CHAPTER
2
Fundamentals of
Electricity
Instructor Name: (Your Name)
Copyright © 2014 Delmar, Cengage Learning
Learning Objectives
• Relate electrical terms including voltage,
current, and resistance to their hydraulic or
pneumatic counterpart
• Explain the relationship among voltage,
current and resistance in an electrical
circuit
• Apply Ohm’s law to simple electrical
circuits to determine an unknown voltage,
current or resistance
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Learning Objectives
• Perform measurements of voltage, current
and resistance using a digital multimeter
(DMM)
• Describe the concept of voltage drop across
a resistance when current flows through the
resistance
• Demonstrate Kirchhoff’s voltage and current
law through voltage and current
measurements obtained from a series, a
parallel, and a series-parallel circuit
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Learning Objectives
• Determine the total circuit resistance in a
series, parallel, and series-parallel
electrical circuit given the individual
resistor values
• Calculate voltage drops and currents
draws in a series, parallel, and seriesparallel circuit, given the individual resistor
values and voltage source
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Atoms, Protons and Electrons
• Atoms are the smallest possible pieces of
any known element
• Atoms are made up of protons, electrons
and neutrons
• Protons have a positive charge and
electrons have a negative charge
• Like charges repel and opposite charges
attract
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Charges Suspended From Strings
Figure 2-6
Charges
suspended
from strings.
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Atoms, Protons and Electrons
Atoms are like a solar system; protons are
the sun with electrons orbiting like planets.
Figure 2-7 Copper Atom.
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Displacing Electrons
• Rubbing two dissimilar materials can strip
electrons from one material and add them
to the other
• One material has a positive charge
(missing electrons)
• One material has a negative charge (too
many electrons)
• An atom with an unequal number of
protons and electrons is called an ion
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Displacing Electrons
• A material with an excess of electrons or
protons will attract a material with the
opposite charge
• An atom with more protons than electrons is
a positive ion
• An atom with more electrons than protons is
a negative ion
• Electrons flow from a location where there is
extra electrons to a location that is missing
electrons
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Figure 2-8 Opposite charges on the paper and comb
cause an attraction.
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Electric Potential
• Electric potential exists between two
materials, one with excess electrons
(negative charge) and one with an absence
of electrons (positive charge)
• Electric potential is know as voltage or
electromotive force (EMF)
• Voltage can be thought of as pressure in an
electrical circuit
• The unit of measurement of voltage or EMF
is the volt
• The symbol for volt is V (truck battery voltage
is usually 12V)
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Electron Flow
• The flow of electricity is known as electric
current
• Current flow is the number of electrons
passing a stationary point in a given period of
time
• 1 coulomb equals 6.25 billon-billion electrons
passing a stationary point in a given period of
time
• Current is measured in amperes
• The symbol for amperes is A
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Direction of Current Flow
• A battery has 2 terminals; positive and
negative
• The positive terminal is marked + and the
negative is marked –
• Conventional theory; positive to negative
• Electron theory; negative to positive
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Figure 2-9 Electron flow is like the movement of cars to the right, while
conventional flow is like the movement of spaces to the left.
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Direct and Alternating Current
• Almost everything on a truck uses direct
current (DC)
• Direct current does not change direction or
amplitude
• Alternating current changes amplitude and
direction of flow continually
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Conductors
• Conductors are made of materials that allow
electron flow with little opposition
• Conductors on trucks are usually made of
copper
• Free electrons are easily dislodged from the
outer most band of an atom and into the
outer most band of the atom next to it
• The outer most band of an atom is called the
valance band
• Atom with one or two electrons in the valance
band make good conductors
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Figure 2-10 Electron flow is like marbles in a tube: a marble that
enters the tube on the left is not the same marble that exits the
tube on the right.
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Conductors (continued)
• Electron flow is believed to be an energy
wave that flows through a conductor
• The energy wave travels near the speed of
light (186,000 miles/second)
• The actual electrons only flow a few inches
per hour
• The energy wave is similar to the energy
wave produced when a pool cue strikes a line
of closely lined up pool balls and only the last
ball rolls
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Resistance
• Resistance is the opposition to the flow of
electric current
• Ohm is the unit of measurement of
electrical resistance
• The symbol for ohms is the Greek letter
omega (Ω)
• Resistors are electrical components with a
specific value of resistance
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Wire Resistance
• All electrical conductors have some
resistance even though it may be a small
amount
• Wire resistance increases with the length
of the wire, a 10mm wire has 10 times the
resistance as a 1mm wire
• A tungsten filament in a light bulb uses
resistance to heat up and light when
electrical current flows through it
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Making Use of Wire Resistance
Figure 2-14 Light bulb filament has a
much smaller diameter than the
wires connected to the light bulb
causing an opposition to the flow of
electric current.
Figure 2-15 Electrical
resistance cause heat and
light when electric current
flows through the light bulb
filament.
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Insulators
• An insulator is a material that offers a
great deal of resistance to the flow of
electrical current
• The resistance of an insulator is said to be
very high
• Insulators have five or more valence
electrons
• Plastic, rubber, and glass are examples of
good insulators
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Ohm’s Law
• Increasing the voltage causes the
electrical current flowing through a fixed
value of resistance to increase
• Increasing the resistance while keeping
the voltage source held at a fixed value
causes the electric current flowing through
the resistance to decrease
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Ohm’s Law (continued)
•
•
•
•
•
•
Voltage ÷ Resistance=Current
1 Volt ÷ 1 Ohm = 1 ampere
E÷R=I
E = Voltage
R = Resistance
I = Amperes
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Memorization Aid
Figure 2-16 Ohm’s law
memorization aid.
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Electrical Maps
• Circuits are the path that electrical current
flows
• Schematics are used to show the layout or
relationship of electrical components
• Schematics are also know as circuit
diagrams or wiring diagrams
• Electrical schematics use symbols to
indicate electrical components found in an
electrical circuit
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Common Electrical Symbols
Figure 2-17
Electric
symbols used
to construct
electrical
schematic.
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Measuring Pressure
• Most pressure gauges display the difference
between atmospheric pressure and the
unknown pressure and reads as PSIG; G=
Gauge
• Some gauges display the difference between
an absolute vacuum and the unknown
pressure and reads as PSIA; A= Absolute
• A differential pressure gauge measures the
difference between two different gauge ports
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Measuring Voltage
• Voltmeters are electrical diagnostic tools that
read voltage
• Voltmeters display the difference in electrical
potential (voltage) between two points
• Most common volt meters are multimeters
capable of reading volts, amperes and
resistance
• Test leads are wires with probes that connect
the volt meter to the circuit being tested
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Typical Multimeter
Figure 2-22
Digital
multimeter.
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Measuring Voltage (continued)
• Test leads are typically colored red and
black
• The red lead is installed at the most
positive point in the circuit
• The black lead is installed at the most
negative point in the circuit
• Volt meters are connected to a circuit in
parallel
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Measuring Voltage With a
Voltmeter
Figure 2-24 Measuring Voltage.
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Measuring Current
• Current flow through a circuit is measured
with an ammeter
• An ammeter is typically included in a
multimeter
• Ammeters are connected in series in a circuit
• Most multimeters have a specific plug for
reading current
• The circuit current can not exceed the
maximum amperage rating of the ammeter
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Measuring Current With an Ammeter
Figure 2-26
Measuring
current with
an ammeter.
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Tech Tip
Modern DMM are fantastic tools but have fooled
almost every technician at one time or another. One
reason for this is just a miniscule amount of voltage
present in a circuit when resistance is being
measured can result in dramatic resistance
measurement errors. The miniscule voltage can be
generated by engine coolant or water in contact with
dissimilar metals, as will be explained in more detail
in later chapters. If resistance measurements do not
make sense, measure the voltage that is present
between the two points of which you want to know
the resistance. If the DMM displays any voltage,
your resistance reading will not be accurate.
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Current Shunts and Amp Probes
• A current shunt is a known low value of
resistance placed in the circuit.
• Voltage is measured across the shunt
• Current can be calculated with Ohm’s law
• Clamp on current probes measure the
strength of the magnetic field surrounding a
wire and converts it to a current value
• Current probes may also be designed as an
input device to a DMM
• Clamp on probes make troubleshooting
easier because the circuit does not have to
be opened to make a amperage reading
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Clamp On Current Probes
Figure 2-28 Clamp-on current probes.
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Measuring Resistance
• Resistance is measured with an ohmmeter
• If the ohmmeter has a range setting, set to
the highest range of unknown resistances
• It is better to isolate the component from
the rest of the circuit
• If a negative resistance reading is
obtained look for voltage on the circuit
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A Pressure Differential Gauge
Figure 2-33
Using a
differential
pressure
gauge.
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Measuring Voltage and Current in a
Series Circuit
Figure 2-40
Measuring
voltage drop
and current
flow.
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Increasing Resistance to 3 Ohms
Figure 2-41
Increasing
resistance.
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Measuring Voltage Drops
Figure 2-44 Difference in the voltage dropped across unequal series resistors.
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Series Circuit
• Series circuits have only one path for
current flow
• The sum of all the voltage drops in a
series circuit is equal to the source voltage
• The total circuit resistance is the sum of all
the resistance in the circuit
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Parallel Circuits
• Parallel circuits contain multiple paths for current
flow
• Voltage drops in a parallel circuit always equal the
source voltage
• The sum of the current flow in each branch of a
parallel circuit is equal to the total current supplied
by the voltage source
• Adding additional parallel resistance decreases
the total circuit resistance
• Total circuit resistance in a parallel circuit is always
less then the smallest resistor
• To calculate total resistance in a parallel circuit use
the product over the sum formula
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Calculating Total Resistance in a Series
Parallel Circuits
Figure 2-50 Solved seriesparallel circuit.
Figure 2-49 Series-parallel
simplification progression.
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Metric Prefixes
Figure 2-51 Common metric prefixes used in electrical system troubleshooting.
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Three Resistors in Parallel
Figure 2-52 Three resistors in parallel. Note the measured resistance
value is less than any of the parallel-connected resistors.
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Summary
• A difference in electric potential between two
points causes electrons to flow between the two
points when a conductive pass is provided
between them. Voltage is electrical potential and is
measured in volts. Voltage can be thought of as
pressure in a hydraulic or pneumatic circuit.
• The flow of electrons is called current and
measured in amperes. An ampere describes a
quantity of electrons passing a stationary point per
second. Electric current is like the fluid that flows
in a hydraulic circuit.
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Summary (continued)
• The opposition of electric current flow is called
resistance and is measured in ohms.
• Ohm’s law mathematically describes the
relationship between voltage, current, and
resistance. Ohm’s law indicates that dividing the
value of the voltage dropped across a resistance
by the value of resistance in ohms yields the
current flow through the resistance in amperes.
Ohm’s law can be manipulated so that when any
two quantities are known (voltage, current, or
resistance) the third unknown quantity can be
calculated.
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Summary (continued)
• A voltmeter is like a pressure differential gauge in
that the difference in electrical potential between
two points is being measured. A voltmeter is
typically placed in parallel across a component to
measure the voltage drop across the component.
• An ammeter is like a flow meter in that the quantity
of electrons passing a stationary point per second
is being measured. An ammeter is always placed
in series with the portion of the circuit that the
current flow is being measured. Placing an
ammeter in parallel with a component can damage
the ammeter.
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Summary (continued)
• An ohmmeter measures resistance and is
only used when there are no voltage sources
in the circuit.
• In a series electrical circuit, there is only one
path for current to flow. The current is the
same in a series circuit. The voltage dropped
across components in a series circuit is
proportional to the component’s resistance
value relative to other components in that
circuit.
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Summary (continued)
• In a parallel electrical circuit, there is more
than one path for current flow. The circuit
is divided among each parallel path. The
greater the resistance of a path, the less
current that flows through that path
compared to other paths. The voltage
dropped across each component in a
parallel circuit is the same, regardless of
the resistance of that component.
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Summary (continued)
• Series-parallel circuits have components
that are connected in parallel and in
series. The voltage drops and branch
currents can be found by determining the
equivalent resistance for the entire circuit.
The equivalent resistance is then used to
determine the total circuit current. The
rules of series and parallel circuits are
then applied to the circuit to determine the
voltage drops and branch currents.
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