Alternating current is of vital importance in electronics for one simple reason: The electric current you can access by plugging a circuit into a wall outlet happens to be alternating current.

Electric current that flows continuously in a single direction is called a direct current, or DC. In a direct current circuit, current is caused by electrons that all line up and move in one direction.

Within a wire carrying direct current, electrons hop from atom to atom while moving in a single direction. Thus, a given electron that starts its trek at one end of the wire will eventually end up at the other end of the wire.

In alternating current, the electrons don't move in only one direction. Instead, they hop from atom to atom in one direction for a while, and then turn around and hop from atom to atom in the opposite direction. Every so often, the electrons change direction. In alternating current, the electrons don’t move steadily forward. Instead, they just move back and forth.

When the electrons in alternating current switch direction, the direction of current and the voltage of the circuit reverses itself. In public power distribution systems in the United States, (including household current), the voltage reverses itself 60 times per second. In some countries, the voltage reverses itself 50 times per second.

The rate at which alternating current reverses direction is called its frequency, expressed in hertz. Thus, standard household current in the United States is 60 Hz.

In an alternating current circuit, the voltage, and therefore the current, is always changing. However, the voltage doesn't instantly reverse polarity. Instead, the voltage steadily increases from zero until it reaches a maximum voltage, which is called the peak voltage.

Then, the voltage begins to decrease again back to zero. The voltage then reverses polarity and drops below zero, again heading for the peak voltage but negative polarity. When it reaches the peak negative voltage, it begins climbing back again until it gets to zero. Then the cycle repeats.

The swinging change of voltage is important because of the basic relationship between magnetic fields and electric currents. When a conductor (such as a wire) moves through a magnetic field, the magnetic field induces a current in the wire. But if the conductor is stationary relative to the magnetic field, no current is induced.

Physical movement is not necessary to create this effect. If the conductor stays in a fixed position but then intensity of the magnetic field increases or decreases (that is, if the magnetic field expands or contracts), a current is induced in the conductor the same as if the magnetic field were fixed and the conductor was physically moving across the field.

Because the voltage in an alternating current is always either increasing or decreasing as the polarity swings from positive to negative and back again, the magnetic field that surrounds the current is always either collapsing or expanding. So, if you place a conductor within this expanding and collapsing magnetic field, alternating current will be induced in the conductor.

It seems like magic! With alternating current, it is possible for current in one wire to induce current in an adjacent wire, even though there is no physical contact between the wires.

The bottom line is this: Alternating current can be used to create a changing magnetic field, and changing magnetic fields can be used to create alternating current. This relationship between alternating current and magnetic fields makes three important devices possible:

  • Alternator: A device that generates alternating current from a source of rotating motion, such as a turbine powered by flowing water or steam or a windmill. Alternators work by using the rotating motion to spin a magnet that’s placed within a coil of wire. As the magnet rotates, its magnetic field moves, which induces an alternating current in the coiled wire.

  • Motor: The opposite of an alternator. It converts alternating current to rotating motion. In its simplest form, a motor is simply an alternator that’s connected backward. A magnet is mounted on a shaft that can rotate; the magnet is placed within the turns of a coil of wire.

    When alternating current is applied to the coil, the rising and falling magnetic field created by the current causes the magnet to spin, which turns the shaft.

  • Transformer: Consists of two coils of wire placed within close proximity. If an alternating current is placed on one of the coils, the collapsing and expanding magnetic field will induce an alternating current in the other coil.