METHODS OF PRODUCING VOLTAGE (ELECTRICITY) Basic Electrical TheoryFigure 16 Producing Electricity from Light Using a Photovoltaic CellThis phenomenon is called the photoelectric effectand has wide applications in electronics, suchas photoelectric cells, photovoltaic cells, optical couplers, and television camera tubes. Threeuses of the photoelectric effect are described below.Photovoltaic: The light energy in one of two plates that are joined together causesone plate to release electrons to the other. The plates build up opposite charges,like a battery (Figure 16).Photoemission: The photon energy from a beam of light could cause a surface torelease electrons in a vacuum tube. A plate would then collect the electrons.Photoconduction: The light energy applied to some materials that are normallypoor conductors causes free electrons to be produced in the materials so that theybecome better conductors.ThermionicEmissionA thermionic energy converter is a device consisting of two electrodes placed near one anotherin a vacuum. One electrode is normally called the cathode, or emitter, and the other is calledthe anode, or plate. Ordinarily, electrons in the cathode are prevented from escaping from thesurface by a potential-energy barrier. When an electron starts to move away from the surface,it induces a corresponding positive charge in the material, which tends to pull it back into thesurface. To escape, the electron must somehow acquire enough energy to overcome this energybarrier. At ordinary temperatures, almost none of the electrons can acquire enough energy toescape. However, when the cathode is very hot, the electron energies are greatly increased bythermal motion. At sufficiently high temperatures, a considerable number of electrons are ableto escape. The liberation of electrons from a hot surface is called thermionic emission.ES-01 Page 24 Rev. 0
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