Electric current is the movement or flow of electric charges. A charge can be either positive or negative. The protons that make up part of the nucleus of every atom have a positive electric charge. The electrons that surround the nucleus have a negative charge. An electric current can consist of positive, negative, or both types of charges.
The American statesman and scientist Benjamin Franklin originated the idea that electric current flows from positive to negative. But other scientists later proved that electric current actually flows in the opposite direction—from negative to positive.
Franklin’s idea also fails to describe the way electric current flows through metals. Each atom of a metal wire has at least one electron that is not held so closely by the nucleus as the others are. Such loosely held electrons can move freely through the metal. But the nucleus cannot move through the wire. Thus, current flowing through a metal wire consists of free electrons.
Conductors and insulators.
Electric current flows most easily through substances called conductors. The number of free electrons in a substance determines how well it conducts current. Such metals as aluminum, copper, silver, and gold are good conductors because they have at least one free electron per atom. Some metals, such as lead and tin, are poorer conductors than other metals because they have less than one free electron per atom. Poor conductors resist the flow of electric current more than good conductors do. Resistance changes electric energy into heat. Engineers use units called ohms to measure resistance (see Ohm).
Substances with no free electrons, such as glass, mica, and rubber, do not normally conduct electric current. They are called insulators. Some substances, including germanium and silicon, are neither good conductors nor insulators. They are called semiconductors (see Semiconductor).
To produce an electric current, some type of nonelectric energy must be converted into an electromotive force (emf). For example, a battery creates an emf by changing chemical energy into electrical potential energy. Thus, a battery has a potential difference (difference in potential energy) between its ends that causes electrons to flow in a conductor. Emf is measured in units called volts. An emf of one volt, when connected to a conductor with a resistance of one ohm, causes 6,241,500,000,000,000,000 electrons to flow past a point in the conductor in one second. This amount of electric current is called one ampere. See Volt; Ampere.
Direct and alternating current.
An electric current is either direct or alternating. Direct current (DC) always flows in the same direction. It is produced by batteries and DC generators. Alternating current (AC) regularly reverses its direction of flow. It is produced by AC generators. Nearly all homes and other buildings use AC.
Each time AC completes two changes of direction, it goes through one cycle. The number of cycles per second is called the frequency of the AC. Frequency is measured in units called hertz. Almost all local power companies in the United States and Canada supply AC with a frequency of 60 hertz.
Direct current
operates automobile electrical systems, locomotives, and some types of motors used in industry. Radios, television sets, and other electronic devices use AC, but they also need DC to operate their internal circuits. Devices called rectifiers easily change AC into DC. DC is also necessary in order to charge storage batteries.
Alternating current
has several advantages over DC. Its major advantage is that power companies can transmit it easily and efficiently. Electric current loses the least amount of energy when traveling at high voltages. But these high voltages are not safe to use in homes. Devices called transformers can easily increase or decrease AC voltage.
A conductor can carry more than one alternating current at a time. A current consisting of two or more individual alternating currents is known as a polyphase current. One common kind of polyphase current is three-phase current, which consists of three individual alternating currents.
