Transcript Chapter 5

Chapter 5
Fundamentals of Electricity
Objectives (1 of 3)
• Define the terms electricity and electronics.
• Describe the atomic structure.
• Outline how some of the chemical and
electrical properties of atoms are defined by
the number of electrons in their outer shells.
• Outline the properties of conductors,
insulators, and semiconductors.
• Describe the characteristics of static
electricity.
Objectives (2 of 3)
• Define what is meant by the conventional and
electron theories of current flow.
• Describe the characteristics of magnetism and the
relationship between electricity and magnetism.
• Describe how electromagnetic field strength is
measured in common electromagnetic devices.
• Define what is meant by an electrical circuit and the
terms voltage, resistance, and current flow.
Objectives (3 of 3)
• Outline the components required to construct
a typical electrical circuit.
• Perform electrical circuit calculations using
Ohm’s law.
• Identify the characteristics of DC and AC.
• Describe some methods of generating a
current flow in an electrical circuit.
• Describe and apply Kirschhoff’s first and
second laws.
Atomic Structure
and Electron Movement (1 of 7)
Atomic Structure
and Electron Movement (2 of 7)
• All matter is electrical in essence.
• All matter is composed of atoms.
• The atom is the smallest particle in a chemical
element.
• The atomic structure of an element determines its
chemical and electrical characteristics.
• The chemical and electrical properties of atoms are
defined by the number of electrons in their outer
shells.
Atomic Structure
and Electron Movement (3 of 7)
• Nucleus
– In the center of every atom is a nucleus.
• Protons
– The nucleus is made up of positively charged matter
called protons.
• Neutrons
– The nucleus contains matter with no charge called
neutrons.
• Electrons
– Negatively charged particles called electrons are
orbiting each atomic nucleus.
Atomic Structure
and Electron Movement (4 of 7)
• Electrons orbit the nucleus in concentric
paths called shells.
• All electrons are alike -- AND -- all protons
are alike.
– So where is the difference?
• Every chemical element has a distinct identity
and is made up of distinct atoms.
– That is, each has a different number of
protons and electrons.
Atomic Structure
and Electron Movement (5 of 7)
• In an electrically balanced atom, the number of
protons equals the number of electrons.
– This means that the atom is in what is described as a
neutral state of electrical charge.
• Ion
– An atom with either a deficit or excess of electrons is
known as an ion.
• Charge can move from one point to another.
– Like charges repel.
– Unlike charges attract.
Atomic Structure
and Electron Movement (6 of 7)
• Electrons (negative charge) are held in their
orbital shells by the nucleus (positive charge)
of the atom.
• Electrons are prevented from colliding with
each other because they all have similar
negative charges that tend to repel each
other.
• A molecule is a chemically bonded union of
two or more atoms.
• A compound is a chemically bonded union of
atoms of two or more dissimilar elements.
Atomic Structure
and Electron Movement (7 of 7)
• All atoms have an electrical charge.
• An atom is balanced when the number of protons
match the number of electrons and is said to be in
an electrically neutral state.
• Electricity is concerned with the behavior of atoms
that have become unbalanced or electrified.
• Electricity may be defined as the movement of free
electrons from one atom to another.
• Current flow is a measurement of the number of
free electrons passing a given point in an electrical
circuit per second.
Conductors and Insulators (1 of 4)
• Electron movement
through a conductor is
referred to as current.
• To produce current
flow, electrons must
move from atom to
atom.
Conductors and Insulators (2 of 4)
• A conductor is
generally a metallic
element that contains
fewer than four
electrons in its outer
shell or valence.
– Copper, aluminum,
gold, silver, iron, and
platinum are classified
as conductors.
Conductors and Insulators (3 of 4)
• An insulator is a
nonmetallic substance
that contains five or
more electrons in its
outer shell or valence.
– Glass, mica, rubber,
and plastic are good
insulators.
Conductors and Insulators (4 of 4)
• Semiconductors are a
group of materials that
cannot be classified
either as conductors or
insulators.
– They have exactly four
electrons in their outer
shell.
– Silicon (Si) is an
example of a
semiconductor.
Current Flow (1 of 3)
• Conventional theory
states that current
flows from positive to
negative.
• Vehicle schematics use
conventional theory
almost exclusively.
Current Flow (2 of 3)
• When the electron was
discovered, scientists
revised the theory of
current flow and called
it electron theory.
• The electron theory
states that current flow
is from negative to
positive.
Current Flow (3 of 3)
• Charge differential or
voltage is a measure of
electrical pressure. It is
referred to as:
–
–
–
–
Charge differential
Voltage (V)
Electro-motive force (EMF)
Potential difference (PD)
• The greater the difference,
the greater will be the rate of
current flow.
Magnetism (1 of 3)
• A bar shaped
permanent magnet has
a north and a south
pole at opposite ends.
• Like poles repel.
• Unlike poles attract.
• The lines of force
surrounding the
magnet are referred to
as flux lines.
Magnetism (2 of 3)
• Flux lines flow in one direction.
• Flux lines exit from the magnets north pole
and enter through the south pole.
• The flux density (concentration) determines
the magnetic force.
Magnetism (3 of 3)
• The flux density is always greatest at the
poles of a magnet.
• Flux lines do not cross each other in a
permanent magnet.
• Flux lines facing the same direction attract.
• Flux lines facing opposite directions repel.
Electromagnetism (1 of 3)
• Current flow through
any conductor creates
a magnetic field.
• Magnetic lines of force
do not change when
the current flow through
a conductor is
constant.
• When current flow
increases, the lines of
force will extend further
from the conductor.
Electromagnetism (2 of 3)
• The intensity and strength of magnetic lines
of force increase proportionally with an
increase in current flow through a conductor.
• Similarly, they decrease proportionally with a
decrease in current flow through the
conductor.
Electromagnetism (3 of 3)
• A rule called the righthand rule is used to
indicate the direction of
the magnetic lines of
force.
Using Electromagnetism (1 of 2)
• A magnetic field exists
when current flows
through a wire.
• When the wire is
coiled, the magnetic
field is intensified.
– When an iron core is
placed in the center of
this coil, the magnetic
field is further
intensified.
Using Electromagnetism (2 of 2)
• Magneto-motive force
is measured in ampereturns (at).
Electrical Current Characteristics (1 of 2)
• Direct current
– Current flows in one direction only.
– Current flow may be:
• Continuous
• Pulsed
– DC current is used almost exclusively in
highway vehicles.
Electrical Current Characteristics (2 of 2)
• Alternating current
– Current cyclically
reverses at high
speed.
– AC current is used in
alternators and by
certain sensors.
Sources of Electricity (1 of 2)
• Chemical
– In a lead acid battery,
voltage is produced by
a chemical reaction
between lead and lead
peroxide plates
submersed in sulfuric
acid.
Sources of Electricity (2 of 2)
•
•
•
•
Static electricity
Thermoelectric
Photoelectric
Piezoelectric
Electromagnetic Induction
• Electromagnetic induction
– Current is produced in a
conductor that is moved
through a stationary
magnetic field.
– Current is produced when
a magnetic field is moved
past a stationary
conductor.
– Electromagnetic induction
is a means of converting
mechanical energy into
electrical energy.
Electrical Circuits
and Ohm’s Law (1 of 5)
• Ohm’s law describes the relationship
between electrical potential, current, and
resistance.
• An electrical circuit must have:
– Power source
– Path
– Load
Electrical Circuits
and Ohm’s Law (2 of 5)
•
•
•
•
Voltage
Current
Resistance
Circuit components
– Power source
– Conductors
– Switches
– Circuit protection devices
Electrical Circuits
and Ohm’s Law (3 of 5)
• Series circuits
– There is a single path for current to flow.
– All of the current flows through each resistor in
the circuit.
• Parallel circuits
– There are multiple paths for current to flow.
– The resistance in each path determines the
current flow through it.
• Series parallel circuits
Electrical Circuits
and Ohm’s Law (4 of 5)
• Electrical circuit terminology
– Short circuit
– Open circuit
– Grounds
– Short to ground
– High resistance circuits
Electrical Circuits
and Ohm’s Law (5 of 5)
• OHM’S law states that an
electrical pressure of 1 volt
is required to move 1 amp of
current through a resistance
of 1 ohm.
• E=IxR
– I = Intensity = current in
amps
– E = EMF (electromotive
force) = pressure in volts
– R = resistance =
resistance in ohms
• This is a mathematical
formula that technicians
MUST know.
Ohm’s Law
Applied to Series Circuits
• All of the current flows
through all of the
resistances in the
circuit
• Total circuit resistance
is the sum of all of the
resistances. Rt =R1 +
R2 etc…
Kirchhoff’s Law of Current
• Current flowing into a
junction or point in an
electrical circuit must
equal the current
flowing out.
Kirchhoff’s Law of Voltage Drops
• Voltage will drop in
exact proportion to the
resistance, and the
sum of the voltage
drops must equal the
voltage applied to the
circuit.
Power
• The unit for measuring power is the watt
usually represented by the letter P.
–P=IxE
– 1 horsepower (HP) = 746 watts
Electric Motor
and Generator Principle (1 of 3)
Electric Motor and
Generator Principle (2 of 3)
• DC motors
– The electric motor converts
electrical energy into
mechanical energy.
– Current-carrying
conductors are arranged
as loops of wire in an
armature.
– The armature is placed
inside a magnetic field.
– When current flows
through the armature,
torque is produced.
Electric Motor
and Generator Principle (3 of 3)
• Generators
– A generator is simply an
electric motor with its
function reversed.
– AC generators produce AC
current which must be
rectified to DC.
– Reluctor-type generators
consisting of a permanent
magnet, a coil of wire, and
a toothed reluctor are used
as shaft speed sensors.
Capacitance
• Capacitors store
electrons.
• A capacitor consists of
two conductors
separated by an
insulating material
called dielectric.
Types of Capacitors
• Power supply filter
• Spike suppressant
• Resistor-capacitor circuits (R-C circuits)
Coils, Transformers, and Solenoids
• Two coils are arranged so that one is subject
to a magnetic field created in the other.
– The input coil is the primary coil.
– The output coil is the secondary coil.
– Step-up transformers have secondary coils
with a greater number of windings.
– Step-down transformers have secondary coils
with a lower number of windings.
Solenoids and Magnetic Switches
• Magnetic switches are
used so that a low
current can control a
high current.
• Solenoids use the
same operating
principle, but are used
to convert electrical
energy into mechanical
movement.
Summary (1 of 7)
• All matter is composed of atoms.
• All atoms have an electrical charge.
– When an atom is balanced (the number of protons
match the number of electrons), the atom can be
described as being in an electrically neutral state.
• All matter is electrical in essence.
– Electricity concerns the behavior of atoms that have
become, for whatever reason, unbalanced or
electrified.
• Electricity may be defined as the movement of free
electrons from one atom to another.
Summary (2 of 7)
• Current flow is measured by the number of free
electrons passing a given point in an electrical circuit
per second.
• Electrical pressure or charge differential is measured in
volts, resistance in ohms, and current in amperes.
• The magnetic properties of some metals such as iron
are due to electron motion within the atomic structure.
• A direct relationship exists between electricity and
magnetism.
– Electromagnetic devices are used extensively on
vehicles.
Summary (3 of 7)
• Magneto-motive force (mmf) is a measure of
electromagnetic field strength.
– Its unit is ampere- turns (At).
• Ohm’s law is used to perform circuit calculations on
series, parallel, and series-parallel circuits.
• In a series circuit, there is a single path for current
flow and all of the current flows through each
resistor in the circuit.
• A parallel circuit has multiple paths for current flow.
– The resistance in each path determines the current
flow through it.
Summary (4 of 7)
• Kirchhoff’s law of voltage drops states that the sum
of voltage drops through resistors in a circuit must
equal the source voltage.
• When current is flowed through a conductor, a
magnetic field is created.
• Reluctance is resistance to the movement of
magnetic lines of force.
– Iron cores have permeability and are used to reduce
reluctance in electromagnetic fields.
• Capacitors are used to store electrons.
– They consist of conductor plates separated by a
dielectric.
Summary (5 of 7)
• Capacitance is measured in farads.
– Capacitors are rated by voltage and by capacitance.
• When current is flowed through a wire conductor,
an electromagnetic field is created.
– When the wire is wound into a coil, the
electromagnetic field strength is intensified.
• The principle of a transformer can be summarized
by describing it as flowing current through a primary
coil and inducing a current flow in a secondary or
output coil.
Summary (6 of 7)
• Transformers can be grouped into three categories:
isolation, step-up, and step-down.
• An electromagnetic switch is used in a truck
electrical circuit to enable a low-current circuit to
control a high-current circuit.
• A relay is an example of an electromagnetic switch.
• A solenoid uses similar operating principles to an
electromagnetic switch except that it converts
electromagnetic energy into mechanical movement.
Summary (7 of 7)
• Solenoids are used extensively in truck
electrical circuits for functions such as starter
engage mechanisms, diesel electronic unit
injector control, automatic transmission clutch
controls, and suspension pilot switches.