Electrical circuits electric current

The graph of Figure 6.8 illustrates the effect of increasing voltage on the electric current between two electrodes immersed in a gas. The circuit is completed by an external resistance, used to limit the current flow. As shown in Figure 6.8, the discharge can be considered in regions, which are described below. 

The term channel induction furnace is appHed to those in which the energy for the process is produced in a channel of molten metal that forms the secondary circuit of an iron core transformer. The primary circuit consists of a copper cod which also encircles the core. This arrangement is quite similar to that used in a utdity transformer. Metal is heated within the loop by the passage of electric current and circulates to the hearth above to overcome the thermal losses of the furnace and provide power to melt additional metal as it is added. Figure 9 illustrates the simplest configuration of a single-channel induction melting furnace. Multiple inductors are also used for appHcations where additional power is required or increased rehabdity is necessary for continuous operation (11). 

Specific Conductance. The specific conductance depends on the total concentration of the dissolved ioni2ed substances, ie, the ionic strength of a water sample. It is an expression of the abiUty of the water to conduct an electric current. Freshly distilled water has a conductance of 0.5—2 ]lS/cm, whereas that of potable water generally is 50—1500 ]lS/cm. The conductivity of a water sample is measured by means of an a-c Wheatstone-bridge circuit with a null indicator and a conductance cell. Each cell has an associated constant which, when multiphed by the conductance, yields the specific conductance. 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

Electric current passing through a metal joint having a moisture content causes electrolysis of water vapour. Copper, being a galvanic metal, forms an electrolytic circuit with other metals and decomposes the joint. Decomposition is corroding and erodes the aluminium metal. 

Fig. 10-5 Protection measure by separation of electrical operational equipment that is connected to the cathodically protected object via the housing, with an FI protection circuit leakage current circuit breaker (see Ref. 14) Tj and isolating transformers (see Ref. 15).

From a basic physics standpoint, a circuit is a complete loop. Electric current can flow only if it returns to its source, i.e., completes the circuit. The path tlirough which tlie current returns to its source is called the "retum or ground. The reason for the tenn "ground is tliat the ctirlh is literally used to provide the return patli no matter what distance separates tlie equipment from the power source. Ground comiections can be made to a cold water system as its components provide a reliable, low-resistance patli for contact witli tlie earth. 

The accidental or unexpected sudden starting of electrical equipment can cause severe injuiy or death. Before ANY inspections or repairs are made (even on the so-called low-voltage circuits) the current should be turned off at the switch box and the switch padlocked in the OFF position. At tlie same time, the switch or controls of the macliine or other equipment being locked out of service should be securely tagged to show which equipment or circuits are being worked on. Lockouts and tagouts should be removed only by the individual(s) who installed them. 

The chemical process that produces an electrical current from chemical energy is called an oxidation-reduction reaction. The oxidation-reduction reaction in a battery involves the loss of electrons by one compound (oxidation) and the gain of electrons (reduction) by another compound. Electrons are released from one part of the batteiy and the external circuit allows the electrons to flow from that part to another part of the batteiy. In any battery, current flows from the anode to the cathode. The anode is the electrode where positive current enters the device, which means it releases electrons to the external circuit. The cathode, or positive terminal of the battery, is where positive current leaves the device, which means this is where external electrons are taken from the external circuit. 

An important property of this or any electrical circuit is the rate that charge moves past a place in the circuit (e.g., out from or into a battery terminal). The electrical current (I) is defined to be the charge (Q) that flows, divided by the time (t) required for the flow I = Q/t. In S.I. units the current (I) is in amperes (A). 

In 1821, Thomas Seebeck, an Estonian physician, discovered the existence of an electric current in a closed circuit consisting of unlike conductors, when the junctions between the conductors were at different temperatures. This discovei y is the basis for ther-... 

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... 

The photoelectric effect (the creation of an electrical current when light shines on a photosensitive material connected m an electrical circuit) was first obseiwed in 1839 by the French scientist Edward Becqiierel. More than one hundred years went by before researchers in the United States Bell Laboratories developed the first modern PV cell in 1954. Four years later, PV was used to power a satellite in space and has provided reliable electric power for space exploration ever since. 

In 1831 Michael Faraday showed that an electromotive force is produced when a wire is moved through the lines of force of a magnet. If the wire is part of a complete circuit, a current flows. In the following years, several inventors made magneto-electric generators ( magnetos ) in which coils of wire were rotated close to the poles of a fixed magnet, or a... 

These ideas, developed for an electrochemical cell, have great importance in chemistry because they are also applicable to chemical reactions that occur in a single beaker. Without an electric circuit or an opportunity for electric current to flow, the chemical changes that occur in a cell can be duplicated in a single solution. It is reasonable to apply the same explanation. 

The photometer is adequately described in Figure 3-2. In the phosphor-photoelectric detector (2.10), the x-ray beam strikes a silver-activated zinc sulfide phosphor to produce blue-violet light that is changed by the multiplier phototube (Type 931-A) into an electric current that is amplified and read on a suitable micro- or milliammeter. A stable power supply for both x-ray tube and detector circuit are essential, as is clear from the circuit diagrams.10... 

If the two junctions of a circuit of two wires of different metals are maintained at different temperatures, Ti > T2, an electric current flows round the circuit, its direction and magnitude depending on the nature of the metals and on the temperatures (Seebeck, 1821). 

Under most conditions, the process is spontaneous/ A chemical potential difference drives the reaction and AG < 0. When the reactants are separated as shown in Figure 9.3, the chemical potential difference can be converted to an electrical potential E. When the electrodes are connected through an external circuit, the electrical potential causes an electric current to flow. Because the electrical potential is the driving force for electrons to flow, it is sometimes... 

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