Basic electrical quantities

As mentioned earlier, the basic electrical quantities, current and voltage drop, are measured directly during a pulse e3q>eriment. With respect to the timescale and the resulting electrical pulses during the discharge, different precautions have to be made to avoid stray pick-up. Therefore, all signal lines are... 

Figure 5. Iridium Set of thermophysical properties obtained from a single pulse-calorimetric experiment on an iridium sample a) basic electrical quantities and the pyrometer signal as a function of experimental duration b) specific enthalpy as a function of temperature c) electrical resistivity, at initial geometry and with volume expansion, as a function of temperature d) thermal conductivity as a function of temperature e) thermal diffusivity as a frmction of temperature f)...

Table 28 Some simple formulas illustrating relationships between a number of basic electrical quantities...

One of the most commonly measured and basic electrical quantities of samples in practice is the resistance R, which depends both on the material, represented by its resistivity P, and the geometry of the measured sample. For a rectangular sample (a cuboid) of... 

Electrical current (I) has been chosen as the basic SI unit in terms of which all other electrical quantities are defined. Unit current, the ampere (A, or amp), is defined in terms of the force exerted between two parallel conductors in which a current of 1 amp is flowing. Since the unit of power, the watt, is the product of current and potential difference,... 

The two most basic electrical parameters of interest, normally, are the carrier concentration and mobility. These quantities may be obtained from the proper measurements of current, electrical field, and applied magnetic field. 

The control of charge flow by an electric quantity is a key issue of today s electronics. The concept to electrically specify the conductivity of a resistor by pure solid state effects was already proposed in 1928 by Julius Edgar Lilien-feld in Germany [1], The basic idea was to control the charge carrier density in a solid by an electric field, applied over a third electrode. However, there is no evidence for a practical realisation by Lilienfeld. The first report about a pure electrically controllable solid state device was the well know Germanium transistor from William Shockley, John Bardeen and Walter Brattain [2]. The new term transistor was later explained as a combination of the words transconductance and varistor . Meanwhile a broad variety of different transistor concepts exists, which, however, can be mainly subdivided in two basic operational principles ... 

Electrical measurement refers to the quantification of electrical properties. As with all forms of measurement, these procedures provide values relative to defined standards. The basic electrical measurements are voltj e, resistance, current, capacitance, and waveform analysis. Other electrical quantities such as inductance and power are generally not measured direcdy but are determined from the mathematical relationships that exist among actual measured properties of an electric circuit. 

As amply demonstrated by the early workers, it is possible to carry out a preparation electrochemically with little other than a suitable electrolysis cell and a source of power such as a battery. The situation is quite different in fields such as analytical or physical chemistry instrumentation becomes important, and may be a controlling factor (2 ). A basic requirement is the ability to measure electrical quantities such as potential, current, and resistance or conductance. 

As in previous chapters we work in the continuum limit employing quantities averaged over macroscopically infinitesimal volume elements and disregarding microscopic local variations associated with the molecular structure (see Brown 1956). These considerations will be limited to processes sufficiently slow to restrict the treatment to time independent or quasistatic fields. The validity of Maxwell s equations of electrostatics is presupposed. The basic electric state variables are the electric field strength vector E, the electric flux density (or electric displacement) vector D, and the electric polarization vector P, related by... 

In the International system of units (SI), now adopted, the basic electric unit is the ampere, A. This is defined as the current I which, if maintained in two parallel conductors of infinite length and negligible circular cross-section, at a distance of 1 metre apart in a vacuum, would produce a force between the conductors equal to 2x10 newton per metre of length. The other common SI electric units are for quantity of electricity, the coulomb, C = As for electric potential, the volt, V = JA s for resistance, the ohm, 11 = VA" and for capacitance, the farad, F=CV ... 

The standard method to synthesize MWCNTs is based on the electric-arc experiment proposed by Ebbesen and Ajayan [8]. Basically, the production system is similar to the one used by Kratschmer et al. [11] to produce macroscopic quantities of C o and the main difference between the two experiments is the inert gas pressure, that must be rather low (20-100 mbar) for an efficient fullerene production [11], but must be increased to 350-700 mbar to generate nanotubes efficiently [8],... 

To illustrate the use of the vector operators described in the previous section, consider the equations of Maxwell. In a vacuum they provide the basic description of an electromagnetic field in terms of the vector quantifies the electric field and 9C the magnetic field The definition of the field in a dielectric medium requires the introduction of two additional quantities, the electric displacement SH and the magnetic induction. The macroscopic electromagnetic properties of the medium are then determined by Maxwell s equations, viz. 

The bulk (or volume) specific resistance p is one of the most useful electric properties that can be measured. Specific resistance is a physical quantity that may differ by more than 1023 in readily available materials. This unusually wide range of conductivity is basic to the wide use of electricity and many electric devices. Conductive materials, such as copper, have p values of about 10-6 ohm cm, while good insulators, such as polytetrafluoroethylene (PTFE) and low-density polyethylene (ldpe), have p values of about 1017 ohm cm. 

Eddy-Current Methods This is one of the earliest NDT methods and is still used Basically, this method reveals any differences tn electrical impedance between parts to be tested and a reference sample. Parts to be examined are passed through a coil or explored with a probe, and a trace appears on a CRT, Since magnetic and electrical characteristics are closely related to metallurgical quantities, a trace position or pattern or a meter reading clearly shows variations in metal hardness and composition, as well as defects. Both ferrous and nonferrous parts can be tested, and various coils, probes, and detector lips are available,... 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, firstly by ion-dipole or ion-induced dipole interactions and secondly by more direct interactions, such as hydrogen bonding to anions or electron pair donation to cations. The latter interactions, thus, depend on the Lewis acidity and basicity, respectively, of the solvents (Table 4.3). The redox properties of the ions at an electrode therefore depend on their being solvated, and the solvent effects on electrode potentials or polarographic half wave potentials, or similar quantities in voltammetry are manifested through the different solvation abilities of the solvents. 

This mention of a family of solvents with particular physical properties prompt us to remark that there are other solvents with special physical quantities requiring some modifications in the methodological formulation of basic PCM. We cite, among others, liquid crystals in which the electric permittivity has an intrinsic tensorial character, and ionic solutions. Both solvents are included in the IEF formulation of the continuum method [20] which is the standard PCM version. 

New spatial forms of carbon - fullerenes, nanotubes, nanowires and nanofibers attract significant interest since the time of their discovery due to their unique physicochemical and mechanical properties [1-3]. There are three basic methods of manufacturing of the carbon nanomaterials (CNM) - laser evaporation, electric arc process, and catalytic pyrolysis of hydrocarbons. However, the multi-stage manufacturing process is a serious disadvantage for all of them. For example, the use of organic solvents (benzol, toluene, etc.) for separation of fullerenes from graphite soot results in delay of the synthesis process and decrease in the final product quantity. Moreover, some environmental problems can arise at this. 

The basic principle of a diffusion pump can be explained with a simple single-stage mercury diffusion pump (see Fig. 7.21). On the system side of the pump (at about 10 2 to 10 3 torr, or better), gas molecules wander around, limited by their mean free path and collisions with other molecules. The lowest section of this diffusion pump is an electric heater that brings the diffusion pump liquid up to its vapor pressure temperature. The vapors of the diffusion pump liquid are vented up a central chimney where, at the top, they are expelled out of vapor jets at supersonic speeds (up to 1000 ft/sec). Below these jets is a constant rain of the pumping fluid (mercury or low vapor-pressure oil) on the gases within the vacuum system. Using momentum transfer/ gas molecules are physically knocked to the bottom of the pump, where they are trapped by the vapor jets from above. Finally, they are collected in a sufficient quantity to be drawn out by the auxiliary (mechanical) pump. 

---