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Explanation of Alternating Current (AC) Electricity - Succeed in Physical Science. Also refer to physics, DC, direct current, magnetism, generator, transformer, current, voltage, hertz, positive, negative, capacitor, inductor, electromagnet, Ron Kurtus, School for Champions. Copyright © Restrictions

Alternating Current (AC) Electricity

by Ron Kurtus (revised 11 September 2005)

Alternating current or AC electricity is the type of electricity commonly used in homes and businesses throughout the world. While the flow of electrons through a wire in direct current (DC) electricity is continuous in one direction, the current in AC electricity alternates in direction. The back-and-forth motion occurs between 50 and 60 times per second, depending on the electrical system of the country. AC is created by an AC electric generator, which determines the frequency. What is special about AC electricity is that the voltage in can be readily changed, thus making it more suitable for long-distance transmission than DC electricity. But also, AC can employ capacitors and inductors in electronic circuitry, allowing for a wide range of applications.

Questions you may have include:

• What is the difference between AC and DC electricity?
• Why do we use AC instead of DC?
• How do we create AC electricity?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

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Back-and-forth movement

Electrons have negative (−) electrical charges. Since opposite charges attract, they will move toward an area consisting of positive (+) charges. This movement is made easier in an electrical conductor, such as a metal wire.

With DC electricity, connecting a wire from the negative (−) terminal of a battery to the positive (+) will cause the negative charged electrons to rush through the wire to the positive charged side. The same thing happens with a DC generator, where the motion of coiled wire through a magnetic field pushes electrons out of one terminal and attracts electrons to the other terminal.

With an AC generator, a slightly different configuration alternates the push and pull of each generator terminal. Thus the electricity in the wire moves in one direction for a while and then reverses its direction when the generator armature is in a different position.

Note: Although DC electricity can be created by a DC generator or a chemical battery, AC electricity is only created with an AC generator. (See Generating Electrical Current for more on that subject.)

Motion of electrons

This illustration gives an idea of how the electrons move through a wire in AC electricity. Of course, both ends of the wire extend to the AC generator or source of power.

AC movement of electrons in wire

The charge at the ends of the wire alternates between negative (−) and positive (+). If the charge is negative (−), that pushes the negatively charged electrons away from that terminal. If the charge is positive (+), the electrons are attracted in that direction.

Rate of change

AC electricity alternates back-and-forth in direction 50 or 60 times per second, according to the electrical system in the country. The frequency is designated as either 50 Hertz (50Hz) or 60 Hertz (60Hz).

(See Worldwide AC Voltages and Frequencies for more information.)

Light bulbs

Many electrical devices--like light bulbs--only require that the electrons move. They don't care if the electrons flow through the wire or simply move back-and-forth. Thus a light bulb can be used with either AC or DC electricity.

Transformer

The major advantage that AC electricity has over DC is that AC voltages can be transformed to higher or lower voltages. This means that the high voltages used to send electricity over great distances from the power station could be reduced to a safer voltage for use in the house.

Changing voltages is done by the use of a transformer. This device uses properties of AC electromagnets to change the voltages.

(See AC Transformers for more information.)

Tuning circuits

AC electricity also allows for the use of a capacitor and inductor within an electrical or electronic circuit. These devices can affect the way the alternating current passes through a circuit. They are only effective with AC electricity.

A combination of a capacitor, inductor and resistor is used as a tuner in radios and televisions. Without those devices, tuning to different stations would be very difficult.

Summary

We commonly use AC electricity to power our television, lights and computers. In AC electricity, the current alternates in direction. AC electricity was proven to be better for supplying electricity than DC, primarily because the voltages can be transformed. AC also allows for other devices to be used, opening a wide range of applications.

Answers to Readers' Questions

Electrify society by applying your knowledge of science

Resources

Following are some resources on this subject:

Web sites

Elements of AC Electricity - Basic electronics tutorial site

Alternating Current - Simple explanation

DC and AC Electricity Resources

Physical Science Resources

Books

Top-rated books on Basic AC Electricity

Top-rated books on AC Electricity

Basic Electricity: Complete Course by Nooger Van Valkenburgh; Delmar Learning; (1995) $35.95 - From a simplified explanation of electronics to advanced topics for mastering the fundamentals of electricity

Teach Yourself Electricity and Electronics by Stan Gibilisco; McGraw-Hill; (2001) $34.95 - Guide for professionals, hobbyists and technicians desiring to learn AC and DC circuits

Miscellaneous

Mini-quiz to check your understanding

1. Why is it called alternating current?

2. What does an electrical transformer do?

3. For what is a combination of a capacitor, inductor, and resistor used?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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