This section
provides information on the following methods of producing
·
electricity:
·
Electrochemistry
·
Static (friction)
·
Induction (magnetism)
·
Piezoelectric (pressure)
·
Thermal (heat)
·
Light
·
Thermionic emission
EO 1.5 DESCRIBE how the following methods produce a
voltage:
a. Electrochemistry
b. Static electricity
c. Magnetic induction
d. Piezoelectric effect
e. Thermoelectricity
f. Photoelectric effect
g. Thermionic
emission
Electrochemistry-
Chemicals can be combined with certain metals to cause a
chemical reaction that will transfer electrons to produce electrical energy.
This process works on the electrochemistry principle. One example
of this principle is the voltaic chemical cell, shown in Figure 11. A chemical reaction
produces and maintains opposite charges on two dissimilar metals that serve as
the positive and negative terminals. The metals are in contact with an
electrolyte solution. Connecting together more than one of these cells will
produce a battery.
Static
Electricity-
Atoms with the proper number of electrons in orbit around them are in a
neutral state, or have a "zero charge." A body of matter consisting
of these atoms will neither attract nor repel other matter that is in its
vicinity. If electrons are removed from the atoms in this body of matter, as
happens due to friction when one rubs a glass rod with a silk cloth, it will
become electrically positive as shown in Figure 12. If this body of matter (e.g.,
glass rod) comes near, but not in contact with, another body having a normal charge,
an electric force is exerted between them because of their unequal charges. The
existence of this force is referred to as static electricity or electrostatic
force.
Magnetic Induction-
A generator is a machine that converts mechanical energy
into electrical energy by using the principle of magnetic induction.
Magnetic induction is used to produce a voltage by rotating coils of wire
through a stationary magnetic field or by rotating a magnetic field through
stationary coils of wire. This is one of the most useful and wide l yem ployed applications
of producing vast quantities of electric power. Magnetic induction will be
studied in more detail in the next two chapters "Magnetism," and
"Magnetic Circuits."
Piezoelectric Effect
By applying pressure to certain crystals (such as quartz
or Rochelle salts) or certain ceramics (like barium titanate), electrons can be
driven out of orbit in the direction of the force. Electrons leave one side of
the material and accumulate on the other side, building up positive and negative
charges on opposite sides, as shown in Figure 14. When the pressure is
released, the electrons return to their orbits. Some materials will react to
bending pressure, while others will respond to twisting pressure. This
generation of voltage is known as the piezoelectric effect. If external wires
are connected while pressure and voltage are present, electrons will flow and
current will be produced. If the pressure is held constant, the current will
flow until the potential difference is equalized. When the force is removed,
the material is decompressed and immediately causes an electric force in the
opposite direction. The power capacity of these materials is extremely small. However,
these materials are very useful because of their extreme sensitivity to changes
of mechanical force.
Thermoelectricity-
Some materials readily give up their electrons and others
readily accept electrons. For example, when two dissimilar metals like copper
and zinc are joined together, a transfer of electrons can take place. Electrons
will leave the copper atoms and enter the zinc atoms. The zinc gets a surplus
of electrons and becomes negatively charged. The copper loses electrons and
takes on a positive charge. This creates a voltage potential across the junction
of the two metals. The heat energy of normal room temperature is enough to make
them release and gain electrons, causing a measurable voltage potential. As
more heat energy is applied to the junction, more electrons are released, and
the voltage potential becomes greater, as shown in Figure 15. When heat is
removed and the junction cools, the charges will dissipate and the voltage
potential will decrease. This process is called thermoelectricity. A
device like this is generally referred to as a "thermocouple."
Thermionic Emission-
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.
