Basic Electrical Theory
S. Christian Mariger Ph.D.
Biological Systems Engineering
Virginia Tech
Objectives
•
•
•
•
•
Identify and describe the scientific
principles related to electricity.
Explain the electron theory and it’s
relationship to electricity.
Describe electrical terminology.
Define Ohm’s law.
Explain electrical power and energy
relationships.
Objectives continued
•
•
Perform electrical calculations.
List and describe the basic types of
electrical circuits.
Electron Theory
• All matter is made
up of atoms.
• Atoms are made
up of:
– Electrons
– Protons
– Neutrons
Each element has its own unique structure.
Electron Theory
• Atoms
– Nucleus
• Protons: positive charge
• Neutrons: neutral charge
Neutrons
– Orbits
• Electrons: negative
charges circle the nucleus
in orbits
Electricity is the flow of electrons
from atom to atom in a conductor.
Electron Theory
• Electrons are in orbits at different
distances from the nucleus.
• Electrons in the orbit the greatest distance
from the nucleus are in the valence shell.
• The outer orbit contains between 1 and 8
electrons.
– Conductors contain < 4 e in the outer orbit.
– Insulators contain > 4 e in the outer orbit.
– Semiconductors contain exactly 4 e.
Electron Theory
• Valence shell electrons
– Some elements have atoms with valence shell
electrons that have a strong magnetic attraction with
the + charges in the nucleus.
– These electrons will not easily move from atom to
atom.
– These elements are poor conductors of electricity
and are called insulators.
• Insulators include: glass, paraffin, rubber, porcelain, wood,
and most plastics.
Electron Theory
• Valence shell electrons:
– Some elements have valence shell electrons
that have a weak magnetic attraction with the
+ charges in the nucleus and will move easily
from atom to atom.
– These materials are good electrical
conductors.
– Silver, Gold, Copper and Aluminum are
examples of conductors.
The Copper Atom
• The copper atom has 29 electrons in four
orbits or shells.
The lone electron
in the outer shell
can easily be
knocked out of
it’s orbit by a free
electron from a
generator or
battery. The
electron once
free will collide
with another
nearby atom in a
chain reaction.
e-
e-
e-
e-
e-
ee-
e-
e-
e-
e-
ee-
e-
e-
e-
e- Cu ee-
e-
e-
e-
e-
e-
e-
ee-
e-
e-
Elements with fewer
than four electrons in
their outer shell are
good conductors.
Almost all metals
have fewer than four
electrons in their
outer shell and thus
are good conductors.
Electrical Current
• Current: the flow of electrons in a
conductor
Free electron
from generator
e-
e-
e-
Cu
e-
Cu
e-
e-
e-
Cu
Cu
e-
Copper Atom
Note that the flow of electrons (e-) is from negative to positive!
e-
e-
Electron vs. Conventional Theory
• In reality we know that electrons flow from
negative to positive (Electron Theory).
• However most electrical diagrams are
based on the idea that electricity flows
from positive to negative (Conventional
Theory).
• This seldom causes problems unless we
are working with a DC circuit.
Two Theories of Current Flow
Electrical Circuit
• Circuit: a continuous conductor that provides a
path for the flow of electrons away from and
back to the generator or source of current.
• Note the “conventional flow” of this diagram.
+
Battery
-
Electrical Voltage
• An electrical generator (or
battery) forces electrons
to move from atom to
atom.
• This push or force is like
the pressure created by a
pump in a water system.
• In an electrical circuit this
pressure (electromotive
force) is called voltage.
• The volt (V) is the unit by
which electrical pressure
is measured.
Electrical Current (Amperage)
• While voltage refers to the electrical
pressure of a circuit, current or amperage
refers to the electrical flow of a circuit.
• Current or amperage is the amount of
electric charges (or electrons) flowing past
a point in a circuit every second.
• One ampere (amp or A) is equal to 6.28
billion billion (or 6.28 x 1018) electrons per
second.
Resistance
• Opposition to flow in electrical circuits is called
resistance (or impedance).
• Measured in Ohms by using an ohmmeter.
• One ohm, or R is the amount of electrical
resistance overcome by one volt to cause one
amp of current to flow.
• Electrical current follows the path of least
resistance.
• Electricity can encounter resistance by the type
of conductor, the size of conductor and even
corrosion on the connections.
Resistance
• Resistance is proportional to the length and the
diameter of the wire being used in the circuit.
• Thick wires (having larger diameter) have less
resistance than thin wires (smaller diameter).
• Longer wires have more resistance than short
wires. If the length of wire is doubled, the
resistance is doubled.
• If the size or cross-sectional area of the wire is
reduced to half, the resistance is doubled.
• Wire Gage (AWG): As wire gage ↑ wire size
(diameter) ↓ .
Resistance
• Temperature also affects resistance.
• As a conductor gets hotter, the amount of
resistance increases.
• Resistance can be good as well as bad.
• In a controlled setting such as electric heat or a
cooking stove, electrical resistance is beneficial.
• Too much friction or resistance in a wire can
result in the insulation melting and increased
potential for a fire hazard.
Resistance in a Conductor
Resistance
R=ρL/A
Where
R = Resistance (Ω)
ρ = Resistivity of wire – a function of
Temperature
L= Length of wire
A= Cross-sectional area of wire
As wire gage ↑ wire size (diameter) ↓
Resistivity (ρ)
When selecting wire it is critical to consider: the length of
the run, the gauge of the voltage of the circuit, the amp
draw and the temperature.
Ohm’s Law
• Identifies the relationship between voltage,
amperage (current) and resistance.
• States that the current (amperage) in a
circuit is directly proportional to the applied
voltage and inversely proportional to the
resistance in a circuit.
• The greater the voltage, the greater the
current.
• If the resistance is doubled, the current will
be half.
Ohm’s Law
Possible results
• When voltage drops, the
amperage drops in the same
proportion if the resistance
remains the same. If the
voltage is cut in half, the
amperage is cut in half.
• When voltage increases,
amperage increases in the
same proportion if the
resistance remains the same.
If voltage is doubled,
amperage is doubled.
• When resistance increases,
amperage decreases in the
inverse proportion as long as
the voltage remains the same.
If resistance doubles, the
amperage is cut in half.
• When resistance decreases,
the amperage goes up in
inverse proportion if the
voltage remains the same. If
resistance is cut in half, the
amperage is doubled.
E = IR or I = E/R or R= E/I
Electrical Power
• Electrical power P is the work done per
time by a current I (amperage) under
pressure (voltage) V or E.
• The unit of measure for electrical power is
the watt (W).
• Watts (P) = volts (E) X amperes (I)
Electrical Power Relationships
•
•
•
•
•
•
Volts
=
watts
Amperes
=
watts
Kilowatts
=
watts
Horsepower (theoretical)
Horsepower (practical)
Electric motor < 1 HP =
/
amperes
/
volts
/
1000
=
746 watts
=
1000 watts
1200 watts/HP
Electrical Energy
• Electrical Energy is Electrical Power P
multiplied by the time of use t.
Electrical Energy = P t (in W or kW)
• Electrical Energy is what you pay for!
Electric Energy Cost ($) = (Pt)($/kWh)
AC Power
• Alternating current: current that reverses
it’s direction at a given frequency (60 Hz).
• Single-phase AC changes direction 120
times every second.
• This is called 60 cycle current, one cycle
for each transition from straight to reverse
and back to straight polarity.
AC Phases
• Phase: is a timed source of electricity
through a conductor (60 cycles/second.)
• Single Phase: is current from one source
with three wires.
– One hot wire
– One neutral wire
– One ground wire
• Three Phase: is actually three single
phases combined (180 cycles/second.)
DC Power
• In Direct current systems electrons (current)
flows in only one direction.
• DC is used in all automotive and mobile
equipment electrical systems.
• Direct current is also used power electric motors
in many “AC” appliances such as the cooling fan
in your computer.
– AC can be “rectified” DC with the use of diodes
(devices that allow current to pass in only one
direction)
Conductors
A simple circuit often contains
• The source with the necessary potential difference to start the flow of
electrons (battery or generator).
• A conductor for the electrons to flow from the source.
• A resistive load (light bulb, electric motor, etc.) to use the flow of
electrons.
• A conductor to connect the load back to the source.
• Often devices such as switches, fuses/circuit breakers are also added.
Types Of Circuits
• There are three basic types of circuits
– Series circuits
– Parallel circuits
– Series-Parallel circuits - Some circuits
combine elements of both types
Series Circuits
• Series Circuits
– Designed so that the current or electricity must flow
through each device or resistor (light bulb for
example) in the circuit.
– If one of the devices such as a lamp for example
burns out, the flow of electricity is stopped.
– This system would not work well for lighting or most
applications. The series principle is used in fuses and
circuit breakers, where it is necessary to stop the flow
of current for safety purposes.
Series Circuits
• Resistors connected in series have the
same current flow through them.
Rs = R1 + R2 + R3 +….
• Resistors connected in series have
different voltage drops across them
(unless they have the same resistance); in
other words, the voltage drop across the
resistor depends on the resistance of the
resistor.
Series Circuits
Parallel Circuits
• Parallel Circuits
– Each device (light bulb for example) has an
independent path for the flow of electricity. If
one device or “bulb” burns out, the remaining
bulbs in the circuit are not affected.
Parallel Circuits
• Resistors connected in parallel have the
same voltage across them.
1/Rp = 1/R1 + 1/R2 + 1/R3 +….
• Resistors connected in parallel have
different current flows through them
(unless they have the same resistance); in
other words, the current flow through the
resistor depends on the resistance of the
resistor.
Parallel Circuit
Series-Parallel Circuit
Branch Circuits
• A circuit may have
many branches, but in
each branch, the
delivery wire and the
return wire are
attached to the
corresponding wire on
the main circuit.
• The main circuit and
branch circuits meet at
a breaker/fuse box or
distribution panel.