► Increasingly, vehicle technicians must have an understanding of the electrical principles involved in vehicle system operation.
► Atoms contain negatively charged electrons moving around a nucleus, in which there are positively charged protons and neutrons with no charge.
► Atoms with excess electrons have a negative charge and create a negative ion; those deficient in electrons have a positive charge and create a positive ion.
► Free electrons can move from one atom to another if an electrical potential is applied.
► Materials with many free electrons are good electrical conductors.
► Copper is the most common conductor.
► Insulators are materials that do not conduct current easily; an example is plastic.
► Semiconductor refers to a material that conducts electricity more easily than an insulator, but not as well as a conductor.
► Free electrons require a pathway or circuit, and a force to act upon them, such as a battery.
► Like charges repel, and unlike charges attract.
► The attraction of free electrons that creates a force is called voltage.
► The four factors that determine electrical resistance level are the type of material and the length, size, and temperature of the conductor.
► Electrical resistance refers to the degree to which a material opposes the passage of an electrical current.
► Resistance is measured in ohms and is constant in an object unless the temperature changes.
► A semiconductor’s ability to conduct electricity depends on negative electrons and holes.
► The number of charge carriers in a semiconductor can be changed by adding small quantities of impurities (doping).
► The PN junction of a semiconductor is located at the depletion layer.
► Semiconductors can prevent or allow current flow, depending on connection to a current source.
► Semiconductor materials include silicon, germanium, gallium-arsenide, and silicon carbide.
► Electrical circuits contain a power supply, a current flow on/off switch, a functional component, a conductive pathway, and a protection device (e.g., a fuse).
► Voltage is the electrical pressure difference between two points in an electrical circuit.
► The ampere (amp) is the unit used to describe how much current is flowing at a given point within a circuit when the functional component is operational.
► The ohm is the unit used to describe electrical resistance in a circuit or component.
► Direct current (DC) flows in one direction only; alternating current (AC) continuously changes its direction of flow.
► Electrical components can work only on AC or DC, but not both.
► Circuits may be described in terms of continuity, open, short, and high resistance.
► Electrostatic energy occurs when two insulators are rubbed together, with one losing electrons to become positively charged and the other gaining electrons to become negatively charged.
► Thermoelectric energy is produced by joining and heating two different metals.
► Electrochemical energy is produced via electrolysis, which is the immersion of two dissimilar metals in a conducting liquid to break down chemicals into ions.
► Photovoltaic energy is produced via solar energy cells.
► Piezoelectric energy is produced when certain crystals are subjected to mechanical stress.
► Electromagnetic induction is created when a conductor cuts across a magnetic field.
► The effects of electricity include light (LED bulbs), heat (headlights), chemical reactions (lead-acid battery), and magnetism (electric motors).
► Electromagnets are used in relays, solenoids, and motors, while electromagnetic induction is used in ignition coils and transformers.

► Relays are used to control circuits that carry high current flow; they can be normally open (NO) or normally closed (NC).
► Solenoids operate similarly to a relay, but create lateral movement rather than closing a circuit.
► Electric motors rely on magnetic fields to create rotary movement.
► Ohm’s law states that the total resistance of a circuit always equals the voltage divided by the amperage.
► The term “work” refers to transforming one form of energy into another.
► Power refers to the rate at which work is done, or the rate of transforming energy.
► The watt is the unit of power.
► Kirchhoff’s current law states that electrical current entering any junction is equal to the sum of the current flowing out of the junction.
► In a series circuit, current can flow in only one path and all electrons flow at the same rate.
► Voltage drop refers to the pressure lost by driving the current through a resistor.
► The electrical properties of a series circuit are as follows: current flow is the same in all parts of the circuit; the applied voltage is equal to the sum of the individual voltage drops; and total circuit resistance is equal to the sum of the individual resistances.
► All components in a parallel circuit are directly connected to the voltage supply; hence the voltage across each component is equal to battery voltage.
► Parallel circuit laws are as follows: the voltage across all branches of a parallel circuit are the same; the total current equals the sum of the current flowing in each branch; the amount of current in each branch is inversely proportional to the resistance of the branch; and the total resistance of a parallel circuit can be calculated as R
T
= (R
1
× R
2
) divided by (R
1
+ R
2
).Or, R
T
= 1/ 1/R
1
+ 1/R
2
+ 1/R
3
.
► Series-parallel circuits contain both a series circuit and a parallel circuit.
► Electrical components must be correctly connected onto circuits and may have numbered or marked terminals to ensure proper connection.
► Circuit protection devices, which break the circuit during excessive current flow, are fuses, fusible links, and circuit breakers.
► Resistors are used to control voltage that reaches various components because they resist the current running through them.
► Resistor types include fixed, variable, thermistors, metal oxide varistors, and ballast resistors.
► Resistors are rated by both resistance value and power rating.
► Resistance value is indicated by colored bands, and tolerance is indicated by the number of identifying bands.
► Variable resistor types are rheostats, potentiometers, and thermistors.
► Thermistors are a type of conductor in which resistance value is affected by temperature.
► Capacitors are used to store electrical energy and are rated by capacitance, which is the amount of change stored in each plate for a given potential voltage between the plates.
► Diodes are used to restrict current flow to one direction only.
► Transistors (NPN and PNP types) are used as switches and to amplify currents.
► Control modules are designed to monitor multiple inputs from sensors and circuits and respond to those inputs.
► Delay circuits can turn on or off an electrical device after a specified time delay.
► Microprocessors are designed to monitor and control most electrical systems on a modern vehicle.
► Most vehicle wires are braided copper with plastic insulation, but other types are shielded wires and ribbon.
► Wire shielding to prevent noise (unwanted electromagnetic induction) can be twisted pair, Mylar tape, or drain lines.
► Proper operation of electrical circuits requires correct wire size.
► Length and diameter of a cable determine its resistance.
► Copper has low resistance value, although as the wire length increases, so does resistance within the wire; therefore, the cross-sectional area needs to increase to overcome resistance.

► Terminals and connectors are fitted to the ends of cables to provide low-resistance cable termination.
► Terminals can be push-on spade terminals, eye ring terminals, and solder-type terminals.
► Connectors can be permanent cable joiners, wiring harness connectors, or male or female connectors.
► Wiring harnesses are used to bind wires together within a sheath of insulating tape or tubing.
► Wiring diagrams are generally split up into systems and subsystems.
► Microprocessors and their assorted components are mounted on circuit boards with conductive tracks designed to connect electronic component leads with one another.
► Electronically controlled systems are integrated to a multiplexed serial communications network, such as a controlled area network bus (CAN-bus).
► Fiber-optic cables can transmit light signals over very long distances.
► Electrical measurement tools include the ammeter, voltmeter, ohmmeter, and digital multimeter.