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, over and above the forces of gravitational or centrifugal. It has been determined that besides mass, electrons and protons carry an electric charge, and 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 exertrepulsion on other electrons.
Thus, it appears to be two kinds of electrical charge. Protons are said to be positive (+) and the electrons are said to be negative (-). The neutron as the name implies, is neutral in charge. The directional quality of the electricity based on the type of charge is called “polarity.” This leads to the basic law of electrostatics 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.Fig.1.1.7 shows the force between charged particles as imaginary electrostatic lines from the positive charge to the negative charge. The conventional method of representing the lines of force is for the arrowheads to point away from the positive charge and point toward the negative 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 its electrostatic field, an electric charge has the ability to move another charge by attraction or repulsion. The ability to attract or repell is called its “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) named in honor of Alessandro Volta, an Italian scientist and the inventor of the “voltaic pile,” the first battery cell. The symbol for potential is V indicating the ability to do the work of forcing electrons to move. Because the volt unit is used, potential difference is called “voltage”. There are many ways to produce voltage, including friction, solar, chemical, and electromagnetic induction. The attraction of bits of paper to a comb that has been rubbed with a wool cloth is an example of voltage produced by friction. 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 called the “Coulomb.” Written in scientific notation is expressed as One Coulomb =6.28 x 10 18 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.
In electrostatic theories as earlier discussed, the concern was mainly the forces between the charges. Another theory that needs explained is that of “motion” in a conductor. The motion of charges in a conductor is defined as an electric current. An electron will be affected by an electrostatic field in the same manner as any negatively charged body. It 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 at which a charge is moved through a conductor. One ampere is a coulomb of charge moving past a point in one second.
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 text books. Some examples of positive charges in motion are applications of current in liquids, gases and semi conductors. 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 and the symbol to represent an Ohm is the Greek letter omega. 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 relating to them 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 (how many free electrons). The more free electrons a material has, the less resistance it offers to current flow.
- Length. The longer the conductor, the higher the resistance. If the length of the wire is doubled as shown in Fig. 1.1.12 (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 Fig. 1.1.12 (b), the resistance for any given length is doubled.
- Temperature. For most materials, the higher the temperature, the higher the resistance. The chart shown in Fig. 1.1.12(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 group of interconnecting paths, capable of carrying electrical currents. It 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 needed to analyze and diagnose electrical circuits. They are Kirchoff’s Laws and Ohm’s Law.
Gustav Kirchoff developed two laws for analyzing circuits. They are stated as:
- 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. None is lost.
- 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 stated, if we started at a particular point in a closed circuit and went around that circuit adding the individual differences in potential until all were considered and the starting point was reached, there would be no extra voltage, and none would be left unaccounted for.
George Simon Ohm discovered one of the most important laws of electricity. It describes the relationship between three electrical parameters: voltage, current and resistance. Ohm’s 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:
When using mathematical equations to express electrical Relation ships, single letters are used to represent them. Resistance is represented by the letter R or the Omega symbol (.) The voltage or difference in potential is represented by the letter E or 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 called a conductor or wire. They move by traveling from atom to atom. Some materials make it easier for electrons to travel and they are called “good conductors. “Examples of good conductors are: silver, copper, gold, chromium, aluminum and tungsten. A material is said to be a good conductor if it has many free electrons. The amount of electrical pressure or voltage, it takes to move electrons through a material depend on how free its electrons are. Although silver is the best conductor it is also expensive. Gold is also a good conductor, but not as good as copper. The advantage gold has is it will not corrode like copper. Aluminum is not as good as copper, but it is less expensive and lighter.
The conductivity of a material determines how good a conductor that material is.Fig.1.1.13 shows some of the common conductors and their relative conductivity to copper.
Conductor Conductivity ( to copper )
Other materials make it difficult for electrons to travel and they are called “insulators.” A good insulator keeps the electrons tightly bound in orbit. Examples of insulators are: rubber, wood, plastics, and ceramics. It is also important to know that 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 allow current flow. The following chart Fig.1.1.14 list some of the more common insulators.
Wax or Paraffin
There is one other item that should be considered when discussing insulators. Dirt and moisture may serve to conduct electricity around an insulator. If an insulator is dirty or there is moisture present, it could cause a problem. The insulator itself is not breaking down, but the dirt or moisture can provide a path for electrons to flow. It is therefore 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 stranded. In most earthmoving applications the wire is stranded copper with a plastic insulation covering the conductor.
There are many sizes of wire. The smaller the wire the larger the identification number. The numbering system is known as the American Wire Gage (AWG).The chart below,Fig.1.1.15 describes the AWG wire size standard.
AWG Diameter ( mils ) Ohm per 1000 ft
10 102.9 0.9989
12 80.8 1.588
14 64.1 2.525
16 50.8 4.016
18 40.3 6.385
20 32.0 10.15
22 25.4 16.14
24 10.0 103.2
26 3.10 1049.0
Use the difference between a 14 AWG wire and an 18 AWG size wire to demonstrate the resistance values .The 14 AWG size wire has a resistance of approximately 2.5 ohms per 1000 feet, while an 18 AWG size wire has a resistance of approximately 6.4 ohms per1000 feet. The 18 AWG wire is36%smaller in diameter, but has approximately three times more resistance.
Resistance can also be affected by other conditions, such as, corrosion, etc., which need to be considered when making resistance measurements.