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Transcript of Chapter 5
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Chapter 5
Fundamentals of Electricity
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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.
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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.
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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.
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Atomic Structure and Electron Movement (1 of 7)
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Current Flow (1 of 3)
• Conventional theory states that current flows from positive to negative.
• Vehicle schematics use conventional theory almost exclusively.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Electromagnetism (3 of 3)
• A rule called the right-hand rule is used to indicate the direction of the magnetic lines of force.
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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.
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Using Electromagnetism (2 of 2)
• Magneto-motive force is measured in ampere-turns (at).
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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.
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Electrical Current Characteristics (2 of 2)
• Alternating current– Current cyclically
reverses at high speed.
– AC current is used in alternators and by certain sensors.
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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.
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Sources of Electricity (2 of 2)
• Static electricity
• Thermoelectric
• Photoelectric
• Piezoelectric
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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.
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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
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Electrical Circuits and Ohm’s Law (2 of 5)
• Voltage
• Current
• Resistance
• Circuit components– Power source– Conductors– Switches– Circuit protection devices
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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
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Electrical Circuits and Ohm’s Law (4 of 5)
• Electrical circuit terminology– Short circuit– Open circuit– Grounds– Short to ground– High resistance circuits
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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 = I x R– 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.
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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…
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Kirchhoff’s Law of Current
• Current flowing into a junction or point in an electrical circuit must equal the current flowing out.
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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.
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Power
• The unit for measuring power is the watt usually represented by the letter P.– P = I x E– 1 horsepower (HP) = 746 watts
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Electric Motor and Generator Principle (1 of 3)
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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.
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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.
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Capacitance
• Capacitors store electrons.
• A capacitor consists of two conductors separated by an insulating material called dielectric.
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Types of Capacitors
• Power supply filter
• Spike suppressant
• Resistor-capacitor circuits (R-C circuits)
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.