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
Contents. • U.S. Government Printing Office; 1945 - 618779
One of the Notable Tech Quotes which has appeared
on RF Cafe is, "The nice thing about standards is that you have so many to choose
from," by computer scientist
Tanenbaum. In the middle of the last century, a change in the fundamental understanding
of current flow precipitated what has become a very large opportunity for people
to misunderstand descriptions of current direction caused by a difference in voltage
potential (voltage) - depending on the era a particular description was written.
Beginning with Benjamin Franklin, electron current flow was assumed to be from positive
to negative, ostensibly but incorrectly, because a positive thing must contain an
excess of something (charge carriers - electrons) and a negative thing must have
a deficiency. Hence, current flowed from an excess source to a deficient sink. We
now know that negative things contain more electrons (relatively) than a positive
thing. "Conventional current" is defined as charge carriers (current) flowing from
positive to negative, whereas "electron flow" refers to charge carriers (current)
moving from negative to positive. Nowadays, however, due to the prevalence of semiconductors,
positively charged "holes" are deemed to be the dominant carriers, so we are back
to the Conventional current system again, but for the opposite reason as Franklin's.
Some of the NAVPERS course content needed to be updated as technology and knowledge
evolved. For instance, what is usually referred to as "conventional current flow"
is defined as being a positive charge moving from the more positive point to the
more negative point in a circuit. We now know that it is electrons that constitute
current flow and they move from the more negative point to the more positive point
in a circuit. So, when you see current flow arrows leaving the source's positive
terminal and reentering the negative terminal, it is "conventional flow."
Conversely, when you see current flow arrows leaving the source's negative terminal
and reentering the positive terminal, it is "electron flow." It is an
important distinction to make when considering magnetic fields generated by current
flow, and induced current from a changing magnetic field (see
Rule page on RF Cafe.
Chapter 3: Electricity in Motion - Moving Electrons
Fresh water must be carried to many parts
of a ship. It is fed to the boilers, to the laundry, to the galleys, to the scuttlebutts,
and to the heads. The water system is complicated-it requires pumps and many pipe
lines to do its job. The electrical system is very much like the water system. Electricity
must be "piped" to the lighting circuits, to the interior communications circuits,
to the battle circuits, and on some ships, to the propulsion circuits. The "pipes"
of an electrical circuit are METAL WIRES and the "'pumps" are the GENERATORS.
Current flowing through a wire is surprisingly like water flowing in a pipe.
If you were to put an electron into one end of a piece of copper wire, this added
electron would unbalance the charges of the molecules in the wire and would act
as a repelling force on the nearest electron in the wire. (Actually, the added electron
gives one end of the wire a higher POTENTIAL than the other end.) The push by the
added electron breaks one electron away from the nucleus of the FIRST molecule and
forces it on to the SECOND molecule. The electron you put in the end of the wire
then fills the empty space left in the first molecule. Now the electron expelled
from the first molecule forces an electron out of the second molecule and into the
THIRD molecule - and so on through the whole length of the Wire.
Figure 11. - Electron flow in a conductor.*
When this shifting of electrons reaches the last molecule in the wire, you have,
in effect, transported one electron the entire length of the wire. Not the same
electron you started with-but, since electrons are all alike anyway, you can say
that there has been a FLOW of one electron through the wire. Figure 11 enormously
magnifies the mechanism of this moving electron. You know that ONE electron by itself
is not enough electricity to be of any use. In an actual circuit, there would be
billions upon billions of moving electrons.
Measuring the size of a waterfall seems easy-if you merely say it's large or
small. Actually, however, there's more to it than meets the eye. First of all, you've
got to have a UNIT OF MEASURE. Drops, ounces, pints, quarts, gallons, or barrels
are all quantity measuring units for a liquid. You'd select the one unit that fits
the problem best-neither too small nor too large. "Gallons" might do the trick.
With the unit of measure-gallons-selected, IS it possible to determine the SIZE
of a waterfall if you're told ONLY THE NUMBER OF GALLONS spilling? How about this:
"Niagara has 5,000,000 gallons of water falling." Exactly how much do you learn
from that statement? Not much! Sure, 5,000,000 gallons is a lot of water, but you
must also know HOW LONG it takes to spill that much. One year, one month, one week,
one day, or one hour! Not much of a falls, if it takes a year. But you know Niagara
is a roaring giant, and the 5,000,000 gallons are spilled in ONE HOUR.
The point is, you must know TWO things in order to measure the size or strength
of a waterfall-the number of MEASURING UNITS moving in a UNIT OF TIME. This is called
TIME RATE OF FLOW. Flow of water is commonly measured in GALLONS PER SECOND, but
it could be measured in "quarts per second" or "barrels per day."
That takes care of a water system-now, how is "flow" measured in an electrical
system? First, select a unit of quantity measure. The electron won't do as a unit
because it's much too small. A larger unit-made up of' 6.3 billion billions of ,
electrons-is the COULOMB. And the coulomb is the standard electrical UNIT OF QUANTITY
Coulombs ALONE can no more measure the STRENGTH of electrical current than can
the gallon ALONE measure the strength of a waterfall. In order to measure electrical
strength, coulombs (quantity) must be hooked up with TIME. COULOMBS PER SECOND correspond
to gallons per second. And "a coulomb per second" equals an AMPERE. The AMPERE IS
THE UNIT OF MEASURE OF CURRENT STRENGTH. ONE COULOMB passing a point in a circuit
in ONE SECOND is ONE AMPERE. One ampere of current means that one coulomb (or 6.3
billion billions of electrons) passes a point in the' circuit each and every second.
Two coulombs each second would be two amperes; and 100 coulombs each second would
be 100 amperes. Likewise, 100 coulombs in 2 seconds would be only 50 amperes (50
coulombs EACH second). AMPERAGE is the measure of the RATE OF FLOW of electrons.
An ordinary light. bulb requires one-half an ampere. But a 36-inch naval searchlight
requires 150 amperes. This shows that the current to a searchlight is 300 times
as large as the current to an ordinary light bulb. The searchlight is about 300
times as strong as the lamp.
Copper wire is used to carry' current because copper has many FREE ELECTRONS
(easily dislodged electrons). Of course, every copper nucleus tries to hang on to
its own electrons including the free ones. And the attraction for the free electrons
must be OVERCOME before current can flow.
The property of "hanging on" is called RESISTANCE. All matter, including a copper
wire, has a certain amount of resistance. When a current flows, the resistance of
the circuit must be overcome by the potential of the circuit. If the POTENTIAL is
large, or the RESISTANCE small-strong current flows. On the other hand, if the POTENTIAL
is small. or the RESISTANCE large-LITTLE current flows.
Conductors and Insulators
Just what makes some materials carry current more easily than others is not thoroughly
known. Most scientists believe that it is because molecules differ in the number
of their free electrons-electrons which can be broken away from a molecule and forced
along to the next molecule. It seems that the molecules of most METALS are loosely
hung together-they have many free electrons. That is, the attraction between electrons
and nucleus is weak, and it is easy to push out electrons. In other words, most
metals have LOW resistances and are called GOOD CONDUCTORS. Most NON-METALS are
just the opposite of this-they have tight molecules which have few tree electrons.
In fact, for all practical purposes, some of the non-metals have no free electrons.
It is almost impossible to force electrons through substances of this kind. Such
non-metals have a HIGH resistance. They are extremely POOR CONDUCTORS and are called
It is wrong to say that all substances are either conductors or insulators. There
is no sharp dividing line. Electricians simply use the BEST conductors for wires
to CARRY current, and the POOREST conductors for insulators to PREVENT the passage
of current. Below is a table listing some of the best conductors and some of the
best insulators (poorest conductors).
Imagine that you are to run power from the dynamo room to the bridge searchlight.
For your wire, you would choose a good conductor. Silver is too. costly, so you'd
probably select copper. You would not be able to use a bare wire because in running
through bulkheads, along overheads, and through decks, a good part of your se3.rchlight
cur-rent would escape through the steel of the ship (a good conductor). To prevent
this loss, you would use a wire coated by an insulator-probably rubber. The copper
carries the current and the rubber prevents the current from escaping out of the
The amount of current that is wanted in any wire depends on the use of the circuit
and the type of the wire. It would be foolish to send one-half an ampere to a searchlight
needing 150 amperes. The amount of current can be controlled in two ways. First,
by the amount of POTENTIAL DIFFERENCE and second, by the amount of RESISTANCE.
Figure 12. - Potential difference and current.
Up to this point, potential has meant the charging of a body or the charging
of one end of a wire. This charging results in a DIFFERENCE of potential between
two bodies or between two ends of a wire. If you refer back to figure 8, you will
see that this difference in potential is easy to calculate. From 0 to +4 is a difference
of 4. From -2 to +3 is a difference of 5. (Note: -2 to 0 is 2, and 0 to +3 is 3.
And 2 + 3 = 5.) To be ABSOLUTELY correct, you should call this POTENTIAL DIFFERENCE.
Electricians often shorten this term to the one word POTENTIAL.
Increasing the pressure in a water pipe increases the flow of water. Likewise,
increasing potential difference on a circuit increases the flow of current. Compare
the drawings in figure 12.
If the resistance to flow remains the same; you can see that when you double
the pressure on the water in a tank, you force twice as much water through the pipe.
:When the pressure is tripled, the amount of water is tripled. Also, when you double
the potential difference, the current is doubled.
For a difference in potential of 2, only one ampere flows, but for a potential
difference (p.d.) of 4, two amperes flow. Calculate how many amperes will flow for
a potential difference of 6. Then check figure 12 to see if you are correct. These
ideas are stated in a fundamental law of electricity -
Current Is Directly Proportional to Potential Difference (Voltage).
Figure 13. - Resistance and current.
The second factor in controlling current is the amount of resistance. If the
potential difference re-mains the same, an INCREASED resistance will DECREASE the
current. Compare the drawings in figure 13. In the water system there are four factors,
determining the resistance to the flow of water:
(1) Diameter of the pipe.
(2) Length of the pipe.
(3) Kind of pipe.
(4) Velocity of flow.
The smaller the pipe, the longer the pipe, the dirtier the inside of the pipe-the
more friction the pipe has. Friction is resistance, so the greater the friction,
the smaller the flow of water through the pipe. Electrical wires are the "pipes"
of an electrical circuit. The resistance of these wires, which is like the friction
of the pipes, depends on four factors:
(2) Length of the wire.
(3) Kind of wire.
(4) Temperature of the wire.
Notice in figure 13 that if the wire is longer or smaller, less current will
flow. This is because the resistance has been increased. Likewise, if the wire is
made of higher resistance material (iron) the current is less. These three factors
are similar to the first three factors in the water - pipe system. Temperature -
the fourth factor which affects the resistance of a wire - may be compared to the
velocity of flow in a water-pipe. For some reason, not entirely Clear to scientists,
the resistance of most conductors increases as the temperature increases. The effect
of temperature change is so small that it may be neglected for all ordinary cases.
If the RESISTANCE of a wire is INCREASED by any on of these four factors - size,
length, material, and temperature - the CURRENT is DECREASED. Thus, you have another
fundamental law of electricity -
Current Is Inversely Proportional to Resistance.
Imagine again that you are running a power cable to the bridge searchlight. A
wire long enough to reach from the dynamo room to the bridge will have considerable
resistance because of its LENGTH. You don't want too much resistance, so you select
a wire LARGE ENOUGH IN DIAMETER to carry the 150 amperes. Naturally, you use a wire
of low resistance material-probably copper-and insulate it.
In this chapter you have studied how a current flows and how its strength is
measured. YOU MUST understand these fundamentals - how resistance and potential
difference affect the strength of a current. Be sure you have these ideas straight.
Chapter 3 Quiz