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Electricity
It was discovered centuries ago that certain types of materials would
mysteriously attract one another being rubbed together. For example: after
rubbing a piece of silk against a piece of glass, the silk and glass would tend
to stick together. Indeed, there was an attractive force that could be
demonstrated even when the two materials were separated:
Glass and silk aren't the only materials known to behave like this. Anyone
who has ever brushed up against a latex balloon only to find that it tries to
stick to them has experienced this same phenomenon. Paraffin wax and wool cloth
are another pair of materials early experimenters recognized as manifesting
attractive forces after being rubbed together:
This phenomenon became even more interesting when it was discovered that
identical materials, after having been rubbed with their respective cloths,
always repelled each other:
It was also noted that when a piece of glass rubbed with silk was exposed to
a piece of wax rubbed with wool, the two materials would attract one another:
Furthermore, it was found that any material demonstrating properties of
attraction or repulsion after being rubbed could be classed into one of two
distinct categories: attracted to glass and repelled by wax, or repelled by
glass and attracted to wax. It was either one or the other: there were no
materials found that would be attracted to or repelled by both glass and wax, or
that reacted to one without reacting to the other.
More attention was directed toward the pieces of cloth used to do the
rubbing. It was discovered that after rubbing two pieces of glass with two
pieces of silk cloth, not only did the glass pieces repel each other, but so did
the cloths. The same phenomenon held for the pieces of wool used to rub the wax:
Now, this was really strange to witness. After all, none of these objects
were visibly altered by the rubbing, yet they definitely behaved differently
than before they were rubbed. Whatever change took place to make these materials
attract or repel one another was invisible.
Some experimenters speculated that invisible "fluids" were being
transferred from one object to another during the process of rubbing, and that
these "fluids" were able to effect a physical force over a distance.
Charles Dufay was one the early experimenters who demonstrated that there were
definitely two different types of changes wrought by rubbing certain pairs of
objects together. The fact that there was more than one type of change
manifested in these materials was evident by the fact that there were two types
of forces produced: attraction and repulsion. The hypothetical
fluid transfer became known as a charge.
One pioneering researcher, Benjamin Franklin, came to the conclusion that
there was only one fluid exchanged between rubbed objects, and that the two
different "charges" were nothing more than either an excess or a
deficiency of that one fluid. After experimenting with wax and wool, Franklin
suggested that the coarse wool removed some of this invisible fluid from the
smooth wax, causing an excess of fluid on the wool and a deficiency of fluid on
the wax. The resulting disparity in fluid content between the wool and wax would
then cause an attractive force, as the fluid tried to regain its former balance
between the two materials.
Postulating the existence of a single "fluid" that was either
gained or lost through rubbing accounted best for the observed behavior: that
all these materials fell neatly into one of two categories when rubbed, and most
importantly, that the two active materials rubbed against each other always
fell into opposing categories as evidenced by their invariable attraction to
one another. In other words, there was never a time where two materials rubbed
against each other both became either positive or negative.
Following Franklin's speculation of the wool rubbing something off of the
wax, the type of charge that was associated with rubbed wax became known as
"negative" (because it was supposed to have a deficiency of fluid)
while the type of charge associated with the rubbing wool became known as
"positive" (because it was supposed to have an excess of fluid).
Little did he know that his innocent conjecture would cause much confusion for
students of electricity in the future!
Precise measurements of electrical charge were carried out by the French
physicist Charles Coulomb in the 1780's using a device called a torsional
balance measuring the force generated between two electrically charged
objects. The results of Coulomb's work led to the development of a unit of
electrical charge named in his honor, the coulomb. If two
"point" objects (hypothetical objects having no appreciable surface
area) were equally charged to a measure of 1 coulomb, and placed 1 meter
(approximately 1 yard) apart, they would generate a force of about 9 billion
newtons (approximately 2 billion pounds), either attracting or repelling
depending on the types of charges involved.
It discovered much later that this "fluid" was actually composed of
extremely small bits of matter called electrons, so named in honor of the
ancient Greek word for amber: another material exhibiting charged properties
when rubbed with cloth. Experimentation has since revealed that all objects are
composed of extremely small "building-blocks" known as atoms,
and that these atoms are in turn composed of smaller components known as particles.
The three fundamental particles comprising atoms are called protons, neutrons,
and electrons. Atoms are far too small to be seen, but if we could look
at one, it might appear something like this:
Even though each atom in a piece of material tends to hold together as a
unit, there's actually a lot of empty space between the electrons and the
cluster of protons and neutrons residing in the middle.
This crude model is that of the element carbon, with six protons, six
neutrons, and six electrons. In any atom, the protons and neutrons are very
tightly bound together, which is an important quality. The tightly-bound clump
of protons and neutrons in the center of the atom is called the nucleus,
and the number of protons in an atom's nucleus determines its elemental
identity: change the number of protons in an atom's nucleus, and you change the
type of atom that it is. In fact, if you could remove three protons from the
nucleus of an atom of lead, you will have achieved the old alchemists' dream of
producing an atom of gold! The tight binding of protons in the nucleus is
responsible for the stable identity of chemical elements, and the failure of
alchemists to achieve their dream.
Neutrons are much less influential on the chemical character and identity of
an atom than protons, although they are just as hard to add to or remove from
the nucleus, being so tightly bound. If neutrons are added or gained, the atom
will still retain the same chemical identity, but its mass will change slightly
and it may acquire strange nuclear properties such as radioactivity.
However, electrons have significantly more freedom to move around in an atom
than either protons or neutrons. In fact, they can be knocked out of their
respective positions (even leaving the atom entirely!) by far less energy than
what it takes to dislodge particles in the nucleus. If this happens, the atom
still retains its chemical identity, but an important imbalance occurs.
Electrons and protons are unique in the fact that they are attracted to one
another over a distance. It is this attraction over distance which causes the
attraction between rubbed objects, where electrons are moved away from their
original atoms to reside around atoms of another object.
Electrons tend to repel other electrons over a distance, as do protons with
other protons. The only reason protons bind together in the nucleus of an atom
is because of a much stronger force called the strong nuclear force which
has effect only under very short distances. Because of this attraction/repulsion
behavior between individual particles, electrons and protons are said to have
opposite electric charges. That is, each electron has a negative charge, and
each proton a positive charge. In equal numbers within an atom, they counteract
each other's presence so that the net charge within the atom is zero. This is
why the picture of a carbon atom had six electrons: to balance out the electric
charge of the six protons in the nucleus. If electrons leave or extra electrons
arrive, the atom's net electric charge will be imbalanced, leaving the atom
"charged" as a whole, causing it to interact with charged particles
and other charged atoms nearby. Neutrons are neither attracted to or repelled by
electrons, protons, or even other neutrons, and are consequently categorized as
having no charge at all.
The process of electrons arriving or leaving is exactly what happens when
certain combinations of materials are rubbed together: electrons from the atoms
of one material are forced by the rubbing to leave their respective atoms and
transfer over to the atoms of the other material. In other words, electrons
comprise the "fluid" hypothesized by Benjamin Franklin. The
operational definition of a coulomb as the unit of electrical charge (in terms
of force generated between point charges) was found to be equal to an excess or
deficiency of about 6,250,000,000,000,000,000 electrons. Or, stated in reverse
terms, one electron has a charge of about 0.00000000000000000016 coulombs. Being
that one electron is the smallest known carrier of electric charge, this last
figure of charge for the electron is defined as the elementary charge.
The result of an imbalance of this "fluid" (electrons) between
objects is called static electricity. It is called "static"
because the displaced electrons tend to remain stationary after being moved from
one material to another. In the case of wax and wool, it was determined through
further experimentation that electrons in the wool actually transferred to the
atoms in the wax, which is exactly opposite of Franklin's conjecture! In honor
of Franklin's designation of the wax's charge being "negative" and the
wool's charge being "positive," electrons are said to have a
"negative" charging influence. Thus, an object whose atoms have
received a surplus of electrons is said to be negatively charged, while
an object whose atoms are lacking electrons is said to be positively
charged, as confusing as these designations may seem. By the time the true
nature of electric "fluid" was discovered, Franklin's nomenclature of
electric charge was too well established to be easily changed, and so it remains
to this day.
REVIEW:
- All materials are made up of tiny "building blocks" known as atoms.
- All atoms contain particles called electrons, protons, and neutrons.
- Electrons have a negative (-) electric charge.
- Protons have a positive (+) electric charge.
- Neutrons have no electric charge.
- Electrons can be dislodged from atoms much easier than protons or
neutrons.
- The number of protons in an atom's nucleus determines its identity as a
unique element.
Lessons In Electric Circuits copyright (C) 2000-2011 Tony R.
Kuphaldt,
under the terms and conditions of the Design
Science License.
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