electric power

Characteristics of electric power by region by country

Africa

Asia and Oceania

Central America

Europe

Middle East

North America

South America

Electric power has three main characteristics:

Voltage

Voltage is often compared to pressure in a water pipe.  The higher the voltage, the thinner the wire needed to carry a given amount of current.  The electricity passing through long distance transmission lines may be run at thousands of volts, which makes it possible to use thinner wires.  The choice of a suitable voltage for homes is a compromise between safety and economy.  Lower voltages are safer but require thicker wires.

Alternating current or direct current?

Electricity from a battery is direct current: the electrons move in one direction. The first power plants also produced direct current.  It is also possible to reverse the direction of the current in a constantly repeating pattern. Electric current in which the direction constantly changes direction is called alternating current.  The wire that is positive becomes negative and the negative wire becomes positive. Electricity sold in the United States goes through a compete cycle 60 times each second; it is 60-hertz (60-Hz) alternating current. For 1/120th of a second the current flows one way, and for the next 1/120th of a second it flows the other way.

Alternating current has almost completely replaced direct current, mainly because the voltage of an alternating current can be easily raised or lowered using a transformer. A transformer depends on having wires in a constantly changing magnetic field, and the changing magnetic field is produced by changing the voltage.

With alternating current there are two instants in every cycle when the voltage is zero; those are the instants when the current is changing direction. From these instants, the voltage slowly increases to a peak and then slowly decreases. The ideal pattern is a sine wave. (For simplicity's sake, we are leaving amperage out of this discussion. The amperes may lead or lag the voltage, depending on the load.)

We have spoken of electrons moving down the wire, but electrical energy is transmitted by the push the electrons give their neighbors, not by their "flow". Imagine a line of people waiting for a bus. The person at the end of the line gives the person in front of him a shove. That person gives the person in front of him a shove, and so on, until the shove reaches the man at the other end of the line. He shoves back, and the shove passes back the line to the original shover. Energy has been transmitted, but everyone is still in the same place. That is alternating current. Even in direct current, the energy travels through wires much faster than the electrons themselves do, just as a wave travels over the surface of the ocean more rapidly than the water itself moves.

Phase

 The voltage falling to zero is perceptible in the flickering of old-fashioned fluorescent lights, and even in the performance of electric motors. To get a more nearly constant source of power without giving up the advantage of alternating current, engineers turned to three-phase power.  Three-phase power requires a minimum of 3 wires; single-phase power can be delivered with two. Having three wires makes three possible pairs (AB, AC, BC). When the voltage across one pair of wires is zero, the voltage across the other two pairs is not.  So a three-phase motor will run more smoothly than a single-phase motor.

Grounding

In some countries the earth itself is used as one of the conductors in the electrical system. The way in which the ground is used differs greatly from country to country.

World Standards

Almost all the electricity now sold in the United States is 60-Hz alternating current. In homes, the voltage is approximately 120 volts, but it may be brought into the house at a higher voltage, and high voltages are used for some power-hungry appliances, such as kitchen ranges, air conditioners and water heaters. (The higher voltage makes it possible to supply the same amount of power with smaller wires.) Three-phase power is rarely found in residences.

The appearance of the receptacle usually indicates what voltage and amperage is available, since the National Electrical Manufacturers Assn. (NEMA) and the NEC specify what type of current each receptacle design is to be used for.

The letters for the plugs and sockets, are arbitrary.

# According to the Denon News (Mar/April 1992), the area east of the Fuji River receives 60-Hz current provided by Tokyo Power and Light, which bought its first equipment from the United States. The area west of the river receives 50-Hz power from Osaka Power and Light, which imported its original generating equipment from Europe.

home

|

materials index

|

search

|

your comments

|

about

|

help

|

privacy terms of use

Copyright © 2000 Sizes, Inc.  All rights reserved.
Last revised: 8 November 2003.