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
Chapter 2 of the U.S. Navy's Basic Navy
Training Courses covers Static Electricity. Chapter 1 is entitled "Matter - Electricity
Does It!," and contains Figures 1 through 4, which explains why Figure 5
is the first one here (in case you were wondering). The first few chapters cover
the fundamentals of electricity and electronics, and the material is as relevant
and true today as it was in the 1940s. Many chapters were originally posted more
than a decade ago and many people have written to comment on the quality of the
Navy's courses. Employers have long preferred former Navy-trained electronics personnel
for technician jobs because of their reputation. Navy hands-on experience combined
with an Associate's degree in electronics technology was (and may still be) an almost
guaranteed ticket to a great civilian career. If you are young and considering a
career in electronics, you might consider enlisting in the military (any branch,
really) for training.
Static Electricity - Repel vs. Attract
Have you ever noticed a girl combing her
hair? It seems to dance around as some strands stand away from the rest. You hear
a faint crackling sound. If it were dark you would be able to see tiny sparks as
the comb moved through her hair. What makes the crackling sound and the sparks?
Electricity - produced by friction, and known as Static Electricity.
The key to understanding this frictional electricity is in the action between
the girl's hair and the comb. Remember, some strands stood out from each other.
These strands had lost some electrons knocked loose by the friction of the comb
against the hair. And the loss of electrons gives the hair a Positive Charge.
Like Charges Repel Each Other
The positive charges on your girl's hair therefore Repel each other. That is,
they push each other away. That's why her hair stands out after it is combed. You
would find that two combs, negatively charged, would Repel each other. If your girl
holds the charged comb close to her charged hair, you'll see that the hair is Attracted
to the comb. Unlike Charges Attract Each Other. These two reactions are a fundamental
law of electricity.
Figure 5 - Charged and uncharged molecules.
Figure 6 - Water and Electrical Potentials.
Figure 7 - Electron flow.
Figure 8 - Electrical charges and electron flow.
Figure 9 - Charging a Leyden jar.
Figure 10 - Electrostatic generator.
In short, here is the picture. Friction has dislodged some electrons. This results
in the charging of Both pieces of matter through the transfer of electrons. One
piece is positive, the other is negative. The two unlike charges attract each other
but the two like charges Repel each other. If the attraction between two unlike
charges is strong enough to bring those charges close to each other, the electrons
jump back where they belong, Discharging both pieces of matter.- This jumping hack
causes the crackling sound you heard and the sparks you saw.
The transfer of electrons to the hair brings both comb and hair back to normal
conditions-that is -every negative electron in the comb or hair is balanced by a
positive Proton. Figure 5 shows two molecules - a Normal or Uncharged molecule and
a Charged molecule. Notice how the charged one is unbalanced-in this case, it has
one extra positive charge. This resulted from friction or perhaps only contact,
between two unlike substances, causing the Removal of Some Electrons from one of
the substances and thus unbalancing the charges. The electrons which are knocked
out of their molecules cling to anything that's handy. These electrons are called
Free Electrons. The object that picked up the electrons will have a negative charge.
Right Here is a good place to get firmly fixed in your mind the idea that a removal
of electrons causes an object to be charged positively and an addition of electrons
causes an object to be charged negatively.
You are familiar with many cases of static electricity. Remember walking across
a heavy rug and then reaching for a door knob or light switch-a spark jumped from
your hand? That was static electricity developed by the friction between your shoes
and the rug. Lightning is static electricity produced by the friction between air
and water particles. A cloud picks up electrons and then discharges them to another
cloud or to the earth.
Why Electrons Move
You are probably wondering why Protons were not transferred from hair to comb;
why Protons aren't scraped off of a rug by shuffling feet. Re-member that Protons
are much heavier than electrons. About 2,000 times as heavy. Compare a pound of
butter to a ton block of granite. Which is most easily moved? This comparison applies
to electrons and Protons. You will always find that the light electrons will move
before the heavy Protons will budge.
Water in a water tank is like a negative static charge. Given a chance to escape,
electrons, like water, will flow from any high level to any lower level. Compare
the two diagrams in figure 6. Note that both water and electrons tend to flow from
where there is a Lot to where there is a Little.
Every charged object has a certain Potential. And when two charged objects have
different Potentials, electrons tend to move because of the Potential Difference.
In short, Potential Difference Causes Electrons to Flow. If you compare the charged
objects in figure 7, you will notice that the electron flow is always toward the
most positive Potential. But there is no electron flow when the Potentials are equal.
Usually charged objects are simply labeled, as in figure 8, with the number of extra
negative or extra positive charges.
The spark from a discharging object is an Electrical Current. When electrons
are held stationary (negative charge) they are Static Electricity; when they move
they are Current Electricity. As you study electricity further, you will find that
most usable electricity is moving electricity. Because it is much easier to push
electrons through a wire than through air, electricians use wire for moving Current.
Sparks, which are electric Currents in air, are usually accidents.
A town may store its water supply in a tank on top of a tower or in a stand-pipe
on a hill. This accomplishes two objectives. The water is stored ready for use and
by keeping it high above ground it has Potential or force to carry it to the consumer.
A Condenser (often called a Capacitor, because it affects capacity) serves the same
purpose with electricity. Condensers, or capacitors, are made of alternate layers
of metal and non-'metal. These are usually in the ,form of thin strips of foil.
A very simple condenser is the Leyden jar. The Leyden jar consists of a glass bottle
partially coated with a metal foil-lead or tin. This foil extends about half way
up the jar, both inside and outside. A knobbed brass rod runs down through a wood
or rubber stopper. It is connected to the inside foil by a dangling metal chain.
The first step in Charging a Leyden jar is to Ground the outside foil. Grounding
is merely running a wire from the foil to the ground. This wire furnishes an easy
path for the electrons from the foil to the earth. Figure 9 shows a Leyden
jar properly grounded and ready for a charge.
Electrons are put on the knob, rod, chain, and inside foil by touching a negatively
charged body to the knob. Electrons piling up on the inside foil Repel the electrons
in the outside foil (like charges Repel) , forcing them into the ground. This grounding
permits a much higher charge because it allows the electrons on the outside foil
to escape. Without the ground wire they would remain on the Outside foil and Repel
any attempt to put electrons on the INSIDE foil. As soon as the Potential of the
inside foil is the same as that of the charging body, electron flow stops. If the
knob is connected to an electrostatic generator (see figure 10) a large charge can
be concentrated in the jar. This is now a Charged condenser. A cloud which is about
to let loose a bolt of lightning is a charged condenser - Some Condenser!
To Discharge the Leyden jar, connect the knob to either the outside foil or ground
by means of a heavily insulated wire. Just as the wire is almost touching the knob,
a heavy spark will jump from the knob to the wire. This spark (or arc) is the flow
of electrons, which were stored on the inside foil at a high Potential. It is quite
possible to discharge a condenser through your body by touching both the outside
foil and the knob. Thus the Current passes through your body. It is likely to be
rather Shocking. You should remember this whenever you work on a condenser. A condenser
discharge is partly responsible for the "hot" spark at a spark plug of a gasoline
engine. If you have ever taken this spark through your body, you know that a condenser
discharge is no joke.
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 2 Quiz