Electric current description

Photovoltaic devices typically consist of a series of thin semiconductor layers that are designed to convert sunlight to dkect-current electricity (see Semiconductors). As long as the device is exposed to sunlight, a photovoltaic (PV) cell produces an electric current proportional to the amount of light it receives. The photovoltaic effect, first observed in 1839, did not see commercial appHcation until the 1950s when photovoltaic modules were used to power early space sateHites. Many good descriptions of the photovoltaic phenomenon are available (7). 

The Mechanism of Electrical Conduction. Let us first give some description of electrical conduction in terms of this random motion that must exist in the absence of an electric field. Since in electrolytic conduction the drift of ions of either sign is quite similar to the drift of electrons in metallic conduction, we may first briefly discuss the latter, where we have to deal with only one species of moving particle. Consider, for example, a metallic bar whose cross section is 1 cm2, and along which a small steady uniform electric current is flowing, because of the presence of a weak electric field along the axis of the bar. Let the bar be vertical and in Fig. 16 let AB represent any plane perpendicular to the axis of the bar, that is to say, perpendicular to the direction of the cuirent. 

No description of British devices corresponding to US electric squibs is found in Brit books on explosives in our possession, such as Refs 36, 38 51. In Ref 38, p 59 is, however, a description of an electric device which probably serves the same purpose. It is an electric powder fuse, which consists of a thick paper tube contg a small chge of Blasting Powder (Brit for Black Powder or Gunpowder), with an ordinary low-tension fusehead fixed at one end. On passing electric current thru the fusehead it flashes and sets off the BkPdr in the tube, which can... 

In the more usual description of the emu system only three base units and three independent dimensions are used. The dimension (electric current) is defined to be the same as that of (force)1/2, so that 1 (emu of current)2 = lgcms 2 = l dyn. The (emu of current) then disappears as a unit, and the constant /i0(ir) is dimensionless and equal to 1, so that it may be omitted from all equations. Thus equation (4b) for the force between current elements in vacuum, for example, becomes simply... 

Ions are formed by the action of particle beams, decomposition of molecules excited by photons, donor-acceptor interactions of suitable compounds and by the effects of an electric current on solutions of supporting electroly-tes. Some special polymerization processes can be initiated by anions and cations generated in this way. The practical importance of all the enumerated methods is so far not large. Nevertheless, these processes continue to be the subject of intense study. A short description of the most interesting of these methods is contained in Sects. 5 and 6.1. Polymerization-initiating anions can even be formed from cations (see Chap. 4, Sect. 5). 

It is not difficult to observe that in all of these expressions we have a multiplication between the property gradient and a constant that characterizes the medium in which the transport occurs. As a consequence, with the introduction of a transformation coefficient we can simulate, for example, the momentum flow, the heat flow or species flow by measuring only the electric current flow. So, when we have the solution of one precise transport property, we can extend it to all the cases that present an analogous physical and mathematical description. Analogous computers [1.27] have been developed on this principle. The analogous computers, able to simulate mechanical, hydraulic and electric micro-laboratory plants, have been experimented with and used successfully to simulate heat [1.28] and mass [1.29] transport. 

The classical description of an electrochemical cell is that it consists of two electrodes immersed in an electrolyte solution. The simplest type of electrode is a metal, so that a simple cell involves two metal solution interfaces. However, experimental study of the electrochemical cell always involves other interfaces which are introduced when the electrodes are connected to the leads of the potential measuring device. When the cell is a source of energy, an electrical current flows in an external circuit as a result of a chemical reaction at each electrode. There are important conventions regarding the presentation of information about these systems. These are outlined in detail below. 

The simplest part of the process to understand is the way the messages pass down the nerve axon itself. Here our earlier description of the nerve fibre, surrounded by its fatty sheath like an insulated electric cable, comes strikingly into its own. In fact, messages pass down the nerve axon in the form of electric currents which begin where the axon springs out of the nerve cell body and travel down the fibre at a rate of about twenty metres a second. 

The optimization of an electrochemical reactor calls for a full description of the process to accomplish the specific objective. The problem of the optimization of the electrocatalyst is of real importance in most of the recently developed technical electrodes that were prepared without detailed studies. It must be borne in mind that the strong experimental conditions in which the large electrical currents and large ionic forces of the electrolytes prevail change the morphology and the composition of the catalyst. 

Structural features of disperse systems, in particular the existence of the electrical double layer (EDL), are responsible for a number of peculiar phenomena related to heat and mass transfer and electric current propagation in such systems. The description of electromagnetic radiation propagation is also included in this chapter. These features are utilized in numerous practical applications and underlie methods used to study disperse systems. These methods include particle size distribution analysis, studies of the surface structure and of near-surface layers, the structure of the EDL, etc. In the most general way the most transfer phenomena can be described by the laws of irreversible thermodynamics, which allow one to carry out a systematic investigation of different fluxes that originate as a result of the action of various generalized forces. 

In a transition from an individual capillary to a real structured disperse system (membrane or diaphragm), one faces complications related to the actual structure of porous medium, in which the transfer of substance and electric current take place. In such systems all previously described basic relationships remain valid, but the radius and length of a single capillary are replaced with coefficients having particular dimensions, referred to as the structure parameters . In general, the determination of these structure parameters is a rather difficult task, but one may expect that in the description of electroosmotic transfer and the electric conductivity of the structured disperse systems these parameters are included in an identical way, similar to the identical dependence of IE and QE on r and /, as shown in eqs (V.32) and... 

The description of oxidation and reduction in terms of electron losses and gains is not merely formal, since very many oxidations and reductions can be carried out by means of an electric current, and in fact are so carried out (see Chapter VII, Electrolytic Methods). If a ferric chloride solution is electrolyzed between inert electrodes, such as platinum, the ferric ion is reduced to ferrous at the cathode ... 

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