Principles of economics 4th edition
Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation.
Infrared radiation emitted by animals detectable with an infrared camera and cosmic microwave background radiation are examples of thermal radiation. If a radiation object meets the physical characteristics of a black body in thermodynamic equilibrium , the radiation is called blackbody radiation.
Wien's displacement law determines the most likely frequency of the emitted radiation, and the Stefan—Boltzmann law gives the radiant intensity. Thermal radiation is also one of the fundamental mechanisms of heat transfer. Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature that is greater than absolute zero.
Thermal energy consists of the kinetic energy of random movements of atoms and molecules in matter. All matter with a nonzero temperature by definition is composed of particles which have kinetic energy, and which interact with each other.
These atoms and molecules are composed of charged particles, i. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons , radiating energy away from the body. Electromagnetic radiation, including visible light, will propagate indefinitely in vacuum. The characteristics of thermal radiation depend on various properties of the surface it is emanating from, including its temperature, its spectral emissivity , as expressed by Kirchhoff's law.
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 a black body is the body's emissivity , so that a black body has an emissivity of unity i. 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 — a good absorber is necessarily a good emitter, and a poor absorber is a poor emitter.
The temperature determines the wavelength distribution of the electromagnetic radiation. For example, the white paint in the diagram to the right is highly reflective to visible light reflectivity about 0.
Thus, to thermal radiation it appears black. 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 is proportional to the absolute temperature T of the black body. Earth's atmosphere is partly transparent to visible light, and the light reaching the surface is absorbed or reflected.
At these lower frequencies, the atmosphere is largely opaque and radiation from Earth's surface is absorbed or scattered by the atmosphere. It is this spectral selectivity of the atmosphere that is responsible for 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.
The incandescent light bulb has a spectrum overlapping the black body spectra of the sun and the earth. Most of the energy is associated with photons of longer wavelengths; these do not help a person see, but still transfer heat to the environment, as can be deduced empirically by observing an incandescent light bulb.
Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in microwave ovens , laser cutting , and RF hair removal. Unlike conductive and convective forms of heat transfer, thermal radiation can be concentrated in a tiny spot by using reflecting mirrors, which concentrating solar power takes advantage of.
Instead of mirrors, Fresnel lenses can also be used to concentrate radiant energy. In principle, any kind of lens can be used, but only the Fresnel lens design is practical for very large lenses. Either method can be used to quickly vaporize water into steam using sunlight. Lighter colors and also whites and metallic substances absorb less of the illuminating light, and as a result heat up less; but otherwise color makes little difference as regards heat transfer between an object at everyday temperatures and its surroundings, since the dominant emitted wavelengths are nowhere near the visible spectrum, but rather in the far infrared.
Emissivities at those wavelengths are largely unrelated to visual emissivities visible colors ; in the far infra-red, most objects have high emissivities. Thus, except in sunlight, the color of clothing makes little difference as regards warmth; likewise, paint color of houses makes little difference to warmth except when the painted part is sunlit.
The main exception to this is shiny metal surfaces, which have low emissivities both in the visible wavelengths and in the far infrared. Such surfaces can be used to reduce heat transfer in both directions; an example of this is the multi-layer insulation used to insulate spacecraft.
Low-emissivity windows in houses are a more complicated technology, since they must have low emissivity at thermal wavelengths while remaining transparent to visible light. Nanostructures with spectrally selective thermal emittance properties offer numerous technological applications for energy generation and efficiency,  e.
These applications require high emittance in the frequency range corresponding to the atmospheric transparency window in 8 to 13 micron wavelength range. A selective emitter radiating strongly in this range is thus exposed to the clear sky, enabling the use of the outer space as a very low temperature heat sink.
Personalized cooling technology is another example of an application where optical spectral selectivity can be beneficial. Conventional personal cooling is typically achieved through heat conduction and convection. However, the human body is a very efficient emitter of infrared radiation, which provides an additional cooling mechanism. Most conventional fabrics are opaque to infrared radiation and block thermal emission from the body to the environment. Fabrics for personalized cooling applications have been proposed that enable infrared transmission to directly pass through clothing, while being opaque at visible wavelengths, allowing the wearer to remain cooler.
The general properties of thermal radiation as described by the Planck's law apply if the linear dimension of all parts considered, as well as radii of curvature of all surfaces are large compared with the wavelength of the ray considered' typically from micrometres for the emitter at K. Indeed, thermal radiation as discussed above takes only radiating waves far-field, or electromagnetic radiation into account.
A more sophisticated framework involving electromagnetic theory must be used for smaller distances from the thermal source or surface near-field thermal radiation. For example, although far-field thermal radiation at distances from surfaces of more than one wavelength is generally not coherent to any extent, near-field thermal radiation i. Planck's law of thermal radiation has been challenged in recent decades by predictions and successful demonstrations of the radiative heat transfer between objects separated by nanoscale gaps that deviate significantly from the law predictions.
This deviation is especially strong up to several orders in magnitude when the emitter and absorber support surface polariton modes that can couple through the gap separating cold and hot objects. However, to take advantage of the surface-polariton-mediated near-field radiative heat transfer, the two objects need to be separated by ultra-narrow gaps on the order of microns or even nanometers.
This limitation significantly complicates practical device designs. Another way to modify the object thermal emission spectrum is by reducing the dimensionality of the emitter itself. Such spatial confinement concentrates photon states and enhances thermal emission at select frequencies. Most importantly, the emission spectrum of thermal wells, wires and dots deviates from Planck's law predictions not only in the near field, but also in the far field, which significantly expands the range of their applications.
The time to a damage from exposure to radiative heat is a function of the rate of delivery of the heat. Thermal radiation 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.
Radiation heat transfer is characteristically different from the other two in that it does not require a medium and, in fact it reaches maximum efficiency in a vacuum. Electromagnetic radiation has some proper characteristics depending on the frequency and wavelengths of the radiation.
The phenomenon of radiation is not yet fully understood. Two theories have been used to explain radiation; however neither of them is perfectly satisfactory. First, the earlier theory which originated from the concept of a hypothetical medium referred as ether.
Ether supposedly fills all evacuated or non-evacuated spaces. The transmission of light or of radiant heat are allowed by the propagation of electromagnetic waves in the ether. Since every body or fluid is submerged in the ether, due to the vibration of the molecules, any body or fluid can potentially initiate an electromagnetic wave.
All bodies generate and receive electromagnetic waves at the expense of its stored energy . The second theory of radiation is best known as the quantum theory and was first offered by Max Planck in Planck claimed that quantities had different sizes and frequencies of vibration similarly to the wave theory. Higher frequencies are originated by high temperatures and create an increase of energy in the 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 the nature of a surface and its temperature. Radiation allows waves to travel from a heated body through a cold nonabsorbing or partially absorbing medium and reach a warmer body again. The interplay of energy exchange by thermal radiation is characterized by the following equation:. An object is called a black body if, for all frequencies, the following formula applies:.
Reflectivity deviates from 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 assumed to reflect in a perfectly specular or 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 a practical situation and room-temperature setting, humans lose considerable energy due to thermal radiation in infra-red in addition to that lost by conduction to air aided by concurrent convection, or other air movement like drafts. The heat energy lost is partially regained by absorbing heat radiation from walls or other surroundings.
Heat gained by conduction would occur for air temperature higher than body temperature. Otherwise, body temperature is maintained from generated heat through internal metabolism. Human skin has an emissivity of very close to 1.
These heat transfer estimates are highly dependent on extrinsic variables, such as wearing clothes, i. 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 STP lab controlled environments.
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.