BASIC ELECTRICAL THEORY 4

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METHODS OF PRODUCING VOLTAGE

 

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.

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 4, Batteries.

Static Electricity

Atoms with the proper number of electrons in
orbit around them are in a neutral state, or havestatic electricity
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.

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 13, or by rotating a
magnetic field through stationary coils of wire. This is one of the most useful and widely employed
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.”

 

magnetic induction

 

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.

 

piezoelectric effect

 

 

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 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.”

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.

 

thermoelectricity

 

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 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.

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 17. The diode can be used to convert
alternating current (AC) flow to a pulsating direct current (DC) flow.

 

vacuum tube diode

 

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