The electrical system is one of the most important systems in a vehicle today. Every year, more and more components and systems use electricity. Technicians who really know and understand automotive electrical and electronic systems will be in great demand.
Our universe is composed of matter, anything that has mass and occupies space. All matter is made from slightly over 100 individual components called elements . The smallest particle that an element can be broken into and still retain the properties of that element is known as an atom .
Figure 31–1 In an atom (left), electrons orbit protons in the nucleus just as planets orbit the sun in our solar system (right). Continued
Electricity is the movement of electrons from one atom to another. The dense center of each atom is called the nucleus . The nucleus contains protons , which have positive charge, and neutrons , electrically neutral (no charge). Electrons surround the nucleus in orbits. Each atom contains an equal number of electrons and protons. Because the number of negative-charged electrons is balanced with the same number of positive-charged protons, an atom has a neutral charge (no charge).
NOTE: As an example of relative sizes of parts of an atom, consider that if an atom were magnified so that the nucleus were the size of the period at the end of this sentence, the whole atom would be bigger than a house.
Positive and Negative Charges Parts of the atom have different charges. Orbiting electrons are negatively charged, protons positively charged. Positive charges are indicated by the “plus” sign (+), and negative charges by the “minus” sign ( ).
Figure 31–2 The nucleus of an atom has a positive () charge and the surrounding electrons have a negative () charge. Continued These same + and signs are used to identify parts of an electrical circuit. Neutrons have no charge at all. They are neutral.
In a normal, or balanced, atom, the number of negative particles equals the number of positive particles. The number of neutrons varies according to the type of atom.
Figure 31–3 This figure shows a balanced atom. Continued
Figure 31–4 Unlike charges attract and like charges repel.
An ordinary magnet has two ends, or poles. One end is the south pole, and the other the north pole. If the opposite poles of the magnets are brought close to each other, south to north, the magnets will snap together because unlike poles attract each other. If two magnets are brought close to each other with like poles together (south to south or north to north), the magnets will push each other apart. This is because like poles repel each other.
Positive and negative charges within an atom are like north and south poles of a magnet. Charges that are alike will repel each other, which is why the negative electrons continue to orbit around the positive protons. They are attracted and held by the opposite charge of the protons. The electrons keep moving in orbit because they repel each other. When an atom loses electrons, it becomes unbalanced. It will have more protons than electrons and will have a positive charge. If it gains more electrons than protons, it will be negatively charged. When an atom is not balanced, it becomes a charged particle called an ion . Ions try to regain balance of equal protons and electrons by exchanging electrons with neighboring atoms. See Figure 31–5. This is the flow of electric current or electricity .
Figure 31–5 An unbalanced, positively charged atom (ion) will attract electrons from neighboring atoms.
Electron Shells Orbit around the nucleus in definite paths. These paths form shells , like concentric rings , around the nucleus. Only a specific number of electrons can orbit within each shell.
Figure 31–6 The hydrogen atom is the simplest atom, with only one proton, one neutron, and one electron. More complex elements contain higher numbers of protons, neutrons, and electrons. Continued If there are too many electrons for the first and closest shell to the nucleus, others will orbit in additional shells until all electrons have an orbit within a shell. There can be as many as seven shells around a single nucleus.
Free and Bound Electrons The outermost electron shell or ring, called the valence shell , is the most important to our study of electricity. The number of electrons in this shell determines the valence of the atom and indicates its capacity to combine with other atoms. If the valence ring of an atom has three or fewer electrons in it, the ring has room for more. The electrons are held very loosely, and it is easy for a drifting electron to join the ring and push another electron away. These loosely held electrons are called free electrons . When a valence ring has five or more electrons, it is fairly full. The electrons are held tightly, and it is hard for a drifting electron to push its way into the ring. These tightly held electrons are called bound electrons . See Figures 31–7 and 31–8.
Figure 31–7 As the number of electrons increases, they occupy increasing energy levels that are further from the center of the atom.
The movement of these drifting electrons is called current . Electric current is controlled, directed movement of electrons from atom to atom within a conductor.
Figure 31–8 Electrons in the outer orbit, or shell, can often be drawn away from the atom and become free electrons. Continued
Conductors Materials with fewer than four electrons in their atom’s outer orbit are Conductors . Copper is excellent as a conductor because it has only one electron in its outer orbit. This orbit is far enough away from the nucleus of the atom that the pull or force holding the outermost electron in orbit is relatively weak.
Figure 31–9 A conductor is any element that has one to three electrons in its outer orbit. Continued
Copper is the conductor most used in vehicles because the price of copper is reasonable compared to the relative cost of other conductors with similar properties.
Figure 31–10 Copper is an excellent conductor of electricity because it has just one electron in its outer orbit, making it easy to be knocked out of its orbit and flow to other nearby atoms. This causes electron flow, which is the definition of electricity. Continued
Insulators The protons and neutrons in the nucleus are held together very tightly. Normally the nucleus does not change. Some outer electrons are held very loosely, and can move from one atom to another. Some materials hold their electrons very tightly; electrons do not move through them very well. These materials are called insulators.
Pure water is an insulator; however, if anything is in the water, such as salt or dirt, then the water becomes conductive. Because it is difficult to keep water from becoming contaminated, water is usually thought of as being capable of conducting electricity, especially high voltage such as from household 110-volt or 220-volt outlets. Is Water a Conductor? Continued
Insulators are materials with more than four electrons in their atom’s outer orbit. Because they have more than four electrons, it becomes easier for these materials to acquire (gain) electrons than to release electrons.
Figure 31–11 Insulators are elements with five to eight electrons in the outer orbit. Continued Insulators include plastics, wood, glass, rubber, ceramics (spark plugs), and varnish for covering (insulating) copper wires in alternators and starters.
How Electrons Move Through a Conductor If an outside source of power, such as a battery, is connected to the ends of a conductor, a positive charge (lack of electrons) is placed on one end of the conductor and a negative charge is placed on the opposite end of the conductor. The negative charge will repel the free electrons from the atoms of the conductor, whereas the positive charge on the opposite end of the conductor will attract electrons. As a result of this attraction of opposite charges and repulsion of like charges, electrons will flow through the conductor.
Figure 31–13 Current electricity is the movement of electrons through a conductor. Continued
Figure 31–14 Conventional theory states that current flows through a circuit from positive (+) to negative (-). Automotive electricity uses the conventional theory in all electrical diagrams and schematics.
Conventional Theory versus Electron Theory It was once thought that electricity had only one charge and moved from positive to negative. This theory of the flow of electricity through a conductor is called the conventional theory of current flow.
This book uses the conventional theory unless stated otherwise. Discovery of the electron and its negative charge led to the electron theory , which states there is electron flow from negative to positive.
Amperes The ampere is the unit used to measure current flow. When 6.28 billion billion electrons (a coulomb ) move past a certain point in 1 second, this represents 1 ampere of current. The ampere is the electrical unit for amount of electron flow just as “gallons per minute” is the unit used to measure water flow. The ampere was named for the French electrician André Marie Ampère (1775–1836).
Figure 31–15 One ampere is the movement of 1 coulomb (6.28 billion billion electrons) past a point in 1 second. Continued
Volts The volt is the unit of measurement for electrical pressure. Named for Alessandro Volta (1745–1827), an Italian physicist. The comparable unit using water as an example would be pounds per square inch (psi). It is possible to have very high pressures (volts) and low water flow (amperes). It is also possible to have high water flow (amperes) and low pressure (volts). Voltage is also called electrical potential , because if there is voltage present in a conductor, there is a potential (possibility) for current flow. Voltage does not flow through conductors, but voltage does cause current (in amperes) to flow through conductors.
Figure 31–17 Voltage is the electrical pressure that causes the electrons to flow through a conductor.
The conventional abbreviations and measurement for voltage are:
Figure 31–18 This digital multimeter set to read DC volts is being used to test the voltage of a vehicle battery. Most multimeters can also measure resistance (ohms) and current flow (amperes). Continued
Volt is the measurement for amount of electrical pressure.
Another term for voltage is Electromotive force , ( EMF ).
The letter V is the generally accepted abbreviation for volt s.
The symbol used in calculations is E, for electromotive force .
Ohms Resistance to the flow of current through a conductor is measured in units called ohms , named after the German physicist Georg Simon Ohm (1787–1854). The resistance to the flow of free electrons through a conductor results from the countless collisions the electrons cause within the atoms of the conductor.
Figure 31–19 Resistance to the flow of electrons through a conductor is measured in ohms. Continued
Conventional abbreviations and measurement for resistance are:
The ohm is the unit of measurement for electrical resistance.
The symbol for ohms is Ω (Greek capital letter omega), the last letter of the Greek alphabet.
The symbol used in calculations is R , for resistance .
Ohms are measured with an ohmmeter .
Figure 31–20 A display at the Henry Ford Museum in Dearborn, Michigan, that includes a hand-cranked generator and a series of light bulbs. This figure shows a young man attempting to light as many bulbs as possible. The crank gets harder to turn as more bulbs light because it requires more power to produce the necessary watts of electricity.
The symbol for power is P . Electrical power is calculated as amperes times volts: P (power) = I (amperes) E (volts)
Watts A watt is the electrical unit for power , the capacity to do work. Named for Scottish inventor, James Watt (1736–1819).
There are several sources of electrical energy, but only a few of them are used in automotive electrical systems.
Continued Friction When certain different materials are rubbed together, the friction causes electrons to be transferred from one to the other. Both materials become electrically charged. These charges are not in motion but stay on the surface where they were deposited. Because the charges are stationary, or static, this type of voltage is called static electricity . Vehicle tires rolling on pavement often create static electricity that interferes with radio reception.
Heat When pieces of two metals are joined together at both ends and one junction is heated, current passes through the metals. Only millionths of an ampere, but enough to use in a temperature-measuring device called a thermocouple .
Figure 31–21 Electron flow is produced by heating the connection of two different metals. Continued Some engine temperature sensors operate in this manner. This form of voltage is called thermoelectricity .
In 1823, German physicist Thomas Johann Seebeck discovered that voltage was developed in a loop containing two dissimilar metals, provided the junctions were maintained at different temperatures. A decade later, French scientist Jean Charles Peltier found electrons moving through a solid can carry heat from one side of the material to the other side. This effect is called the Peltier effect . A Peltier effect device is often used in portable coolers to keep food items cool if the current flows in one direction and to keep items warm if the current flows in reverse.
Light In 1839, Edmond Becquerel noticed that shining a beam of sunlight over two different liquids developed electric current. When certain metals are exposed to light, some light energy is transferred to free electrons of the metal. This excess energy breaks electrons loose from the metal. They can be collected and made to flow in a conductor.
Figure 31–22 Electron flow is produced by light striking a light-sensitive material. Photoelectricity is widely used in light-measuring devices such as photographic exposure meters and automatic headlamp dimmers. Continued
Pressure The first demonstration of a connection between macroscopic piezoelectric phenomena and crystallographic structure was published in 1880 by Pierre and Jacques Curie. When subjected to pressure, certain crystals, such as quartz, develop a potential difference, or voltage, on the crystal faces. This current is used in phonograph pickups, crystal microphones, underwater hydrophones, and certain stethoscopes.
Figure 31–23 Electron flow is produced by pressure on certain crystals. The voltage created is called piezoelectricity . Some automobile engine control sensors, such as the knock sensor (KS), use piezoelectricity to create voltage or to vary resistance and control a computer input signal. Continued
Chemistry Two different materials (usually metals) placed in a conducting and reactive chemical solution create a difference in potential, or voltage, between them. This principle is called electrochemistry and is the basis of the automotive battery.
Conductors and Resistance All conductors have some resistance to current flow. Several principles of conductors and their resistance include the following:
If the conductor length is doubled, its resistance doubles This is why battery cables are designed as short as possible.
If the conductor diameter is increased, resistance is reduced This is the reason starter motor cables are larger in diameter than other wiring in the vehicle. See Chapter 7 for further details on wiring sizes.
As the temperature increases, the resistance of the conductor also increases This is the reason for installing heat shields on some starter motors. The shield helps protect the conductors (copper wiring inside the starter) from excessive engine heat and reduces resistance of starter circuits. Because a conductor increases in resistance with increased temperature, the conductor is called a positive temperature coefficient ( PTC ) resistor.
Materials used in the conductor have an impact on its resistance Silver has the lowest resistance of any conductor, but is expensive. Copper is the next lowest in resistance and it is reasonably priced. See the following chart for a comparison of materials.
Most electrical and electronic devices use resistors of specific values to limit and control the flow of current. Resistors can be made from carbon or from other materials that restrict the flow of electricity and are available in various sizes and resistance values. Most resistors have a series of painted color bands around them. These color bands are coded to indicate the degree of resistance.
Figure 31–25 This figure shows a typical carbon resistor. Continued
Variable Resistors Two types of mechanically operated variable resistors are used in automotive applications. A potentiometer is a three-terminal variable resistor where the majority of the current flow travels through the resistance of the unit and a wiper contact provides a variable voltage output.
Figure 31–26 A three-wire variable resistor is called a potentiometer. Continued Potentiometers are most commonly used as throttle position (TP) sensors on computer-equipped engines.
Figure 31–27 A two-wire variable resistor is called a rheostat.
Another type of mechanically operated variable resistor is the rheostat . A rheostat is a two-terminal unit in which all of the current flows through the movable arm. A rheostat is commonly used for a dash light dimmer control.