METHODS OF PRODUCING VOLTAGE (ELECTRICITY)
Basic Electrical Theory
Figure 16 Producing Electricity from Light Using a Photovoltaic Cell
This phenomenon is called the photoelectric effect and has wide applications in electronics, such
as photoelectric cells, photovoltaic cells, optical couplers, and television camera tubes. Three
uses of the photoelectric effect are described below.
Photovoltaic: The light energy in one of two plates that are joined together causes
one 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 to
release electrons in a vacuum tube. A plate would then collect the electrons.
Photoconduction: The light energy applied to some materials that are normally
poor conductors causes free electrons to be produced in the materials so that they
become better conductors.
A thermionic energy converter is a device consisting of two electrodes placed near one another
in a vacuum. One electrode is normally called the cathode, or emitter, and the other is called
the anode, or plate. Ordinarily, electrons in the cathode are prevented from escaping from the
surface 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 the
surface. To escape, the electron must somehow acquire enough energy to overcome this energy
barrier. At ordinary temperatures, almost none of the electrons can acquire enough energy to
escape. However, when the cathode is very hot, the electron energies are greatly increased by
thermal motion. At sufficiently high temperatures, a considerable number of electrons are able
to escape. The liberation of electrons from a hot surface is called thermionic emission.