reactions. In 1862–64 James Clerk Maxwell developed equations for the electromagnetic field which suggested that waves in the field would travel with a speed that was very close to
21580-626: The oscillations of the two fields are on average perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave . Electromagnetic radiation is commonly referred to as "light", EM, EMR, or electromagnetic waves. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. In order of increasing frequency and decreasing wavelength,
21746-412: The other properties in that it is bidirectional in nature. In other words, this property depends on the direction of the incident of radiation as well as the direction of the reflection. Therefore, the reflected rays of a radiation spectrum incident on a real surface in a specified direction forms an irregular shape that is not easily predictable. In practice, surfaces are often assumed to reflect either in
21912-401: The particle of light was given the name photon , to correspond with other particles being described around this time, such as the electron and proton . A photon has an energy, E , proportional to its frequency, f , by where h is the Planck constant , λ {\displaystyle \lambda } is the wavelength and c is the speed of light . This is sometimes known as
22078-485: The planetary greenhouse effect , contributing to global warming and climate change in general (but also critically contributing to climate stability when the composition and properties of the atmosphere are not changing). Burning glasses are known to date back to about 700 BC. One of the first accurate mentions of burning glasses appears in Aristophanes 's comedy, The Clouds , written in 423 BC. According to
22244-428: The pressures on either side of it are equal. If the adiabatic wall is more complicated, with a sort of leverage, having an area-ratio, then the pressures of the two systems in exchange equilibrium are in the inverse ratio of the volume exchange ratio; this keeps the zero balance of rates of transfer as work. A radiative exchange can occur between two otherwise separate systems. Radiative exchange equilibrium prevails when
22410-428: The radiation's power and its frequency. EMR of lower energy ultraviolet or lower frequencies (i.e., near ultraviolet , visible light, infrared, microwaves, and radio waves) is non-ionizing because its photons do not individually have enough energy to ionize atoms or molecules or to break chemical bonds . The effect of non-ionizing radiation on chemical systems and living tissue is primarily simply heating, through
22576-470: The red part of the spectrum, through an increase in the temperature recorded with a thermometer . These "calorific rays" were later termed infrared. In 1801, German physicist Johann Wilhelm Ritter discovered ultraviolet in an experiment similar to Herschel's, using sunlight and a glass prism. Ritter noted that invisible rays near the violet edge of a solar spectrum dispersed by a triangular prism darkened silver chloride preparations more quickly than did
22742-499: The respective intensive parameters of the system and surroundings are equal. This definition does not consider the most general kind of thermodynamic equilibrium, which is through unselective contacts. This definition does not simply state that no current of matter or energy exists in the interior or at the boundaries; but it is compatible with the following definition, which does so state. M. Zemansky also distinguishes mechanical, chemical, and thermal equilibrium. He then writes: "When
22908-412: The same points in space (see illustrations). In the far-field EM radiation which is described by the two source-free Maxwell curl operator equations, a time-change in one type of field is proportional to the curl of the other. These derivatives require that the E and B fields in EMR are in-phase (see mathematics section below). An important aspect of light's nature is its frequency . The frequency of
23074-464: The same time (see wave-particle duality ). Both wave and particle characteristics have been confirmed in many experiments. Wave characteristics are more apparent when EM radiation is measured over relatively large timescales and over large distances while particle characteristics are more evident when measuring small timescales and distances. For example, when electromagnetic radiation is absorbed by matter, particle-like properties will be more obvious when
23240-472: The source, the power density of EM radiation from an isotropic source decreases with the inverse square of the distance from the source; this is called the inverse-square law . This is in contrast to dipole parts of the EM field, the near field, which varies in intensity according to an inverse cube power law, and thus does not transport a conserved amount of energy over distances but instead fades with distance, with its energy (as noted) rapidly returning to
23406-422: The spectral intensity, I λ {\displaystyle I_{\lambda }} as follows, E λ ( λ ) = π I λ ( λ ) {\displaystyle E_{\lambda }(\lambda )=\pi I_{\lambda }(\lambda )} where both spectral emissive power and emissive intensity are functions of wavelength. A "black body"
23572-439: The speed in a medium to speed in a vacuum. Electromagnetic radiation of wavelengths other than those of visible light were discovered in the early 19th century. The discovery of infrared radiation is ascribed to astronomer William Herschel , who published his results in 1800 before the Royal Society of London . Herschel used a glass prism to refract light from the Sun and detected invisible rays that caused heating beyond
23738-511: The surface of contiguity may be supposed to be permeable only to heat, allowing energy to transfer only as heat. Then the two systems are said to be in thermal equilibrium when the long-range forces are unchanging in time and the transfer of energy as heat between them has slowed and eventually stopped permanently; this is an example of a contact equilibrium. Other kinds of contact equilibrium are defined by other kinds of specific permeability. When two systems are in contact equilibrium with respect to
23904-593: The surrounding subsystems are so much larger than the system that the process can affect the intensive variables only of the surrounding subsystems, and they are then called reservoirs for relevant intensive variables. It can be useful to distinguish between global and local thermodynamic equilibrium. In thermodynamics, exchanges within a system and between the system and the outside are controlled by intensive parameters. As an example, temperature controls heat exchanges . Global thermodynamic equilibrium (GTE) means that those intensive parameters are homogeneous throughout
24070-406: The surroundings. The allowance of such operations and devices in the surroundings but not in the system is the reason why Kelvin in one of his statements of the second law of thermodynamics spoke of "inanimate" agency ; a system in thermodynamic equilibrium is inanimate. Otherwise, a thermodynamic operation may directly affect a wall of the system. It is often convenient to suppose that some of
24236-405: The surroundings. Consequent upon such an operation restricted to the surroundings, the system may be for a time driven away from its own initial internal state of thermodynamic equilibrium. Then, according to the second law of thermodynamics, the whole undergoes changes and eventually reaches a new and final equilibrium with the surroundings. Following Planck, this consequent train of events is called
24402-468: The surroundings." He distinguishes such thermodynamic equilibrium from thermal equilibrium, in which only thermal contact is mediating transfer of energy. Another textbook author, J.R. Partington , writes: "(i) An equilibrium state is one which is independent of time ." But, referring to systems "which are only apparently in equilibrium", he adds : "Such systems are in states of ″false equilibrium.″" Partington's statement does not explicitly state that
24568-633: The surroundings." It is evident that they are not restricting the definition to isolated or to closed systems. They do not discuss the possibility of changes that occur with "glacial slowness", and proceed beyond the time period allotted for experimentation. They note that for two systems in contact, there exists a small subclass of intensive properties such that if all those of that small subclass are respectively equal, then all respective intensive properties are equal. States of thermodynamic equilibrium may be defined by this subclass, provided some other conditions are satisfied. A thermodynamic system consisting of
24734-408: The surroundings: where T denotes the absolute thermodynamic temperature, P the pressure, S the entropy, V the volume, and U the internal energy of the system. In other words, Δ G = 0 {\displaystyle \Delta G=0} is a necessary condition for chemical equilibrium under these conditions (in the absence of an applied voltage). Thermodynamic equilibrium
24900-421: The system must be isolated; Callen does not spell out what he means by the words "intrinsic factors". Another textbook writer, C.J. Adkins, explicitly allows thermodynamic equilibrium to occur in a system which is not isolated. His system is, however, closed with respect to transfer of matter. He writes: "In general, the approach to thermodynamic equilibrium will involve both thermal and work-like interactions with
25066-410: The system was in equilibrium . In a section headed "Thermodynamic equilibrium", H.B. Callen defines equilibrium states in a paragraph. He points out that they "are determined by intrinsic factors" within the system. They are "terminal states", towards which the systems evolve, over time, which may occur with "glacial slowness". This statement does not explicitly say that for thermodynamic equilibrium,
25232-454: The system will be in neither global nor local equilibrium. For example, it takes a certain number of collisions for a particle to equilibrate to its surroundings. If the average distance it has moved during these collisions removes it from the neighborhood it is equilibrating to, it will never equilibrate, and there will be no LTE. Temperature is, by definition, proportional to the average internal energy of an equilibrated neighborhood. Since there
25398-419: The system, no changes of state are occurring at a measurable rate." There are two reservations stated here; the system is isolated; any changes of state are immeasurably slow. He discusses the second proviso by giving an account of a mixture oxygen and hydrogen at room temperature in the absence of a catalyst. Münster points out that a thermodynamic equilibrium state is described by fewer macroscopic variables than
25564-467: The temperature gradient of a scene and are commonly used to locate objects at a higher temperature than their surroundings. In a dark environment where visible light is at low levels, infrared images can be used to locate animals or people due to their body temperature. Cosmic microwave background radiation is another example of thermal radiation. Blackbody radiation is a concept used to analyze thermal radiation in idealized systems. This model applies if
25730-524: The temperature of a body is high enough, its thermal radiation spectrum becomes strong enough in the visible range to visibly glow. The visible component of thermal radiation is sometimes called incandescence , though this term can also refer to thermal radiation in general. The term derive from the Latin verb incandescere , 'to glow white'. In practice, virtually all solid or liquid substances start to glow around 798 K (525 °C; 977 °F), with
25896-489: The term "black body" does not always correspond to the visually perceived color of an object). These materials that do not follow the "black color = high emissivity/absorptivity" caveat will most likely have functional spectral emissivity/absorptivity dependence. Only truly gray systems (relative equivalent emissivity/absorptivity and no directional transmissivity dependence in all control volume bodies considered) can achieve reasonable steady-state heat flux estimates through
26062-410: The term associated with the changing static electric field of the particle and the magnetic term that results from the particle's uniform velocity are both associated with the near field, and do not comprise electromagnetic radiation. Electric and magnetic fields obey the properties of superposition . Thus, a field due to any particular particle or time-varying electric or magnetic field contributes to
26228-475: The transmitter or absorbed by a nearby receiver (such as a transformer secondary coil). In the Liénard–Wiechert potential formulation of the electric and magnetic fields due to motion of a single particle (according to Maxwell's equations), the terms associated with acceleration of the particle are those that are responsible for the part of the field that is regarded as electromagnetic radiation. By contrast,
26394-427: The transmitter to affect them. This causes them to be independent in the sense that their existence and their energy, after they have left the transmitter, is completely independent of both transmitter and receiver. Due to conservation of energy , the amount of power passing through any spherical surface drawn around the source is the same. Because such a surface has an area proportional to the square of its distance from
26560-425: The two systems have the same temperature. The A collection of matter may be entirely isolated from its surroundings. If it has been left undisturbed for an indefinitely long time, classical thermodynamics postulates that it is in a state in which no changes occur within it, and there are no flows within it. This is a thermodynamic state of internal equilibrium. (This postulate is sometimes, but not often, called
26726-524: The wave theory, however, Einstein's ideas were met initially with great skepticism among established physicists. Eventually Einstein's explanation was accepted as new particle-like behavior of light was observed, such as the Compton effect . As a photon is absorbed by an atom , it excites the atom, elevating an electron to a higher energy level (one that is on average farther from the nucleus). When an electron in an excited molecule or atom descends to
26892-413: The wavelength distribution of the electromagnetic radiation. The distribution of power that a black body emits with varying frequency is described by Planck's law . At any given temperature, there is a frequency f max at which the power emitted is a maximum. Wien's displacement law, and the fact that the frequency is inversely proportional to the wavelength, indicates that the peak frequency f max
27058-471: The whole system, while local thermodynamic equilibrium (LTE) means that those intensive parameters are varying in space and time, but are varying so slowly that, for any point, one can assume thermodynamic equilibrium in some neighborhood about that point. If the description of the system requires variations in the intensive parameters that are too large, the very assumptions upon which the definitions of these intensive parameters are based will break down, and
27224-418: Was allowed by the propagation of electromagnetic waves . Television and radio broadcasting waves are types of electromagnetic waves with specific wavelengths . All electromagnetic waves travel at the same speed; therefore, shorter wavelengths are associated with high frequencies. All bodies generate and receive electromagnetic waves at the expense of heat exchange. In 1860, Gustav Kirchhoff published
27390-422: Was an experimental anomaly not explained by the wave theory: the photoelectric effect , in which light striking a metal surface ejected electrons from the surface, causing an electric current to flow across an applied voltage . Experimental measurements demonstrated that the energy of individual ejected electrons was proportional to the frequency , rather than the intensity , of the light. Furthermore, below
27556-565: Was reported that a thermometer detected a lower temperature when a set of mirrors were used to focus "frigorific rays" from a cold object. In 1791, Pierre Prevost a colleague of Pictet, introduced the concept of radiative equilibrium , wherein all objects both radiate and absorb heat. When an object is cooler than its surroundings, it absorbs more heat than it emits, causing its temperature to increase until it reaches equilibrium. Even at equilibrium, it continues to radiate heat, balancing absorption and emission. The discovery of infrared radiation
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