Electronics Information for Armed Services Vocational Aptitude Battery (ASVAB) Study Guide (page 2)
We use electricity every day of our lives. Light bulbs produce light from electricity. Many ovens use electricity to produce heat. Our televisions and computers use electricity. Our cars use electricity to ignite the fuel in the engine. Batteries produce electricity to power games and calculators. This article will add to your knowledge of electronics and review what you already know so that you can get a top score on the Electronics Information subtest of the ASVAB.
The electricity used in the United States is predominately produced from three resources: fossil fuels, such as oil, natural gas, and coal; nuclear materials, primarily uranium; and hydropower from water.
Almost 80 percent of the electrical power used in the United States is produced from the burning of fossil fuels. Fossil fuels are burned to produce heat. The heat is used to produce steam that turns a turbine. The turbine transforms rotational mechanical energy into electric energy which in turn is fed into a power grid.
Nuclear power plants produce about 7 percent of the electricity we consume. A nuclear power plant uses a nuclear reaction (fission) to produce heat that generates steam. The process described above is then used to convert steam into electricity.
About 15 percent of the electricity we use is from hydropower. The kinetic energy of falling water is used to turn turbines that produce the electrical power.
Kinetic Energy: the energy produced by a body in motion.
Basic Electrical Theory
Understanding electricity and electronics is not dependent on understanding the complex structure of the atom—understanding the basics is sufficient. All materials on Earth are made up of atoms. An atom is made up in part of electrons and protons. These two subatomic particles each have an electric charge, or electric polarity. The charge of an electron has a negative polarity while the charge of a proton has a positive polarity. Electricity is essentially the management of positive and negative electric charges.
Most everyone has experienced the buildup of electric charge when shuffling across a carpet. Your body develops a static charge. It is static because the charge is not moving. When you touch a light switch, the static charge moves, creating a current. You have produced and used electricity.
The symbol for electric charge is q or Q. Charge is measured in coulombs, C. A coulomb of electrons has a negative charge and a coulomb of protons has a positive charge. A coulomb is defined as 6.25 × 1018 electrons or protons:
the charge of 6.25 × 1018 electrons = Q = 1C
Example: What is the charge, in coulombs, of one electron?
You remember that
6.25 × 1018 electrons = 1 Coulomb
To get the charge of one electron, divide both sides by 6.25 × 1018:
1 electron = .16 × 10–18 C
An electric charge has the potential to do work by forcing another charge to move. Opposite charges attract each other and like charges repel each other, just like magnets. Thus, a positive and a negative charge would attract each other, while two negative charges would repel each other. The potential of an electric charge to do work is the voltage or the potential difference. A battery produces a voltage. This voltage can be thought of as the force that moves electrons from one terminal to the other. This force is called the electromotive force (emf). The accepted symbol for voltage is V. The schematic symbol for a DC voltage is:
Voltage: the potential of an electric charge to do work.
All batteries have two terminals, a positive and a negative one. On a flashlight battery, for example, one end (usually marked with a + sign) is the positive terminal, and the other end (usually marked with a – sign) is the negative terminal.
When a battery is connected to a load with wires, the potential difference, or voltage, between the two terminals (the two opposite charges) forces a third charge to move. The charge in motion is called an electric current. Current is produced when a potential difference moves an electric charge. Picture a battery connected with wires to a light bulb:
The battery produces a voltage, which forces the free electrons in the wire to move. The mobile free electrons moving in the wire are the current. The current is always a continuous flow of electrons, and at every point in the circuit, the current is the same.
Load: the resistance in an electric circuit.
Electric current is measured in amperes. An ampere is defined to be 6.25 × 1018 electrons moving past any given point in one second. This is the same as one coulomb of charge moving past any given point in one second. The symbol for current is I or i. Mathematically, current is expressed as:
- I = where I is current in amperes (A), Q is charge in coulombs (C), and T is time in seconds (s).
Example: What is the current if 10 coulombs of charge flow through a light bulb every 5 seconds?
The charge, or number of electrons, can be determined using the equation above:
- I =
- multiply both sides of the equation by time
Therefore, if we solve for charge, or Q, this same equation tells us that charge is equal to current multiplied by time. In other words, charge equals the amount of current during a given time period.
Example: How many electrons are flowing through the light bulb when the current is 2 amperes?
Alternating Current (AC) and Direct Current (DC)
A battery is an example of a direct voltage source. The terminals of the battery always maintain the same polarity, so the current flow from one terminal to the other is always in the same direction. On the other hand, an alternating voltage source periodically reverses its polarity. The current resulting from an alternating voltage also periodically changes its direction of flow.
The electricity generated in a power plant is by nature an alternating voltage. The magnetic fields developed in a rotating turbine always produce an alternating voltage. The voltage we most often use in our homes is 110 volt 60 Hz. The 60 Hz, or Hertz, refers to the frequency that an alternating voltage changes polarity. In this case the polarity changes from positive to negative and back to positive 60 times a second.
One advantage of producing an alternating voltage is that it is more easily changed to a different value than a direct voltage can be changed. This is very important because power plants produce thousands of volts, while we can safely use just 110 or 220 volts in our homes. Most of our appliances then convert the 110 or 220 volts to even a smaller voltage. Simple transformers are used to step up or down alternating voltages. A direct voltage must first be converted to an alternating voltage before its value can be changed. This adds complexity and cost to using direct voltages.
Another benefit of using alternating voltages and currents is that they can be easily and inexpensively converted into direct voltage and current. A diode is a semiconductor device that allows current to flow in only one direction.When the direction of current flow changes, the diode acts like an insulator and stops the current. Two or four diodes can be used to transform alternating voltages and currents into direct voltages and currents. This process is referred to as rectifying an alternating voltage.
Basic electrical theory is most easily understood by studying direct voltages and currents. The study of alternating voltages and currents can become very complex. The rest of this chapter will discuss only direct voltages and currents.
Conductors, Insulators, and Semiconductors
A copper wire is an example of a conductor. A conductor is a material that has electrons that can easily move.Metals are very good conductors. Copper is used to make most of the wires we use because it has high conductance and is relatively inexpensive. Aluminum was used in the 1950s to make wires for our homes because it was less expensive than copper; however, it is not as good a conductor.
An insulator is a material whose electrons do not move freely. Glass, rubber, wood, and porcelain are all examples of insulators. Insulators are used to prevent the flow of current.
A semiconductor is a material that conducts less than a metal conductor but more than an insulator. Silicon is the most recognized semiconductor. Most transistors, diodes, and integrated circuits are produced from semiconductor materials such as silicon or germanium.
Resistance is the opposition to current. A copper wire has very little resistance; therefore it is a good conductor. Insulators have a large resistance. The symbol for resistance is R. Resistance is measured in ohms. The symbol for ohms is the Greek letter omega, Ω. The schematic symbol for resistance is:
A good copper wire has a resistance of about onehundredth of an ohm, or 0.01 Ω per foot. For comparison, the resistive heating element used in a medium-size hair dryer has a resistance of about 14 Ω.
Resistors are fabricated using many different materials. The most common types of resistors are wire-wound resistors, carbon-composition resistors, and film resistors.Wire-wound resistors are generally used in high-power applications. Carbon resistors are the most common. They are used in most electronic circuits due to their low cost. Carbon resistors can't typically be built with an exact resistance value. Film resistors are used when a more exact resistance is needed. Resistors are easily built with resistance values from 0.01 Ω to many millions of ohms.
Analog Electrical Circuits
All electrical circuits have the three following components:
- A potential difference or voltage.
- A closed path for current to flow from one side of the potential difference to the other.
- Resistance, which is often referred to as a "load."
Ohm's law defines the relationships between voltage, current, and resistance in a simple electrical circuit.
The illustration below shows a flashlight, where the voltage source is a battery and the load is a lightbulb:
Ohm's law states that:
- potential difference (or voltage) = current × resistance
- V = I × R
This can be rewritten as:
Example: The battery in the flashlight below supplies 4.5 volts and the light bulb has a resistance of 1.5 Ω. How much current flows in the circuit?
Ohm's law states that current (remember that current is measured in amperes) equals voltage over resistance:
According to Ohm's law, 3 amperes of current flows through this circuit.
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