PHYSICS
OF ELECTRICITY
ELECTRON THEORY:
In 1911, Ernest Rutherford published a
theory of atomic structure that described
an atom as having a central positive
nucleus surrounded by orbiting negative
electrons. This model suggested that most
of the mass of the atom was contained in a
small nucleus, and that the rest of the
atom was mostly empty space. Rutherford
came to this conclusion following the
results of an experiment that involved
firing radioactive particles through
minutely thin metal foils, most notably
gold. Rutherford found that although the
vast majority of particles passed straight
through the foil, some of them were
deflected, leading him to his theory that
most of the atom was made up of empty
space.
Here is a drawing of Rutherford’s concept.
A further development of this idea gave
the electrons specific orbits around the
nucleus in “shells,” and can be used to
explain how certain elements combine
with one another to make compounds.
Although newer atomic theories have been
developed, Rutherford’s idea still makes
an excellent model to use in explaining
electrical theory, and in predicting how
electrons work in producing electricity.
In its simplest concept, electricity is
formed when electrons move from one
atom to another. This can happen in a
number of different ways. Most often
electrons move through a substance called
a conductor, meaning that material
conducts, or moves, electrons from one
place to another.
Virtually all commonly used metals will
conduct electricity. The most often used
in electronics are copper (Cu), aluminum
(Al), silver (Ag), and gold (Au). Iron (Fe),
is used to make electronic devices like
transformers, but generally not for wiring.
Silver and gold are most often used for
contact points in switches or high end
connectors. Gold plated connectors are
not as subject to corrosion as other types.
Although copper is the metal of choice for
most wiring, high-voltage power lines are
generally made from aluminum because it
costs less and is not as heavy.
Solid metals are not the only substances
that will conduct electricity. One other
very commonly used method uses
compounds dissolved in water. Many
solutes form ions such as can be made
from dissolving ordinary table salt in water.
The sodium and chlorine atoms form
positive and negative ions in the water, and
these can be used to conduct electricity.
1
E is the symbol used for mathematical
computations using volts.
v is the symbol used to express an
amount of voltage. Example : E=12v.
Where does the force to move electrons
come from? The easiest example is the cell
or battery. How do batteries work?
Some elements and/or compounds will
not conduct electrons at all (or very
poorly). These are known as insulators. If
every element conducted electrons there
would be no way to contain or control
electricity. Rubber, plastics, glass, ceramics,
and air are frequently used insulators.
Some elements are said to be semiconductors in that they only pass electrons
under certain conditions. Transistors,
diodes, and integrated circuit chips are all
made from semi-conducting materials like
silicon or germanium.
One common method is to use two
dissimilar metals that are electrically
connected to one another by a solution
containing ions. This type is often called a
wet cell storage battery and is the type used
in a car. A battery is actually a number of
cells linked together. Although most
electrical units are named for European
scientists, Benjamin Franklin coined the
term “battery” which to him meant a
series of glass plates, each with a static
electrical charge on it. Each plate in a
storage battery is a cell. A number of cells
makes up a battery of them. If placed in
series with one another, the group of cells
will produce a higher voltage.
The name given to a forced movement of
electrons is EMF or Electro Motive Force.
The amount of the force applied is
measured in VOLTS.
In this sort of battery, (notice that there is
a series of plates) the + ions gather at the
Cu electrode (an electrode is an ending
point for an electrical pathway), while the
-ions gather at the Zn electrode.
If a conductor is placed across the two
terminals, electrons will flow through it
from the negative terminal to the positive
terminal.
2
The flow of electrons is known as the
current. (in the same way that flowing
water has a current)
Current is measured in Amperes, or Amps.
I is the symbol used for mathematical
computations using amps.
A is the symbol used to express an
amount of current. Example : I= 20A.
Technically, 1 Amp is defined as 1
Coulomb of electrons moving past a given
point in one second. This is generally not
very useful information in and of itself,
but the mathematical relationship between
Volts and Amps is very important.
It is important to remember that Amps
refers to an amount of electrons flowing,
whereas Volts are used to measure the
pressure of the flow.
The wet cell is not the only type of battery.
A dry cell gets its name because no liquids
are involved, and they are much less messy
than a wet cell. You could imagine the
difficulty of carrying around a flashlight
filled with sulfuric acid. Instead a gel
solution is used to stabilize the ionic
solution and keep it in the case. Even so,
these types of batteries will leak eventually,
and the acid in them can be quite
destructive.
There are a number of commonly used
dry cell batteries that are called by these
names, D, C, AA, and AAA. All of these
types have the same voltage, 1.5v, but the
D cell has more active materials in it and
will maintain a charge longer than the
AAA. As the materials inside the battery
are used up by the chemical process of
producing current, the battery is
discharged, the pressure of the voltage
decreases, and eventually becomes too low
to be useful. This process is exemplified by
the way a flashlight becomes increasingly
dim and then goes out. Some batteries can
be recharged by externally applying a
current that runs backward through the
cell, reversing the chemical process. This is
the norm with wet cell car batteries, but
also other dry types such as lithium cells.
It is possible to make your own battery
from zinc, copper and salt water. You can
buy sheets of copper roof flashing at a
hardware store, and many different types
of fabric can be used to store salt water for
the battery. The zinc plate is a bit more
difficult to come by in a pure form, but
fortunately zinc is also the main ingredient
used in galvanizing steel to prevent it from
rusting. Zinc plated steel roof flashing is
also available. Since only the surface of the
material is used to create the battery, it
doesn’t matter very much that the interior
of the zinc plate is actually steel.
3
chemical compounds, to burning coal, to
hydro-electric.
This is just one cell of the battery. If you
use sheets of material approximately one
inch square, the cell should provide about
0.25v of electricity at an extremely low
amperage. To get a more useful, higher
voltage, stack a number of the cells
together in series. The voltage of each cell
can then be added together. Series circuits
are a concept discussed in detail later on.
Wet the fabric with salt water, and
sandwich the battery together as shown in
the drawing. Use a rubber band to hold
the assembly together. Rubber is a good
insulator, and will not short out the
battery cells.
Originally, scientists thought that
electrons flowed from the positive
terminal to the negative terminal, and
many sources still use that model. In
reality electrons move from negative to
positive. Schematic diagrams often use the
conventional current flow standard rather
than the actual.
For the purposes of our work, and for the
analysis we will do, it makes no real
difference which way the electrons are
actually moving.
Voltage potential always exists, even when
there is no actual current flow. Energy has
been stored in the battery and is waiting
for a chance to be expended. If you carry a
heavy weight to the top of a building and
hold it over the edge of the roof there is a
potential for it to fall even though it may
not be moving at the time. In the case of
the heavy weight, energy is stored by
raising it to the roof. Electrical energy can
be formed in many ways, from volatile
Electrical circuits are created when current
flows through wires and other devices to
make a complete pathway from the
negative terminal of a battery to the
positive terminal. Schematic drawings are
used to show the electrical connections
between various electronic components.
They don’t show the actual placement of
the parts relative to one another, just how
they are connected for electrical purposes.
Lines represent conductors. Other
components are shown with graphic
symbols, some of which are shown below.
4
once through a circuit without proper
limits, and will often destroy part of the
circuit in the process.
Here is a very simple schematic drawing
showing just a battery and a conductor:
Conductors are used to make a completed
pathway, and one is shown here as a line.
The symbol for a battery represents a
number of cells linked together. The
conductor could be a wire, but it could
also be a trace on a printed circuit board.
This circuit is not very useful because it
does no work. It would actually be a short
circuit unless the conductor were very long,
or otherwise had a lot of resistance to it.
“Shorts” allow electrons to move all at
In this circuit, closing the switch causes
the bulbs to glow. The filament in the
lamp glows because a great deal of current
is flowing through it. The pressure of the
voltage can make the bulb dimmer or
brighter. Notice that the bulbs are not
connected one after the other, in series,
like the cells of a battery. Instead, they are
connected across two parallel lines.
Resistance is defined as the opposition to
current flow. This resistance can come
from a number of sources. All conductors
will resist the flow of electrons to some
degree or the other. Remember that
insulators conduct electrons so poorly that
they are generally regarded as not
conducting at all, so for our purposes they
have infinite resistance.
5
The three units of voltage, amperage, and
resistance are of great importance in
electrical work.
The tiny tungsten filament in the light
bulb resists the flow of electricity, and in
the process, changes some of the electrical
energy into heat energy. If the voltage
pressure is high enough and the wire gets
hot enough, it will incandesce, or glow.
Most materials will do that, especially
metals, as shown by the nichrome wire in
a toaster oven, or the elements on the top
of an electric range. As for the light bulb,
the vacuum in the glass envelope keeps the
tungsten from burning up through
oxidation. The curl in the filament spreads
it out and creates a larger amount of light.
The screw part at the bottom is used for
connection purposes.
Increase the pressure of E to make the bulb
brighter.
Decrease the pressure of E to make the bulb
dimmer.
The amount of resistance to electron flow
in the wire filament is measured in ohms
using the uppercase omega as a symbol, Ω.
R is the symbol used for mathematical
computations using resistance.
Ω is the symbol used to express an
amount of resistance. Example : 150 Ω.
TERMS USED IN THIS SECTION:
Ampere
Atom
Battery
Cell
Circuit
Conductor
Dry cell
Electromotive force
Electron
Filament
Incandesce
Insulator
Ion
Ohm
Ohm’s Law
Omega
Resistance
Resistor
Schematic
Semi-conductor
Short circuit
Volt
Wet cell
In general, a larger wire will present less
resistance. A longer wire will have more
resistance than a shorter one, which is
fairly intuitive if you think about it.
6