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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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Capacitors 79
BRASS BALL Figure 6-2 Leyden jar capacitor.
charge depending upon the air’s humidity. When the inside and outside foil were electrically connected, an electric discharge would occur.
More than one Leyden jar could be connected together. In Figure 6-3, where the Leyden jars are connected in series (inner foil connected to outer foil of preceding jar) and the two discharge balls approach each other, the discharge wouldn’t be heavy, but would discharge quite a distance, as the emf of each Leyden jar would add to each of those in series; thus, an increased potential would result.
Figure 6-3 Leyden jars connected in series.
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When Leyden jars are paralleled as in Figure 6-4 (inner foils connected together), the discharge emf would be the same as for each individual Leyden jar and the discharge gap would be only one-third of that in Figure 6-3, but the spark would be thicker due to the current of the Leyden jars adding up (the current would be three times as heavy as in Figure 6-3).
Figure 6-4 Leyden jars connected in parallel.
Leyden jars are really used only for experiments nowadays, but they very effectively illustrate the theory of capacitors.
The most commonly used form of capacitor is composed of layers of foil separated by wax paper as the dielectric, or some other similar dielectric. See Figure 6-5. The foil is cut into long narrow strips, separated by waxed paper, and then rolled up into a cylinder as shown in Figure 6-6. This of course is not the only form that capacitors appear in.
Figure 6-5 A commonly used capacitor in finished form (schematic).
Capacitors 81
Figure 6-6 A commonly used form of capacitor.
A more recent type of capacitor is the electrolytic capacitor. The plates of this type of capacitor are polarized and marked anode (+) and cathode ( —). The anode is the aluminum foil with large surface area, while the cathode is usually an aluminum container, but not necessarily so. There are both wet and dry types of electrolytic capacitors. The most common type is the wet type, with a solution of borax and boric acid in water. In another type, the electrolyte is ammonium citrate in water. The dry type uses a paste electrolyte of boric acid, glycerine, and ammonia. Paper or gauze packed between the anode and cathode is impregnated with the solution.
A DC source of emf is connected to the electrolytic capacitor, with the positive (+) to the anode and the negative ( —) to the cathode. A slight current is established through the electrolyte, which produces a thin film on the surface of the anode, polarizing the capacitor. This film also serves as the dielectric. This is termed forming the capacitor.
If the polarity is reversed, the film will break down and the capacitor will be destroyed, often blowing up.
There is also an electrolytic capacitor that is not polarized; it is used in starting single-phase motors. These AC electrolytic capacitors can’t be subjected to AC for long periods or too often in quick succession.
A capacitor doesn’t pass electrons. (Lest one contest this statement, since there is no perfect insulator, we probably should state that basically a capacitor passes no electrons.)
As stated previously, the two plates of a capacitor become charged, one negative and one positive. A good way to demonstrate this is with a capacitor, a two-way switch, a DC source of emf, and a galvanometer. (We have not discussed galvanometers as yet, as these will be covered in the chapter on meters, but a galvanometer is a delicate meter for registering current.)
When the switch A in Figure 6-7 is closed, the galvanometer will show current as waveform B of Figure 6-8. The peak is the maximum flow point and then the flow tapers off. Thus the voltage doesn’t all appear across the capacitor at first but must build up as the current drops. Then, when switch B in Figure 6-7 is closed, the galvanometer will show current as waveform C of Figure 6-8. The voltage across the capacitor drops to zero as the current goes to zero and the negative charges move to the other plate of the capacitor. Thus, a
82 Chapter 6
Figure 6-7 Charging and discharging a capacitor.
capacitor opposes changes in voltage. (The circuit property that opposes changes in current is called inductance and will be discussed later.)
Figure 6-8 Current impulses of the circuit in Figure 6-7.
Figure 6-9 illustrates a capacitor in series with a light bulb and a DC source of emf from a battery. When switch A is closed, the lamp will light up for an instant and then go out. This is due to the current to charge the plates of the capacitor. When the capacitor is charged, no more current will flow.
Figure 6-9 A lamp connected in series with a capacitor and a DC source.
Capacitors 83
Figure 6-10 shows a capacitor in an AC circuit, with a switch and lamp. When the switch is closed, the lamp will light and continue to stay lit. Actually, the current doesn’t flow through the capacitor, but the plates are alternately charged positive and negative, giving the effect of current flowing through the capacitor. This happens so rapidly that the filament of the lamp doesn’t have time to cool down, and thus it appears as if current flows steadily. During one cycle, the current flows first in one direction and then flows in the other direction. Thus, with 60 cycles, there are 120 alternations per second.
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