Radiation from electrical conductors

A thermocouple is a junction between two different electrical conductors. Electrons have lower free energy in one conductor than in the other, so they flow from one to the other until the resulting voltage difference prevents further flow. The junction potential is temperature dependent because electrons flow back to the high-energy conductor at higher temperature. If a thermocouple is blackened to absorb radiation, its temperature (and hence voltage) becomes sensitive to radiation. A typical sensitivity is 6 V per watt of radiation absorbed. 

A few comments about the validity of tlie diffuse approximation are in order. Although real surfaces do not emit radiation in a perfectly diffuse manner as a blackbody does, they often come close. The variation of emissivity with direction for both electrical conductors and nonconductors is given in Fig. 12 26. Here 0 is tlie angle measured from the normal of the surface, and thus 0 = 0 for radiation emitted in a direction normal to the surface. Note that Sg remains nearly constant for about 0 < d0° for conductors such as metals and for 6 < 70° for nonconductors such as plastics. Therefore, the directional emissivity of a sur face in the normal direction is representative of the hemispherical emissivity of the surface. In radiatioit analysis, it is common practice to assume the surfaces to be diffuse emitters with an emissivity equal to the value in the normal (6 = 0) direction. 

Unfortunately the electromagnetic theory is only valid under a series of limiting suppositions, so that the emissivities calculated from it frequently differ from reality. Despite this, it provides important, qualitative statements that can be used for the extrapolation from measurements or to estimate for missing data. We will not discuss the electromagnetic theory, see for this [5.4], but will use some of its results in the treatment of emissivities of electrical insulators and electrical conductors (metals). These two material groups differ significantly in their radiation behaviour. 

For practical purposes, only a mean value of the emissivity or absorptivity over the direction is required. Sieber (1941) obtained experimental data on total emissivity of opaque materials depending on the temperature of the source. Many authors (Ginzburg, 1969 Kreith, 1965) have reproduced these results graphically (Figure 18.1). The behaviour of electrical conductors and nonconductors with temperature of the radiator can be approximately interpreted from the dependency of the monochromatic emissivity on wavelength and the relationship between temperature of the radiator and the wavelength. 

When lead, which is very soft, is freshly cut, it has shiny blue-white sheen, which soon oxidizes into its familiar gray color. Lead is extremely malleable and ductile and can be worked into a variety of shapes. It can be formed into sheets, pipes, buckshot, wires, and powder. Although lead is a poor conductor of electricity, its high density makes it an excellent shield for protection from radiation, including X-rays and gamma rays. 

After 14 years on the faculty of Imperial College, Jacobs moved from London, England, to London, Ontario, where his research program focused on the optical and electrical properties of ionic crystals, as well as on the experimental and theoretical determination of thermodynamic and kinetic properties of crystal defects.213 Over the years his research interests have expanded to include several aspects of computer simulations of condensed matter.214 He has developed algorithms215 for molecular dynamics studies of non-ionic and ionic systems, and he has carried out simulations on systems as diverse as metals, solid ionic conductors, and ceramics. The simulation of the effects of radiation damage is a special interest. His recent interests include the study of perfect and imperfect crystals by means of quantum chemical methods. The corrosion of metals is being studied by both quantum chemical and molecular dynamics techniques. 

Semiconductors operate on a different principle. When radiation falls on them, they change from a nonconductor to a conductor. No temperature change is involved in the process only the change in electrical resistance is important. This takes place over an extremely short period of time. Response times of the order of nanoseconds are common. This enables instruments to be designed with very short scanning times. It is possible to complete the scan in a few seconds using such detectors. These kinds of instruments are very valuable when put onto the end of a GC and used to obtain the IR spectra of the effluents. Such scans must be made in a few seconds and be completely recorded before the next component emerges from the GC column. 

The electric signal is mainly attenuated by reflection from the casing surfaces, and absorption inside the material radiation scattering is insignificant. The absorption A depends on the conductor thickness t (m) and the signal frequency f (Hz) according to... 

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