Vern J. OstdiekDonald J. Bord

Chapter 7Electricity(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.