Vern J. Ostdiek Donald J. Bord Chapter 7 Electricity (Section 6)

7.6 AC and DC The current flows out of the positive (+) terminal of the power supply, moves through the circuit, and flows into the negative (–) terminal of the power supply. If the total resistance in the circuit doesn’t change, the size of the current remains constant (as long as the battery doesn’t run down). A graph of the current I versus time t is simply a horizontal line.

7.6 AC and DC In an AC power supply, the polarity of the two output terminals switches back and forth—the voltage alternates. This causes the current in any circuit connected to the power supply to alternate as well. It flows counterclockwise, then clockwise, then back to counterclockwise, and so on.

7.6 AC and DC All the while, the size of the current is increasing, then decreasing, and so forth. A graph of the current in an AC circuit shows this variation in the size and direction of the current. When I goes below zero, it means that the direction has reversed.

7.6 AC and DC One can even think of AC as a kind of “wave” causing the charges in a conductor to oscillate back and forth. Almost all public electric utilities in the United States supply 60-hertz AC. The voltage between the two slots in a wall outlet oscillates back and forth 60 times per second. In Europe, the standard frequency of AC is 50 hertz.

7.6 AC and DC Some electronic devices (such as lightbulbs) can operate on AC or DC, whereas others require one or the other. Electric motors and generators must be designed to operate on or produce either AC or DC. There are devices that can convert an AC voltage to a DC voltage and vice versa. Batteries can produce direct current only. For this reason, automobiles have DC electrical systems. The alternator in an automobile generates AC, which is then converted into DC to be compatible with the battery.

7.6 AC and DC Alternating current has one distinct advantage over DC: simple, highly efficient devices called transformers can “step up” or “step down” AC voltages. This makes it possible to generate AC at a power plant at some intermediate voltage, step it up to a very high voltage (typically more than 300,000 volts) for economical transmission, and then step it down again to lower voltages for use in homes and industries.

7.6 AC and DC There is no counterpart of the transformer for DC. Another important use of AC is in electronic sound equipment. One example: if a 440-hertz tone is recorded on tape and then played back, the “signal” going to the speaker will be an alternating current with a frequency of 440 hertz.

Summary Electrons, protons, and certain other subatomic particles possess a physical property called electric charge that is the basic source of electrical and magnetic phenomena. Forces act between any objects that possess a net electric charge, positive or negative. Objects with like charges experience repulsive forces; objects with unlike charges experience attractive forces.

Summary The electrostatic force, expressed by Coulomb’s law, is responsible for binding electrons to the nucleus in atoms, for the amber effect (such as static cling), and for a number of other natural or technological phenomena. Electric charges produce electric fields in the space around them. This field is the agent for the electrostatic force, just as the gravitational field is the agent of the gravitational attraction between objects.

Summary Most useful applications of electricity involve electric currents. They most often involve the flow of electrons through metal wires driven by an electrical power supply, such as a battery. The flow of charge is analyzed using the physical concepts voltage, current, and resistance. Ohm’s law states that the current in a circuit equals the voltage divided by the resistance.

Summary The power consumption in a circuit depends on the voltage and the current. The electrical energy needed to cause a current to flow through a resistive element is converted into internal energy. This ohmic heating is usefully exploited directly by space heaters and toasters, as well as by incandescent lightbulbs to produce light.

Summary Fuses and circuit breakers are used to automatically disconnect a circuit if the current is large enough to cause excessive ohmic heating. At extremely low temperatures, many materials become superconductors—they have zero resistance. Consequently, no energy is lost to heating when electric currents flow through them. Superconductors now in use are still primarily limited to special-purpose scientific and medical instruments.

Summary There are two types of electric current: alternating (AC) and direct (DC). Because batteries produce DC, battery-powered devices generally employ DC. Transformers can be built to “step up” or “step down” an AC voltage from one value to another. This makes AC particularly convenient for electrical supply networks such as electric utilities.