Volume properties, electrical conductors

An applied electric field can be the electric held component of an electromagnetic wave, in which case electronic excitations or other optical responses may ensue. These are the topic of the next chapter. Here, the concern is with electrostatics, specihcally, the dielectric, or insulative, properties of materials. In an electrical conductor, an applied electric held, E, produces an electric current - ions, in the case of an ionic conductor, or electrons, in the case of an electronic conductor. Electrical conductivity has already been examined in earlier chapters. In insulating solids, the topic of the current discussion, the response to an applied electric held is a static spatial displacement of the bound ions or electrons, resulting in an electrical polarization, P, or net dipole moment (charge separahon) per unit volume, which is a vector quantity. In a homogeneous linear and isotropic medium, the polarization and electric held are aligned. In an anisotropic medium, this need not be so. The fth component of the polarization is related to the jth component of the electric held by ... 

We start our discussion with simple concepts from the band theory for solids, discuss what can break the symmetry of one-dimensional systems, introduce electrical conductivity and superconductivity, present the Mulliken charge transfer theory for solution complexes and its extension to solids, then discuss briefly the simple tt electron theory for long polyenes. Other articles in this volume review the detailed interplay between structure and electronic properties of conductors and superconductors [206], and electrical transport in conducting polymers [207],... 

Surface and Volume Resistivity. Electrical resistivity of laminate materials is differentiated between specific surface resistance and volume resistance. The surface resistance (7 characterizes the electrical resistance between two conductors along the surface of the laminate material. The volume resistance p describes the electrical resistance between two layers of conducting copper along the z-axis of the laminate material.The higher the values for both of these electrical properties, the better, as this ensures proper isolation of individual copper conductors in the printed circuit board. 

The resistance of most plastics to the flow of direct current is very high. Both surface and volume electrical resistivities are important properties for applications of plastics insulating materials. The volume resistivity is the electrical resistance of the material measured in ohms as though the material was a conductor. Insulators will not sustain an indefinitely high voltage as the applied voltage is increased, a point is reached where a drastic decrease in resistance takes place accompanied by a physical breakdown of the insulator. This is known as the dielectric strength, which is the electric potential in volts, which would be necessary to cause the failure of a 1/8-in. thick insulator (Chapter 4, ELEC-TRICAL/ELECTR ONICS PRODUCT). 

The temperature of a substance in a particular state of aggregation (solid, liquid, or gas) is a measure of the average kinetic energy possessed by the substance molecules. Since this energy cannot be measured directly, the temperature must be determined indirectly by measuring some physical property of the substance whose value depends on temperature in a known manner. Such properties and the temperature-measuring devices based on them include electrical resistance of a conductor (resistance thermometer), voltage at the junction of two dissimilar metals (thermocouple), spectra of emitted radiation (pyrometer), and volume of a fixed mass of fluid (thermometer). 

Because cellular polyethylene is comprised of roughly equal volumes of resin and gas, its properties are different from those of ordinary unfoamed polyethylene. The cellular product has a much lower dielectric constant and therefore lower electrical losses. The composition of polyethylene (dielectric constant 2.3) and an inert gas (dielectric constant 1.0) has a dielectric constant of 1.5. In terms of electrical insulation the lower dielectric constant permits a reduction in space between inner and outer conductors without changing the characteristic impedance. For this reason it is possible to reduce the attenuation by... 

Properties.—Physical.—It is a bluish-white metal brilliant upon freshly cut surfaces very soft and pliable not very malleable or ductile crystallizes in octahedra a poor conductor of electricity a better conductor of heat. When exx anded by heat, it does not, on cooling, return to its original volume. 

In regions of tissue where there are no sources, S is zero. In these cases, the divergenceless of / is equivalent to the law of conservation of current that is often invoked when analyzing electrical circuits. Another property of a volume conductor is that the current density and the electric field, E (V/m), are related linearly by Ohm s Law,... 

Before discussing the conductivity of tissue, consider one of the simplest and most easily understood volume conductors sahne. The electrical conductivity of sahne arises from the motion of free ions in response to a steady electric field, and is on the order of 1 S/m. Besides conductivity, another property of saline is its electrical permittivity, s (S sec/m). This property is related to the dielectric constant, /c (dimensionless), by e = /cso, where Sq is the permittivity of free space, 8.854 x 10 S sec/m. Dielectric properties arise from bound charge that is displaced by the electric field, creating a dipole. They can also arise if the appHed electric field aligns molecular dipoles (such as the dipole moments of water molecules) that are normally oriented randomly. The DC dielectric constant of sahne is similar to that of water (about... 

We may define the value of the lead vector at every point in the volume conductor. If we then place the lead vectors to the points for which they are defined, we have a field of lead vectors throughout the volume conductor. This field of lead vectors is called the lead field /l. The lead field illustrates the behavior of the sensitivity in the volume conductor and is a very powerful tool in analyzing the properties of electric and magnetic leads (see Figure 27.1b). 

The ability of a polymer to flow or to oppose an electrical current under the application of an external electric field is known as its electrical property. In a good conductor, electrons or ions are able to flow easily through the material. If the material resists the flow, it is described as an insulator. Most polymers are insulators. The volume resistance of a polymer is the resistance of an electric field (current) between the opposite faces of a unit... 

As a result of the electrical activity of the heart, electrical potentials appear at all points of the body surface. These potentials reflect both the electrical sources within the heart, and the electrical properties of the torso volume conductor. The... 

The insulation resistance between two conductors or plated holes is the ratio of the voltage to the total current between the conductors. Two measures of electrical resistance are volume and surface resistivities. Since these properties can vary with temperature and humidity, testing is normally performed at two standardized environmental conditions, one involving humidity conditioning, the other involving elevated temperature. Humidity conditioning subjects the sample to 90 percent relative humidity and 35°C for 96 hours (96/35/90).The elevated temperature conditioning normally subjects the sample to 125°C for 24 hours (24/125). 

The volume resistivity of a material is the ratio of the potential gradient parallel to the current in the material to the current density. Insulating materials are generally used to insulate and support components of an electric network from each other and from the ground. Therefore, it is desirable to have the insulation resistance as high as possible, consistent with acceptable mechanical, physical, chemical, and thermal properties. Volume resistivity measurements are designed to isolate the inherent properties responsible for the dc insulating qualities of a plastic material. Resistivities above 10 ohm cm are considered to be insulators those with values of 10 to 10 ohm cm are partial conductors. 

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