Here is the "Electricity - Basic Navy Training Courses" (NAVPERS 10622) in its
entirety. It should provide one of the Internet's best resources for people
seeking a basic electricity course - complete with examples worked out. See
Table of Contents.
¶ U.S. GOVERNMENT PRINTING OFFICE; 1945 - 618779
(* VERY IMPORTANT NOTE:
Current flow here is defined as from negative to positive, which is
WHAT IT IS
opposite of today's convention. Modern convention of positive-to-negative
current flow, with negative-to-positive electron flow requires a
"right-hand rule." See
page on RF Cafe).
You have learned that potential difference causes electrons to flow through
a conductor. But-you'll have to know about another "electron-mover." Because it is an "electron-moving-force,"
scientists have named it ELECTROMOTIVE FORCE (emf). Mechanical force is usually measured in pounds, but emf is
measured in VOLTS. Just as pounds of force make water flow through a pipe, so emf makes current flow through a
The three terms - POTENTIAL DIFFERENCE, ELECTROMOTIVE FORCE and VOLTAGE - are often used interchangeably. You
will hear electricians say, "VOLTAGE of the generator"; or "EMF of the generator" and "VOLTAGE of the circuit" or
"POTENTIAL of the circuit." However, note the technical distinctions in these terms. To be ABSOLUTELY CORRECT, EMF
should be applied only to the force produced by a generator or battery. Example - "The emf of the generator is 120
volts." POTENTIAL or POTENTIAL DIFFERENCE applies to a total circuit or a part of a circuit. Example - "The
potential difference or drop (p.d.) is 63 volts." The term, VOLTAGE, applies to the number of volts concerned in
either case. Example--"The lamp has a voltage of 120 volts."
Electricians, and sometimes books, confuse
these three terms. Don't let it bother you. Just REMEMBER that to all practical purposes, emf, potential, and
voltage mean the same thing-THE FORCE THAT MAKES CURRENT FLOW.
WHERE IT COMES FROM
When a scientist studies a moving automobile, this is what he sees –
First, a gasoline tank full of
chemical energy. Second, an engine which burns this gasoline and uses the heat energy released to turn a
crankshaft. Third, the mechanical energy of the turning crank-shaft transferred as a force 'on the wheels which
move the automobile.
The automobile engine, then, is simply a machine which converts chemical energy into
mechanical energy. A steam engine is similar. It takes the chemical and heat energy out of coal or oil and
converts it to mechanical energy in the form of force on moving parts. No matter what kind of engine or motor you
select, you will find that each of them converts one kind of energy into another kind and then transfers the
energy to a force which does the work.
Electrical energy - emf - can be produced by the conversion of four
kinds of energy - mechanical energy, chemical energy, frictional energy, and heat energy. To change these forms of
energy into emf requires "engines" just as the automobile requires an engine to convert chemical energy into
mechanical energy. These electrical "engines" are the batteries and generators of your circuits.
EMF FROM MECHANICAL ENERGY - THE GENERATOR
The GENERATOR is the "engine" which converts mechanical energy into electrical energy. It is the most
economical and by far the most common source of emf.
Generators consist of two parts - a stationary FRAME
and a rotating ARMATURE. The armature is connected to a source of mechanical energy, called a PRIME MOVER -
usually a turbine, or a gasoline or diesel engine. The prime mover furnishes the energy which rotates the
armature. Then the armature, by a process to be explained later, converts the mechanical energy to electrical
energy. Figure 25 is a representation of a ship's power plant. Notice the different forms of energy as each
machine makes a conversion. Finally, at the generator, the energy is in the form of electricity ready to be sent
out on the ship's wires to be used to run motors, light lights, power radios, and heat galley stoves.
Figure 25. - Ship's power - oil to electricity.
The exact mechanism by which a generator converts mechanical energy to electrical energy is very complex. The
generator is treated in detail in a later chapter of this book. The important idea to remember here is that
GENERATORS FURNISH A CONTINUOUS EMF TO A CIRCUIT.
EMF FROM CHEMICAL ENERGY - THE BATTERY
Much energy is stored by nature in chemical compounds (combinations of elements). Coal, wood, and oil have
tremendous stores of energy which are released as heat when these compounds are burned. Oxygen and hydrogen have
so much energy that they explode when they combine. The electrician is interested in these - substances only
because he can get some of this energy as emf.
Releasing electrical energy from chemical energy is
surprisingly simple. If two dissimilar metals - copper and zinc, for example - are placed in certain chemical
solutions, an emf results. This is the principle employed in all CELLS and BATTERIES - a battery is simply TWO OR
MORE cells connected together.
HOW A CELL WORKS
When two or more atoms of different elements combine, they produce a molecule of a COMPOUND. For example,
atoms of the elements carbon and oxygen combine to form molecules of carbon dioxide. Carbon dioxide is a COMPOUND,
consisting of a combined form of the elements carbon and oxygen.
When a compound dissolves in certain substances - notably water - it breaks up into CHARGED PARTICLES. These
charged particles are called IONS. Ions are NOT the same as atoms-ions are charged and atoms are not. You will
remember that atoms contain an equal number of protons and electrons and therefore are neutral. But an ion of a
dissolved compound either loses or gains one or more electrons. If you were to dissolve one molecule of sodium
chloride - common table salt - in water, it would split into a sodium ion and a chloride ion. But the chloride ion
holds on to one of the sodium ion's electrons. This gives the chloride ion a negative charge and the sodium ion a
positive charge. It has been proved experimentally that a solution containing ions will conduct an electric
current. The ions seem to "ferry" the current through the solution. This should explain to you why salt water is
so likely to produce short circuits aboard ships. Because compounds that form ions in solution will conduct
electric currents, they are called ELECTROLYTES.
Figure 26. - Voltaic cell.*
All this leads you to an understanding of the workings of a cell. If any two dissimilar metal plates are
placed in an electrolyte, the ions will develop an emf at the plates. If the dissimilar plates are then connected
by means of a conductor outside the solution, the emf will force a current through this conductor. This is called
a VOLTAIC cell (after Volta, an early Italian experimenter).
Here's an example of how the Voltaic cell works. Immerse a copper plate and a zinc plate in a solution of
ammonium chloride, as in figure 26. The positive ammonium ions pick up electrons from the copper plate. This
reduces the number of electrons on the COPPER PLATE and gives it a POSITIVE charge. The chloride ions give up
their excess electrons to the zinc plate. This gives the ZINC plate, an excess of electrons and, therefore, a
NEGATIVE charge. Notice that you have two charged plates - one positive and one negative. If a wire is connected
to the two plates, the potential difference between its two ends will cause a current to flow.
A number of
changes occur in both the plates and electrolyte as the current flows. The most important change occurs at the
zinc plate. Electrons are constantly being lost by the zinc plate as the current flows, and the zinc atom changes
to a zinc ion - which dissolves in the solution. In short, the zinc is EATEN A WAY by the action. When the zinc is
completely ionized (dissolved) - the cell's emf ceases. Because the action of the cell uses up a primary part, the
cell is called a PRIMARY cell. Such a cell cannot be recharged.
The most common primary cell is one you
have seen many times - the "dry cell." Figure 27 shows a cross section view of a dry cell. The two plates, called
ELECTRODES, are zinc and carbon. Notice how the zinc electrode is shaped to form a cylindrical can. Thus, the
electrode serves as the cell case. The electrolyte is ammonium chloride dissolved in water and mixed with paste.
The paste is merely to prevent the electrolyte from spilling.
The chloride ions lose electrons to the zinc
plate, giving it a negative charge. The ammonium ions pick up electrons from the carbon rod giving this electrode
a positive charge. In this type of cell about 1.5 volts of emf are developed. Once the outside circuit is
completed, the zinc begins to dissolve. Since this is a primary cell, the action ceases when all the zinc is used
up. Usually the paste electrolyte will leak from a "dead" cell because the zinc container is eaten away. To avoid
messy leakage, dry cells should be removed from flashlights, lanterns, and radios when the gear is stowed or the
batteries are worn out.
Figure 27. - Cross section of a dry cell.
There are many kinds of primary cells - differing from each other in the materials used for electrodes and
electrolytes. The amount of emf produced by each depends on the materials composing the electrodes and
Figure 28. - Lead-acid storage battery.
Primary cells are useful for only a short time. Their chemical energy is used up and they must be discarded.
SECONDARY cells also lose their chemical energy, BUT, they can be restored by passing an electric current through
them. The lead storage cell is a common example of the secondary cell. It is used in battery form (two or more
cells connected together) in automobiles, motor launches and submarines.
Figure 29. - Charging and discharging the lead storage battery.*
The lead storage cell has electrodes of lead and lead oxide immersed in a solution of sulphuric acid. The
complete construction is shown in figure 28. During use, both plates are changed to lead sulphate, and much of the
sulphuric acid is converted to water. These changes mean that the chemical energy of the cell has been converted
to electrical energy. The cell is DISCHARGED. If a current from another source is passed through this cell IN THE
OPPOSITE DIRECTION TO THAT OF DISCHARGE, THE CHEMICAL ENERGY IS RESTORED. That is, the plates again become lead
and lead oxide and the water is changed back to sulphuric acid. The cell is now CHARGED and ready to deliver an
emf. Figure 29 shows the difference between a charged and discharged cell. With proper care, a secondary cell can
be charged and discharged many many times.
EMF FROM FRICTION
Friction between two unlike substances results in a potential difference between those substances. This
potential difference is an emf which will move electrons toward the lower potential. You are familiar with this
kind of emf production-it is the STATIC CHARGE. Although, this was the earliest discovered method of producing an
emf, there is relatively little practical use for static electricity. Most of it is wasted as static discharges.
EMF FROM HEAT
When two unlike metals, such as platinum and rhodium, are bound together and heated, they produce an emf. This
arrangement of two metals is called a THERMOCOUPLE The strength of the emf produced by any thermo-couple is
proportional to the temperature. Thermo-couples are used in steel furnaces, boiler flues, stacks, and molten
metals to measure extremely high temperatures. Notice in figure 30 that the two wires are twisted together at .one
end and then welded. This is the end to be heated. The balance of the wires are insulated by porcelain beads to
prevent a short circuit.
A very sensitive voltmeter is used to measure the emf produced by the thermo-couple. In the example used-a
platinum and rhodium thermocouple-a temperature of 920 °F. will produce 0.003672 volt but a temperature of 1980
°F. will produce 0.010534 volt. Notice that increasing the temperature increases the voltage of the emf. When a
thermo-couple is used to measure temperature, the voltmeter is calibrated IN DEGREES INSTEAD OF IN VOLTS. Thus the
voltmeter reads directly the temperature of the thermo-couple.
The thermocouple, used to measure
temperature, is the only practical use made of emf produced by heat.
Figure 30. - Thermocouple construction.
Chapter 5 Quiz