Introduction to Electricity
Flashlights, electric drills, and motors are electric. Computers and televisions are refered as electronic.
Any component that works with electricity is electric, including both flashlights and electric drills, but not all electric components are electronic. The term electronic refers to semiconductor devices known as electron devices. These devices depend on the flow of electrons for operation.
To better understand electricity, it is necessary to have a basic knowledge of the fundamental atomic structure of matter. Matter has mass and occupies space. Matter can take several forms or states. The three common forms are a solid, a liquid and a gas. This course will provide a basic understanding of the theoretical principles needed to develop a foundation for studying and working with electrical circuits and components as a Caterpillar technician.
Matter and Elements
Matter takes up space and has weight when matter is subjected to gravity. Matter consists of extremely tiny particles grouped together to form atoms. There are approximately 100 different naturally, occurring atoms called elements. An element is a substance that cannot be decomposed any further by chemical action. Most elements have been found in nature. The following are examples of some of the natural elements: copper, lead, iron, gold and silver. Other elements (approximately 14) have been produced in the laboratory. Elements can only be changed by an atomic reaction or nuclear reaction. Elements can be combined to make compounds. The atom is the smallest particle of an element that still has the same characteristics as the element. Atom is the greek word meaning a particle too small to be subdivided.
Although an atom has never been seen, the structure fits evidence that has been measured accurately. The size and electric charge of the invisible particles in an atom are indicated, by how much the atoms are deflected by known forces. The present atomic model, with a nucleus at the center was proposed by Niels Bohr in 1913. The atomic model is patterned after the solar system with the sun at the center and the planets revolving around it.
The center of an atom is called the nucleus. The nucleus is composed primarily of particles called protons and neutrons. Orbiting around every nucleus are small particles called electrons. These electrons are much smaller in mass than either the proton or neutron. Normally, an atom has an equal number of protons in the nucleus and electrons around the nucleus. The number of protons or electrons is called the atomic number. The atomic weight of an element is the total number of particles, protons, and neutrons that are in the nucleus.
Illustration 3 shows the structure of two of the simpler atoms. Illustration 3(a) is an atom of hydrogen, which contains 1 proton in its nucleus balanced by 1 electron in the orbit or shell. The atomic number for a hydrogen atom is 1. Illustration 3(b) shows a simple atom of helium, which has 2 protons in its nucleus balanced by 2 electrons in orbit. The atomic number for helium is 2 and the atomic weight would be 4 (2 protons + 2 neutrons).
Scientists have discovered many particles in the atom. In order to explain basic electricity, only three particles will be discussed: electrons, protons and neutrons. To better understand the basics of electricity, we will use an atom of copper as an example.
Illustration 4 shows a typical copper atom. The nucleus of the atom is not much bigger than an electron. In the copper atom the nucleus contains 29 protons (+), 35 neutrons. The copper atom has 29 electrons (-) orbiting the nucleus. The atomic number of the copper atom is 29 and the atomic weight is 64. When a length of copper wire is connected to positive and negative source, such as a dry cell battery, the following process occurs.
An electron (-) is forced out of orbit and attracted to the positive (+) end of the battery. The atom is now positive (+) charged because it now has a deficiency of electrons (-). The atom in turn attracts an electron from a neighbor. The neighbor in turn receives an electron from the next atom, and so on until the last copper atom receives an electron from the negative end of the battery.
The result of this chain reaction is that the electrons move through the conductor from the negative end to the positive end of the battery. The flow of electrons will continue as long as the positive charges and negative charges from the battery are maintained at each end of the conductor.
There are two types of forces at work in every atom. Under normal circumstances, these two forces are in balance. The protons and electrons exert forces on one another. These are over and above gravitational or centrifugal forces. Besides mass, electrons and protons carry an electric charge. These additional forces are attributed to the electric charge that they carry. However, there is a difference in the forces. Between masses, the gravitational force is always one of attraction while the electrical forces both attract and repel. Protons and electrons attract one another, while protons exert forces of repulsion on other protons, and electrons exert repulsion on other electrons.
There appears to be two kinds of electrical charge. The protons are said to be positive (+). The electrons are said to be negative (-). The neutron carries a neutral charge. Polarity is based on directional flow of electricty. The type of change determines direction. This leads to the basic law of electrostatic which states, UNLIKE charges attract each other, while LIKE charges repel each other.
Charges and Electrostatics
The attraction or repulsion of electrically-charged bodies is due to an invisible force called an electrostatic field, which surrounds the charged body. Illustration 7 shows the force between charged particles as imaginary electrostatic lines from the negative charge to the positive charge.
When two like charges are placed near each other, the lines of force repel each other as shown below.
Because of the force of the electrostatic field, an electric charge has the ability to move another charge by attraction or by repulsion.
The ability to attract or repell is called potential. When one charge is different from the other, there must be a difference in potential between them.
The sum difference of potential of all charges in the electrostatic field is referred to as electromotive force (EMF). The basic unit of potential difference is the volt (V). The volt is named in honor of Alessandro Volta an Italian scientist. Volta invented the voltaic pile, the first battery cell. The symbol for potential is V, that indicates the ability to force electrons to move. Because the volt unit is used, the potential difference is called voltage.
The following conditions will produce voltage: friction, solar, chemical and electromagnetic induction. A photocell, such as on a calculator, would be an example of producing voltage from solar energy.
A need existed to develop a unit of measurement for electrical charge. A scientist named Charles Coulomb investigated the law of forces between charged bodies and adopted a unit of measurement that is called the Coulomb. When the Coulomb is written in scientific notation this measurement is expressed as One.
Coulomb = 6.28 ×1018 electrons or protons. Stated in simpler terms, in a copper conductor, one ampere is an electric current of 6.28 billion electrons passing a certain point in the conductor in one second.
Another theory that needs to be explained is the theory of motion in a conductor. The motion of charges in a conductor is an electric current. An electron will be affected by an electrostatic field in the same manner as any negatively charged body. An electron is repelled by a negative charge and attracted by a positive charge. The drift of electrons or movement constitutes an electric current.
The magnitude or intensity of current is measured in amperes. The unit symbol is A. An ampere is a measure of the rate when a charge is moved through a conductor. One ampere is a coulomb of charge that moves past a point in one second.
Conventional versus Electron Flow
There are two ways to describe an electric current flowing through a conductor. Prior to the use of atomic theory to explain the composition of matter, scientists defined current as the motion of positive charges in a conductor from a point of positive polarity to a point of negative polarity. This conclusion is still widely held in some engineering standards and textbooks. Some examples of positive charges in motion are applications of current in liquids, gases, and semiconductors. This theory of current flow has been termed conventional current.
With the discovery of using atomic theory to explain the composition of matter, it was determined that current flow through a conductor was based on the flow of electrons (-), or negative charge. Therefore, electron current is in the opposite direction of conventional current and is termed electron current.
Either theory can be used, but the more popular conventional theory describing current as flowing from a positive (+) charge to a negative (-) charge will be used in this course.
George Simon Ohm discovered that for a fixed voltage, the amount of current flowing through a material depends on the type of material and the physical dimensions of the material. In other words, all materials present some opposition to the flow of electrons. That opposition is termed resistance. If the opposition is small, the material is labeled a conductor. If the opposition is large, it is labeled an insulator.
The Ohm is the unit of electrical resistance. The symbol to represent an Ohm is the Greek letter omega, Ohms. A material is said to have a resistance of one ohm, if a potential of one volt results in a current of one ampere.
It is important to remember that electrical resistance is present in every electrical circuit, including components, interconnecting wires, and connections. Electrical circuits and the laws that relate to the electrical circuits will be discussed later in this unit.
As resistance works to oppose current flow, it changes electrical energy into other forms of energy, such as, heat, light, or motion. The resistance of a conductor is determined by four factors:
- Atomic structure is the amount of free electrons. The more free electrons a material has, the less resistance that is offered to current flow.
- Length. The longer the conductor, the higher the resistance. If the length of the wire is doubled, as shown in Illustration 12(a), the greater the resistance between the two ends.
- Width (cross sectional area). The larger the cross sectional area of a conductor, the lower the resistance (a bigger diameter pipe allows for more water to flow). If the cross section area is reduced by half, as shown in Illustration 12(b), the resistance for any given length is increased by a factor of 4.
- Temperature. For most materials, the higher the temperature, the higher the resistance. Illustration 12(c) shows the resistance increasing as the temperature rises. Please note, there are a few materials whose resistance decreases as temperature increases.
Electrical Circuits and Laws
An electrical circuit is a path or a group of interconnecting paths, that are capable of carrying electrical currents. The electrical circuit is a closed path that contains a voltage source or sources. There are two basic types of electrical circuits: series and parallel. The basic series and parallel circuits may be combined to form more complex circuits, but these combinational circuits may be simplified and analyzed as the two basic types. It is important to understand the laws that are needed to analyze electrical circuits and to diagnose electrical circuits. They are Kirchoff's Laws and Ohm's Law.
Gustav Kirchoff developed two laws for analyzing circuits. The two laws are stated below:
- Kirchoff's Current Law (KCL) states that the algebraic sum of the currents at any junction in an electrical circuit is equal to zero. Simply stated, all the current that enters a junction is equal to all the current that leaves the junction.
- Kirchoff's Voltage Law (KVL) states that the algebraic sum of the electromotive forces and voltage drops around any closed electrical loop is zero. Simply said, the addition of all differences of potential in a closed circuit will equal zero.
George Simon Ohm discovered one of the most important laws of electricity. The law describes the relationship between three electrical parameters: voltage, current and resistance. Ohms' law is stated as follows: The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance. The relationship can be summarized by a single mathematical equation:
Current = Electromotive Force/Resistance
or, stated in electrical units: I= Volts/Ohms
Single letters are used to represent mathematical equations that express electrical relationships. Resistance is represented by the letter R or the Omega symbol (Ohms). The voltage or the difference in potential is represented by the letter E or the letter V (electromotive force). Current is represented by the letter I (intensity of charge). Using these laws to calculate circuits will be discussed later in this course.
In electrical applications, electrons travel along a path that is called a conductor or a wire. The electrons move by traveling from atom to atom. Some materials make it easier for electrons to travel.These are called good conductors.
The following materials are examples of good conductors :
A material is said to be a good conductor if the material has many free electrons. The amount of electrical pressure or voltage it takes to move electrons through a material depends on how free the electrons are.
Although silver is the best conductor, silver is also expensive. Gold is a good conductor, but gold is not as good as copper. The advantage gold has is gold will not corrode like copper. Aluminum is not as good as copper, but is less expensive and lighter.
The conductivity of a material determines the quality of the material. Illustration 13 shows some of the common conductors and the relative conductivity to copper.
Other materials make it difficult for electrons to travel. These materials are called insulators. A good insulator keeps the electrons tightly bound in orbit. The following examples of insulators are: rubber, wood, plastics and ceramics. It is possible to make an electric current flow through every material. If the applied voltage is high enough, even the best insulators will break down and will allow current flow. The following chart, shown in Illustration 14, lists some of the more common insulators.
Dirt and moisture may serve to conduct electricity around an insulator. A dirty insulator or moisture could cause a problem. The insulator is not breaking down, but the dirt or moisture can provide a path for electrons to flow. It is important to keep the insulators and contacts clean.
A wire in an electrical circuit is made up of a conductor and an insulator. The conductor is typically made up of copper and the insulator (outside covering) is made of plastic or rubber. Conductors can be a solid wire or a stranded wire. In most earthmoving applications, the wire is stranded copper with a plastic insulation. This insulation covers the conductor.
There are many sizes of wire. The smaller the wire is, the larger the identification number. The numbering system is known as the American Wire Gage (AWG). Illustration 15 describes the AWG wire size standard.
Resistance can also be affected by other conditions, such as, corrosion. These conditions need to be considered when resistance measurements are made.