Conductors, Insulators, and Electron Flow
The electrons of different types of atoms have different degrees of freedom
to move around. With some types of materials, such as metals, the outermost
electrons in the atoms are so loosely bound that they chaotically move in the
space between the atoms of that material by nothing more than the influence of
room-temperature heat energy. Because these virtually unbound electrons are free
to leave their respective atoms and float around in the space between adjacent
atoms, they are often called free electrons.
In other types of materials such as glass, the atoms' electrons have very
little freedom to move around. While external forces such as physical rubbing
can force some of these electrons to leave their respective atoms and transfer
to the atoms of another material, they do not move between atoms within that
material very easily.
This relative mobility of electrons within a material is known as electric conductivity.
Conductivity is determined by the types of atoms in a material (the number of
protons in each atom's nucleus, determining its chemical identity) and how the
atoms are linked together with one another. Materials with high electron
mobility (many free electrons) are called conductors, while materials
with low electron mobility (few or no free electrons) are called insulators.
Here are a few common examples of conductors and insulators:
- dirty water
- (dry) cotton
- (dry) paper
- (dry) wood
- pure water
It must be understood that not all conductive materials have the same level
of conductivity, and not all insulators are equally resistant to electron
motion. Electrical conductivity is analogous to the transparency of certain
materials to light: materials that easily "conduct" light are called
"transparent," while those that don't are called "opaque."
However, not all transparent materials are equally conductive to light. Window
glass is better than most plastics, and certainly better than "clear"
fiberglass. So it is with electrical conductors, some being better than others.
For instance, silver is the best conductor in the "conductors"
list, offering easier passage for electrons than any other material cited. Dirty
water and concrete are also listed as conductors, but these materials are
substantially less conductive than any metal.
Physical dimension also impacts conductivity. For instance, if we take two
strips of the same conductive material -- one thin and the other thick -- the
thick strip will prove to be a better conductor than the thin for the same
length. If we take another pair of strips -- this time both with the same
thickness but one shorter than the other -- the shorter one will offer easier
passage to electrons than the long one. This is analogous to water flow in a
pipe: a fat pipe offers easier passage than a skinny pipe, and a short pipe is
easier for water to move through than a long pipe, all other dimensions being
It should also be understood that some materials experience changes in their
electrical properties under different conditions. Glass, for instance, is a very
good insulator at room temperature, but becomes a conductor when heated to a
very high temperature. Gases such as air, normally insulating materials, also
become conductive if heated to very high temperatures. Most metals become poorer
conductors when heated, and better conductors when cooled. Many conductive
materials become perfectly conductive (this is called superconductivity)
at extremely low temperatures.
While the normal motion of "free" electrons in a conductor is
random, with no particular direction or speed, electrons can be influenced to
move in a coordinated fashion through a conductive material. This uniform motion
of electrons is what we call electricity, or electric current. To
be more precise, it could be called dynamic electricity in contrast to static
electricity, which is an unmoving accumulation of electric charge. Just like
water flowing through the emptiness of a pipe, electrons are able to move within
the empty space within and between the atoms of a conductor. The conductor may
appear to be solid to our eyes, but any material composed of atoms is mostly
empty space! The liquid-flow analogy is so fitting that the motion of electrons
through a conductor is often referred to as a "flow."
A noteworthy observation may be made here. As each electron moves uniformly
through a conductor, it pushes on the one ahead of it, such that all the
electrons move together as a group. The starting and stopping of electron flow
through the length of a conductive path is virtually instantaneous from one end
of a conductor to the other, even though the motion of each electron may be very
slow. An approximate analogy is that of a tube filled end-to-end with marbles:
The tube is full of marbles, just as a conductor is full of free electrons
ready to be moved by an outside influence. If a single marble is suddenly
inserted into this full tube on the left-hand side, another marble will
immediately try to exit the tube on the right. Even though each marble only
traveled a short distance, the transfer of motion through the tube is virtually
instantaneous from the left end to the right end, no matter how long the tube
is. With electricity, the overall effect from one end of a conductor to the
other happens at the speed of light: a swift 186,000 miles per second!!! Each
individual electron, though, travels through the conductor at a much
If we want electrons to flow in a certain direction to a certain place, we
must provide the proper path for them to move, just as a plumber must install
piping to get water to flow where he or she wants it to flow. To facilitate
this, wires are made of highly conductive metals such as copper or
aluminum in a wide variety of sizes.
Remember that electrons can flow only when they have the opportunity to move
in the space between the atoms of a material. This means that there can be
electric current only where there exists a continuous path of conductive
material providing a conduit for electrons to travel through. In the marble
analogy, marbles can flow into the left-hand side of the tube (and,
consequently, through the tube) if and only if the tube is open on the
right-hand side for marbles to flow out. If the tube is blocked on the
right-hand side, the marbles will just "pile up" inside the tube, and
marble "flow" will not occur. The same holds true for electric
current: the continuous flow of electrons requires there be an unbroken path to
permit that flow. Let's look at a diagram to illustrate how this works:
A thin, solid line (as shown above) is the conventional symbol for a
continuous piece of wire. Since the wire is made of a conductive material, such
as copper, its constituent atoms have many free electrons which can easily move
through the wire. However, there will never be a continuous or uniform flow of
electrons within this wire unless they have a place to come from and a place to
go. Let's add an hypothetical electron "Source" and
Now, with the Electron Source pushing new electrons into the wire on the
left-hand side, electron flow through the wire can occur (as indicated by the
arrows pointing from left to right). However, the flow will be interrupted if
the conductive path formed by the wire is broken:
Since air is an insulating material, and an air gap separates the two pieces
of wire, the once-continuous path has now been broken, and electrons cannot flow
from Source to Destination. This is like cutting a water pipe in two and capping
off the broken ends of the pipe: water can't flow if there's no exit out of the
pipe. In electrical terms, we had a condition of electrical continuity
when the wire was in one piece, and now that continuity is broken with the wire
cut and separated.
If we were to take another piece of wire leading to the Destination and
simply make physical contact with the wire leading to the Source, we would once
again have a continuous path for electrons to flow. The two dots in the diagram
indicate physical (metal-to-metal) contact between the wire pieces:
Now, we have continuity from the Source, to the newly-made connection, down,
to the right, and up to the Destination. This is analogous to putting a
"tee" fitting in one of the capped-off pipes and directing water
through a new segment of pipe to its destination. Please take note that the
broken segment of wire on the right hand side has no electrons flowing through
it, because it is no longer part of a complete path from Source to Destination.
It is interesting to note that no "wear" occurs within wires due to
this electric current, unlike water-carrying pipes which are eventually corroded
and worn by prolonged flows. Electrons do encounter some degree of friction as
they move, however, and this friction can generate heat in a conductor. This is
a topic we'll explore in much greater detail later.
- In conductive materials, the outer electrons in each atom can
easily come or go, and are called free electrons.
- In insulating materials, the outer electrons are not so free to
- All metals are electrically conductive.
- Dynamic electricity, or electric current, is the uniform
motion of electrons through a conductor. Static electricity is an
unmoving, accumulated charge formed by either an excess or deficiency of
electrons in an object.
- For electrons to flow continuously (indefinitely) through a conductor,
there must be a complete, unbroken path for them to move both into and out
of that conductor.
Lessons In Electric Circuits copyright (C) 2000-2011 Tony R. Kuphaldt,
under the terms and conditions of the Design