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alternating current (AC), direct current (DC)
AC current is a specific type of electric current in which the direction of the current's flow
is reversed, or alternated, on a regular basis. Direct current is no different electrically from
alternating current except for the fact that it flows in the same direction at all times.
Alternating current was chosen early in the 20th century as the North American standard
because it presented fewer risks and promised higher reliability than competing DC systems
of the day. Many of DC's deficiencies were later corrected, but not until a substantial North
American infrastructure had already been developed. DC is the European standard.
Electric power distribution requires a circuit, usually represented as two wires leading to a
device that uses electricity. In AC current, one wire is negative and the other is either is
positive or neutral (ground). The two wires take turns at sending electricity. In North
America, AC current uses a standard "rhythm" in which each side gets its turn 60 times
each second, thus the 60Hz designation given to standard AC current. This switching of
polarity takes the form of a rhythmic pulse in the electrical current that occurs within the
normal audible range. This is why you can actually hear this rhythm in circuits such as
fluorescent lighting ballasts and audio equipment as a low buzzing tone. This buzz is
referred to as "sixty cycle hum". Prior to the 1970s, two AC power schemes were used in
North America. One offered energy at 45-50Hz, the other at 60Hz. "Fifty-cycle power",
occasionally referred to as "rural power", is now obsolete and the 60Hz standard is now
used throughout North America.
In DC circuits, the electricity is always the same polarity, which means that in a two-wire
circuit, one "wire", or side of the circuit, is always negative, and the negative side is always
the one that sends the electricity. There is no hum because there is no cyclic change in
current flow. DC current is more effective for long-distance, high-voltage transmission
because it results in less energy lost in transmission, but the cost of converting DC current
to AC is relatively high, so DC is typically cost-effective only for long-distance
Electrical devices that convert electricity directly into other forms of energy can operate
just as effectively from AC current as from DC. Lightbulbs and heating elements don't care
whether their energy is supplied by AC or DC current. However, nearly all modern
electronic devices require direct current for their operation. Alternating current is still used
to deliver electricity to the device, and a transformer is included with these devices to
convert AC power to DC power (usually at much lower than the supplied voltage) so that
electronic devices can use it.
Down through the centuries in which electricity remained a natural mystery, and later a fashionable novelty, it
turned up only in the form we would term today direct current (DC), that is, with electrons moving in one
direction only. The first, cumbersome batteries (called voltaic piles) and mechanical curiosities that built up static
charge (like Leyden jars) provide electrons that stream in one direction. Even the famous experiments of
Benjamin Franklin utilized a direct current supply—lightning.
There's certainly nothing inferior about a direct current, unless you are trying to solve practical engineering
problems concerned with generating power and distributing it over great distances. A few visionaries, Tesla
foremost among them, comprehended both that the new science of electricity must be, literally, transformed and
that the means already existed in theory—as well as in some wheezy devices usually found in physics labs of
that era. The solution lay in alternating currents (AC).
What Is Alternating Current?
An AC source produces currents that flow in one direction and then the other, continuously cycling through peak
values in either direction, i.e., first positive, then negative, and so on. The advantages—which turn out to be
nothing short of revolutionary—are not immediately obvious; they derive chiefly from that magnetic property of
currents, induction.
Direct currents don't cause much inductive action. When a switch is thrown and current first flows in a DC circuit,
a magnetic field builds up. The field can induce a current to flow in any nearby wire, but only briefly, just during
the few instants it takes for the current to get moving. In fact, Michael Faraday was led to his discoveries in
induction by first noticing the momentary currents induced by a DC source he had turned on. Once the field is
built up, induction stops; the field's force lines are stationary and no longer carrying a change of energy through
space and cutting across nearby wires.
With an alternating current the magnetic state of affairs is never a settled one. Each time current direction
reverses, so must the pole orientation of its associated magnetic field. The entire field collapses and rebuilds in
the magnetically opposite direction. If current alternates continuously, the field is never static. Alternating
currents do, in a sense, copy their changes of energy into nearby circuits, making energy available there.
Though all very clever, it may seem this isn't a prize winning trick; why not just connect the two circuits with a
piece of wire? Why complicate matters with induction?
Transforming AC
It's not just a question of getting power to a nearby circuit; induction can be made to change the form in which
power is delivered, it can be transformed, in the electrical sense. Manipulating the way fields are concentrated—
usually by making coils of the conductor—will change the properties of currents and voltages that a source (the
primary) induces in another, nearby set of coils (the secondary). For example, power present in the primary as a
large current at a low voltage may be transformed into low current at high voltage in the secondary.
AC Advantages
Generally, engineers would much prefer to send power over long lines at a very high voltage, with comparatively
lower current, but deliver it to most users at a safer, lower voltage. Transformers make that possible. Resistance
in AC circuits works differently, too, so that with good design, losses in power lines are dramatically lower than in
DC lines. (The first DC power stations could only serve an area within a few mile radius.)
The same basic AC ideas, a magnetic transfer and transformation of power, can also make highly efficient,
reliable motors. One obvious advantage, though there are many, is the spinning part, the rotor, need not be
connected physically to any electrical contacts; ever changing fields in the stator (stationary part) convey the
power. Nor are AC devices limited to a single AC source; several may be supplied simultaneously in a polyphase