This
article provides information on the following methods of producing electricity:
•
Electrochemistry
•
Static (friction)
•
Induction (magnetism)
•
Piezoelectric (pressure)
•
Thermal (heat)
•
Light
•
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 1. 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.
Figure
1 Voltaic Chemical Cell
Example:
A battery can maintain a potential difference between its positive and negative
terminals by chemical action. Various types of cells and batteries will be
studied in more detail in Module Batteries.
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 2. 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.
Figure
2 Static Electricity
Example:
Have you ever walked across a carpet and received a shock when you touched a
metal door knob? Your shoe soles built up a charge by rubbing on the carpet,
and this charge was transferred to your body. Your body became positively
charged and, when you touched the zero-charged door knob, electrons were
transferred to your body until both you and the door knob had equal charges.
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, as shown in Figure 3, or by rotating a magnetic field through stationary
coils of wire. This is one of the most useful and widelyemployed applications
of producing vast quantities of electric power.
Figure
3 Generator – Electromagnetics Induction
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 4. 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.
Figure
4 Pressure Applied to Certain Crystals Produces an Electric Charge
Example:
One example is the crystal phonograph cartridge that contains a Rochelle salt
crystal. A phonograph needle is attached to the crystal. As the needle moves in
the grooves of a record, it swings from side to side, applying compression and
decompression to the crystal. This mechanical motion applied to the crystal
generates a voltage signal that is used to reproduce sound.
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 5. 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."
Figure
5 Heat Energy Causes Copper to Give up Electrons to Zinc
The
thermoelectric voltage in a thermocouple is dependent upon the heat energy
applied to the junction of the two dissimilar metals. Thermocouples are widely
used to measure temperature and as heat-sensing devices in automatic
temperature controlled equipment.
Thermocouple
power capacities are very small compared to some other sources, but are
somewhat
greater than those of crystals. Generally speaking, a thermocouple can be
subjected to higher temperatures than ordinary mercury or alcohol thermometers.
Photoelectric
Effect
Light
is a form of energy and is considered by many scientists to consist of small
particles of energy called photons. When the photons in a light beam strike the
surface of a material, they release their energy and transfer it to the atomic
electrons of the material. This energy transfer may dislodge electrons from
their orbits around the surface of the substance. Upon losing electrons, the
photosensitive (light sensitive) material becomes positively charged and an
electric force is created, as shown in Figure 6.
Figure
6 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 6). 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.
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.
The
electrons that have escaped from the hot cathode form a cloud of negative
charges near it called a space charge. If the plate is maintained positive with
respect to the cathode by a battery, the electrons in the cloud are attracted
to it. As long as the potential difference between the electrodes is
maintained, there will be a steady current flow from the cathode to the plate.
The simplest example of a thermionic device is a vacuum tube diode in which the
only electrodes are the cathode and plate, or anode, as shown in Figure 7. The
diode can be used to convert alternating current (AC) flow to a pulsating
direct current (DC) flow.
Figure
7 Vacuum Tube Diode
Summary
The
important information contained in this article is summarized below.
•
Electrochemistry - Combining chemicals with certain metals causes a chemical
reaction that transfers electrons.
•
Static electricity - When an object with a normally neutral charge loses
electrons, due to friction, and comes in contact with another object having a
normal charge, an electric charge is exerted between the two objects.
•
Magnetic induction - Rotating coils of wire through a stationary magnetic field
or by rotating a magnetic field through a stationary coil of wire produces a
potential.
•
Piezoelectric effect - Bending or twisting certain materials will cause
electrons to drive out of orbit in the direction of the force. When the force
is released, the electrons return to their original orbit.
•
Thermoelectricity - Heating two joined dissimilar materials will cause a
transfer of electrons between the materials setting up a current flow.
•
Photoelectric effect - Dislodging of electrons from their orbits by light beams
creates positively-charged objects.
•
Thermionic emission - Freeing electrons from a hot surface causes electrons to
escape.
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