Electric current in metal

One of the most exciting and perhaps unexpected discoveries in science within the last decade has been the observation of superconductivity (the complete absence of resistivity to electric current) in metal oxides at temperature < 90 K. This tempera-... 

The concept of quantization enabled physicists to solve problems that nineteenth-century physics could not. One of these involved the thermal properties of solids when they are heated to incandescence. The other involved the induction of electrical current in metals when they are exposed to only specific frequencies of electromagnetic radiation. 

What is different about electric current in metals compared with current in electrolyte solutions ... 

It would appear that the technique shows promise but more work needs to be done on the use of magnetic devices for the reduction of scaling problems before the technology can be used with confidence. Furthermore the possibility of inducing electric currents in metallic structures, that could result in enhanced corrosion (see Chapter 10), must be carefully considered. 

Students should be aware from everyday life about distinguishing materials as electrical conductors and insulators. An added feature of metallic conduction is that no chemical changes are taking place at the same time as current flows. From physics, students should know that the electric current in metallic conductors is the flow of electrons through the metal, which is caused by an electric potential difference (an electric field) across the conductor. 

Electric currents in metallic conductors and semiconductors are due to the motions of electrons, whereas electric currents in electrolyte solutions are due to the motions of ions. Ohm s law asserts that the current in a conducting system is proportional to the voltage imposed on the system ... 

Current electricity is a flow of electrically charged particles. In electric currents in metallic conductors, the charged particles are electrons in molten salts or in aqueous solutions, the particles are both negatively and positively charged ions. 

Metal contained in the channel is subjected to forces that result from the interaction between the electromagnetic field and the electric current in the channel. These inward forces produce a circulation that is generally perpendicular to the length of the channel. It has been found that shaping the channels of a twin coil inductor shown in Figure 10 produces a longitudinal flow within the channel and significantly reduces the temperature difference between the channel and the hearth (12). 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

The ions move between electrodes in the electrolyte due to voltage potential gradients. The velocity of these chemical currents increases with temperature. Hence, electrolytic conductivity increases as temperature goes up. This is the opposite of electrical currents m metallic conductors, which increase as the temperature goes down. 

Seebeck s outstanding scientific achievement was the discovei"y of one of the three classical thermoelectric effects, which are the Seebeck, the Peltier, and the Thomson effects. Seebeck s discovery was the first, dating from 1822—1823, followed by that of Jean-Charles-Athanase Peltier in 1832 and that of William Thomson in 1854. Seebeck obseiwed that an electric current in a closed circuit comprised different metallic components if he heated the junctions of the components to different temperatures. He noted that the effect increases linearly with the applied temperature difference and that it crucially depends on the choice of materials. Seebeck tested most of the available metallic materials for thermoelectricity. His studies were further systematized by the French physicist... 

Metals and semiconductors are electronic conductors in which an electric current is carried by delocalized electrons. A metallic conductor is an electronic conductor in which the electrical conductivity decreases as the temperature is raised. A semiconductor is an electronic conductor in which the electrical conductivity increases as the temperature is raised. In most cases, a metallic conductor has a much higher electrical conductivity than a semiconductor, but it is the temperature dependence of the conductivity that distinguishes the two types of conductors. An insulator does not conduct electricity. A superconductor is a solid that has zero resistance to an electric current. Some metals become superconductors at very low temperatures, at about 20 K or less, and some compounds also show superconductivity (see Box 5.2). High-temperature superconductors have enormous technological potential because they offer the prospect of more efficient power transmission and the generation of high magnetic fields for use in transport systems (Fig. 3.42). 

Electroplating is a process by which a metal such as copper is coated with another metal, such as silver or chromium. The transfer of metal atoms is driven by an electrical current. In an electroplating process, a spoon... 

In the laser flash method, the heat is put in by laser flash instead of electric current in the stepwise heating method mentioned above. Thus this method may be classified as a stepwise heating method. A two-layered laser flash method was developed by Tada et al. " The experimental method and the data analysis, including a case involving radiative heat flow, are described in detail in the review article by Waseda and Ohta. A thin metal plate is placed at the surface of a melt. A laser pulse is irradiated onto a metal plate of thickness / having high thermal conductivity. The sample liquid under the metal plate and the inert gas above the plate are designated as the third and first layers, respectively. The temperature of the second layer becomes uniform in a short time" and the response thereafter is expressed by... 

The further fate of the solvated electrons depends on solution composition. When the solution contains no substances with which the solvated electrons could react quickly, they diffuse back and are recaptured by the electrode, since the electrochemical potenhal of electrons in the metal is markedly lower than that of solvated electrons in the solution. A steady state is attained after about 1 ns) at this time the rate of oxidahon has become equal to the rate of emission, and the original, transient photoemission current (the electric current in the galvaihc cell in which the illuminated electrode is the cathode) has fallen to zero. Also, in the case when solvated electrons react in the solution yielding oxidizable species (e.g., Zn " + Zn" ),... 

FIGURE 39 Corrosion. Corrosion is the process of gradual deterioration of metals and alloys as a result of their interaction with the environment. The corrosion process is a reversal of metallurgical processes, whereby metals are recovered from the minerals in which they occur in nature (a). It is an electrolytic process, brought about by the passage of electric currents. Any metal or alloy contains sites in which there are slight local compositional differences. When such compositional differences are exposed to a humid or wet environment, extremely small electrolytic cells as the one shown in (b) are created in each cell, an electric current drives the otherwise nonspontaneous corrosion reactions. In a surface undergoing corrosion there are millions of electrolytic cells. 

A rectifier, or diode, passes electrical current in one direction (the forward bias direction), but blocks it in the other direction (reverse bias). For a molecule between two electrodes in a metal I molecule I metal sandwich, there are three distinct processes that can give rise to such an asymmetrical conduction. 

Bismuth is more resistant to electrical current in its solid state than it is in its liquid form. Its thermal conductivity is the lowest of all metals, except mercury. Even though it is considered a metal-like element, it is a very poor conductor of heat and electricity. 

Classical Free-Electron Theory, Classical free-electron theory assumes the valence electrons to be virtually free everywhere in the metal. The periodic lattice field of the positively charged ions is evened out into a uniform potential inside the metal. The major assumptions of this model are that (1) an electron can pass from one atom to another, and (2) in the absence of an electric field, electrons move randomly in all directions and their movements obey the laws of classical mechanics and the kinetic theory of gases. In an electric field, electrons drift toward the positive direction of the field, producing an electric current in the metal. The two main successes of classical free-electron theory are that (1) it provides an explanation of the high electronic and thermal conductivities of metals in terms of the ease with which the free electrons could move, and (2) it provides an explanation of the Wiedemann-Franz law, which states that at a given temperature T, the ratio of the electrical (cr) to the thermal (k) conductivities should be the same for all metals, in near agreement with experiment ... 

A metal differs sharply from a dielectric by its electron energy spectrum at absolute zero. The basic state of a metal is contiguous to a continuous spectrum of states. For this reason, an arbitrarily weak electric field causes an electric current in the metal which depends on transition of the system to states which are arbitrarily close in energy to the basic state. On the other hand, the electron energy spectrum of a dielectric is characterized by the existence of a finite gap, a certain difference in energies between the basic state with minimum energy (In which there is no current) and adjacent excited states in which one of the electrons of the dielectric becomes free and electrical conductivity appears. 

Zeolites have been also used in the production of fuel cell electrodes, fundamentally as catalyst supports for metals such as Ag, Pd, Rh, Pt, Ru, and Ni [174], Besides, zeolite membranes have been used for the separation of the anode and the cathode sections of a fuel cell, because they allow the transfer of ions through it, for example, to carry an electric current in methanol fuel cells using a basic electrolyte such as carbonate, where the zeolite prevents the escape of methanol [174,179],... 

Metal ions cannot move in the metal electrode but can move through the solution, producing electric current in the solution under an applied potential. 

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