Fundamentals of
Electricity
Circuits 1
Fall 2005
Harding University
Jonathan White
Outline
Benjamin Franklin / History of Electronics
 Atoms/Electrons

– Electron shells and orbits
– Valence electrons
Charge
 Current
 Voltage
 Electrical Ground
 Resistance
 Electric circuits

History of Electronics

Began with Benjamin Franklin in 1747
– He determined that electricity was a single force, with
positive and negative aspects
– Coined over 25 new terms, including armature, battery,
and conductor
– Famous kite-flying experiment in a thunderstorm was
performed in 1752, near the end of his work in the field

Ben Franklin arbitrarily assumed that the actual
carriers of electrical current had a positive
electrical charge.
– While this assumption was wrong, all his experiments
still worked, and this assumption is still often used
today.
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However, the theories of electricity go back a lot
farther than this.
History 1
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600 B.C. – Thales of Miletus writes about amber
becoming charged by rubbing.
1600 – English scientist William Gilbert coins the
term electricity from the Greek word for amber.
Experiments with magnets, coining the terms
electric force, magnetic pole, and electric
attraction.
1745 – Dutch physicist Pieter van Musschenbroek
invented the “Leyden Jar”, a device that stored
static electricity. This was the first capacitor.
1747 – William Watson discharged a Leyden Jar
through a circuit. This begins the comprehension
of current and circuits.
1800 – Alessandro Volta invents the first electric
battery. He also proved that electricity could
travel over wires.
History 2
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1820 – Oersted and Ampere observe that a coil
of wires acts like a magnet when a current is
passed through it
1821 – Faraday invents the first electric motor
1826 – Ohm states a relationship between
potential, current, and circuit resistance
1873 – Maxwell writes equations that described
the electromagnetic field
1876 – Edison Electric Light Co. founded
1879 – First commercial power station opens in
San Francisco, uses a Brush generator and arc
lights
1883 – Transformer invented
1886 – Alternating current electric system
developed by William Stanley
1897 – Electron discovered by J.J. Thompson
Atoms
 Atom
– Smallest particle of an
element that retains all the
characteristics of that element.
– Consists of positively charge protons
and uncharged neutrons in the nucleus,
and several negatively charge electrons
that surround the nucleus.
 Electrons
occupy specific energy
levels, or “shells” around the nucleus
Electrons

Each shell is discrete and electrons will try and occupy the
lowest energy level available to them.
– Negative electrons that are close to the nucleus are attracted
to the positive nucleus and are tightly bound.
– Electrons farther out are loosely bound, and hence have a
higher potential energy
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However, there is a limit to the number of electrons that
each shell can hold.
– Number of electrons possible = 2N2, where N is the shell
number.
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The outermost shell is known as the valence shell, and the
electrons in it are called valence electrons.
The factor that becomes important is that those elements
with only 1 or 2 electrons in their outermost shells don’t
hold on the them very strongly. Therefore it requires little
energy to pull these electrons from their parent atoms and
move them someplace else.
The Copper Atom
29 electrons that orbit the nucleus in 4
shells.
 When a copper atom gains sufficient
thermal energy, it can break away from
the parent and become a free electron.
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– This happens at room temperature for copper.
Categories of Materials
Conductors: readily allow current, large
number of free electrons, 1 – 3 valence
electrons. What’s the best conductor?
Exp: silver, copper, gold, aluminum, iron.
 Semiconductors: 4 valence electrons in
their structures. Exp: silicon,
germanium.
 Insulators: poor conductors of electric
current, more than 4 valence electrons.
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Electrical Charge

2 types, positive and negative. Charge of a proton is equal in
magnitude to the charge of an electron.
– Symbolized by letter Q.
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Static electricity is the presence of a net positive or negative
charge.
Like charges repel, unlike charges attract.
– This attraction or repulsion is a force called an electromagnetic field.
– This force and gravity are the only forces that we humans can
experience directly.
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Charge is measured in coulumbs
– One coulumb is the total charge possesed by 6.25 X 1018 electrons

Like matter, charges follow an inverse square law.
– F1,2 = q1 q2/(4πE0 r2)
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Where q1, q2 are measured in coulumbs, r is the distance between the
charges measured in meters, and E0 is a fundamental constant of nature, =
~8.885419 x 10-12 Farads/meter
However, charge is different than matter. Does anti-matter have a
negative mass? Also, two masses always seem to attract each
other.
Charge 2
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The charge on an electron is always the same for every
electron in the universe.
If you have one electron and one proton, then the two
charges cancel each other out and the atom is said to have
no charge. Also, the electron and proton will cancel out
each other exactly.
The charge on an electron is a fundamental quantity - a
constant of nature.
Where is charge used?
– When you plug in a light bulb, charge flows from the socket,
through the connecting wire, and then through the bulb
filament, heating it up and giving off light. The electrons
aren’t destroyed, but they do lose energy.
– Your car battery stores energy by storing charge on the
battery plates. When you start your car, charge flows from
your battery to the engine, providing enough energy for the
vehicle to run.
Current
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Current – charge in motion.
– The “flowing” of charges through something.
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We typically think of charge flowing through a wire, but it
can also flow through water, air and even vacuum.
You can think of current as water flowing through the
interior of a pipe, though current actually flows though the
empty spaces between atoms in a wire.
Current is represented by the mathematical symbol i.
– i = Q/t, or, current is equal to the number of electrons that
flow past a point in a given amount of time.
– Current is measured in amperes, which is equal to
coulombs/sec. Amperes is abbreviated with the letter A.
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Current is a “through” variable, meaning that in order to
measure it, you need the current to go through something.
Current 2
 Charge
comes in discrete packets,
but it is useful to assume that it can
take continuous values. Then, we
can imagine making the time frame
very small and find the current at an
instant. Then, i(t) = dQ(t)/dt
 It takes energy to make charges flow
through something. The energy that
makes current is called voltage.
Voltage
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Can be thought of as the driving force behind the current
(though it isn’t really a force).
Voltage is the energy per unit charge.
Current flows through electrical elements when a voltage
appears across the terminals of the element, similar to
when water flows through a pipe when a pressure
difference appears across the pipe.
– Voltage is an “across” variable. We talk about pressure
differences and voltage differences.
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Voltage is related to potential energy. Voltage is defined as
the electrical potential energy that a charge has by its
position in space.
– If you pull two charges apart, you put potential energy into the
system
– That potential energy can be converted into other forms of
energy
– Energy can neither be created or destroyed, only transferred
Voltage 2
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A mass m, h meters above the Earth has
potential energy mgh. A mass at h=0 has 0
potential energy. A charge at electrical ground
has 0 potential engergy. Voltage is the potential
energy per unit charge, or V= W/Q.
If we move a charge from point A to point B, and
put a given number of joules of work into the
charge, we will recover exactly the same number
of joules from the charge if it moves back from
point B to point A. If we move the charge
through any closed path or circuit, there will be
no net energy input to the system and no net
energy recovered from the charge.
Voltage and Batteries
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Batteries are voltage sources.
Batteries can be thought of as charge pumps.
– They take a charge and though chemical reactions pump
them up to a certain voltage, or potential energy level.
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As the charge flows through the circuit, this
potential energy can be used by the circuit to do
work. The charge loses energy as it goes
through the loads.
– Heat up a filament
– Make a motor turn.
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Energy gained from the battery = energy lost by
the loads.
– Law of conservation of energy
Ground
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Reference voltage from which all other measurements are
measured – the potential of the Earth.
– Defined as having 0 V potential energy with respect to the rest
of the circuit.
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In physics equations, ground level is used as the point of 0
potential energy when lifting a weight, another thing
electrical systems have in common with mechanical
systems.
In wiring for houses, the ground is physically connected to
the Earth – a place of 0 potential energy when compared to
the rest of the wiring.
Ground provides a return path for the current back to the
source because all the ground points are electrically the
same point and provide a zero resistance path
Resistance
 Opposition
to the flow of current.
 When there is current through any
material that has resistance, heat is
produced by the collisions of
electrons and atoms.
 Can be thought of as partially closed
valve in our pipe system – it restricts
the flow of water.
Circuits
 Consists
of a voltage source, a load,
and a path for current between the
source and the load.
– A load is a device on which work is done
by the current through it.
 Open
circuits versus closed circuits.
Review
 Charge
– Definition: Fundamental property of
matter based on the absence or excess
of electrons.
– Symbol: Q
– Measured in Coulombs.
– 1C = total charge of 6.25 X 1018
electrons
– 2 types, positive and negative
Review
 Current
– Definition: Charge in motion
– Symbol: I
– Measured in amperes (A).
– I = Q/T, current is the amount of charge
that passes a point in a given amount of
time. Or, i(t) = dq / dt
– Current is a “through variable”.
– The water that’s flowing through a pipe.
Review
 Voltage
– Definition: Energy per unit charge.
– Symbol: V
– V= W/Q , voltage is the energy per
unit charge. Where, W is the energy
expressed in joules, Q is in Coulombs.
– Voltage is an across variable.
– The pressure that pushes the water
through the pipe.
Ground and Resistance
 Ground
– Definition: Reference point that is at 0
volts with respect to all other points in
the circuit.
 Resistance
– Opposition to the flow of electrons.
– The resistance caused by the collision of
electrons and atoms causes heat to be
given off.