The more accident investigations I
perform, the more I am convinced that a thorough
understanding of basic electricity is critical in
solving tough cases. In the past twenty years, I have
taught basic electricity more than any other subject. A
majority of my students are lawyers. In order for them
to understand how an electrical accident happened, they
must know the basics of electricity.
What is electricity?
To answer this question, I first consulted the IEEE
Standard 100, The Authoritative Dictionary of IEEE
Standards Terms. With nearly 35,000 entries, it
doesn’t define the term electricity. Note that
back in 1992, IEEE Standard 100 was called the IEEE
Standard Dictionary of Electrical and Electronic Terms.
I presume that at some time after 1992, the authors of
IEEE Standard 100 realized that trying to keep up with
all the new electrical and electronic terms was becoming
too difficult. Keeping up with the terms used in just
the IEEE Standards was within their grasp and control,
hence the revised title and scope. The need for the use
of well-defined industry standard terms in litigation is
critical. Don’t be surprised if a definition in IEEE
Standard 100 is different from the definition within
another standards authority’s publication. I then
consulted The American Heritage Dictionary.
"Electricity. The class of physical phenomena
arising from the existence and interactions of electric
charge." Sounds like a definition of static
electricity. Today, I’m not talking about static
electricity. That’s another can of worms. I’m
talking about the electricity that makes my
refrigerator, TV, air conditioner and lights work. As
electrical inspectors, we typically speak of electricity
using terms like voltage, current, amps, resistance,
watts, and watt-hours. The electrically
uneducated general public often uses the term electricity in place of the more specific terms with which we are
familiar. The following are examples of the use of the
term electricity by the general public. "How much
electricity is in those wires?" "The
electricity in the stove jumped out and bit me."
"My TV burned up because you gave me too much
electricity." Note that if we substitute the term
voltage for electricity in the previous three sentences,
we immediately have a better understanding what the
person is talking about. When people ask me, "What
is electricity?" I tell them that electricity is
the presence of voltage.
Why do some materials conduct
electricity better than others?
A common analogy some people think of when discussing
current flowing in a wire is water flowing in a pipe.
Though there are some similarities between the two
ideas, conduction of current through a wire is very
different from the flow of water within a pipe. Water
can flow through a pipe because there is a space inside
the pipe filled with air. The water can flow through the
pipe by pushing the air out. There is no space inside a
wire. The wire is solid metal. So, how does current flow
through a wire? All materials known to man are made up
of one or a combination of up to 103 elements, as of
1967. You may recall chemistry teachers referring to the
Periodic Table, a spreadsheet that listed all the
elements by symbol and atomic number. All the elements
are made up of atoms. Atoms are so small that we cannot
see them, even with the most powerful microscopes. For
centuries, scientists have tried to predict what an atom
looks like in order to explain its behavior. The image
of the atom that works well for me in describing
electricity is the Bohr atom. In 1913, a Danish
physicist by the name of Niels Bohr, described the atom
as made up of a nucleus surrounded by electrons.
The
nucleus is made up of protons and neutrons. Electrons
travel around the nucleus in very specific orbits
similar to the planets orbiting around the sun. The
maximum number of electrons that can orbit around the
nucleus in the closest orbit is two. In the next orbit,
the maximum number is eight. In the third orbit, the
maximum number is eighteen. The atomic number of the
element is the number of electrons that make up the
atoms of the element. Elements that easily conduct
current have only a few electrons in the outermost
orbit. Current is the motion of electrons from atom to
atom within the material. Current is measured in
amperes, amps for short and abbreviated as A. Though the
number of electrons within an atom cannot change, an
atom can pass an electron to another atom if it receives
one at the same time. All materials conduct current to
some degree. Some conduct better than others. The four
best conductors of current at room temperature are
silver, copper, gold and aluminum. These elements are
much more willing to pass on their electrons than other
materials. These materials are commonly called
conductors because they have very low resistance to
passing on electrons. Materials that do not pass on
their electrons easily are called insulators. Their
resistance to pass on electrons is very high. Some
examples of insulators are dry air, porcelain, glass,
rubber, and plastics like PVC and polyethylene. A
chamber that has been evacuated, i.e., all or most of
the air removed, is a very good insulator because there
are no or very few atoms left to pass on their
electrons. Materials that fall between conductors and
insulators relative to their resistance to passing on
electrons, are often referred to as semiconductors.
The Circuit
For current to flow within a material, the electrons
within the material’s atoms have to be passed on from
one atom to the next. To visualize what is happening,
let us imagine a straight piece of copper wire six
inches long. Let us imagine that the diameter of the
wire is the diameter of the atoms within the wire. The
wire is so small that the atoms that make up the wire
are lined up like people standing in line. Each copper
atom has one electron in its outer orbit. Let us imagine
that something tries to give an electron to the atom at
one end of the wire. The only way that atom can accept
the electron is if it passes one on to the next atom at
the same time. At any instant in time, each copper atom
can only have one electron in its outer orbit. The
second atom must also give up an electron to the third
atom in order for it to accept an electron from the
first atom. Since the atom at the other end of the wire
cannot pass an electron to another copper atom, the
passing on of electrons will not happen. As you can see,
the line of copper atoms must form a complete loop or
closed circuit in order for the electrons to be passed
on. When the loop is broken, the current stops flowing.
This is referred to as an open circuit.
The Push
When we want to move an object, we exert a force on it.
When we want to move electrons, we exert a special force
called electro motive force or EMF. EMF is measured in
volts (abbreviated as V). EMF can be produced by
chemical reaction as in a battery. EMF can also be
produced by an energy conversion device like a generator
which converts mechanical power to electrical power, or
a solar voltaic cell (solar cell) which converts
electromagnetic radiation (sunlight) to electrical
power. The devices that produce EMF try to move the
electrons of the atoms within the device. In direct
current (DC) devices, the device tries to push the
electrons out one terminal and tries to suck them in on
the other terminal.
The magnitude of the force exerted
on the electrons is measured in volts and is usually
referred to as the voltage not the EMF magnitude. No
current flows externally from the device that produces
EMF unless there is a completed external circuit. Even
when a battery is not connected to a circuit of highly
conductive material, there is a circuit completed by the
air around the battery. Since dry air is a very good
insulator, the amount of current that flows is so small
it is not measurable.
Please send me your comments on this
series. If you have questions about basic electricity or
general questions about the NESC, please e-mail me at dave.young@conectiv.com .
National Electrical Safety Code and NESC
are registered trademarks of the Institute of Electrical and
Electronics Engineers. National Electrical Code and NEC
are registered trademarks of the National Fire Protection
Association.
Dave is a consulting engineer with Conectiv Power
Delivery of Wilmington, Delaware, where he has been working with and
teaching all aspects of the NESC for over 33 years. He is a member of
the NESC Interpretations Subcommittee and represents the Edison Electric
Institute on the NESC Overhead Line Clearances Subcommittee 4. Dave is
also an inspector member of the IAEI. |
