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What is an Alternating Current?

By Adam Hill
Updated May 17, 2024
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Alternating current (AC) electricity is the type of electricity most commonly used in homes and businesses all over the world. It is said to “alternate” because it reverses direction in an electrical circuit at regular intervals, usually many times per second. Alternating current is created by an electric generator, which determines the frequency of these oscillations. In the United States, alternating current is generated at 60 hertz, meaning that the current alternates 60 times per second.

There are a number of reasons why most electrical power plants produce AC rather than DC or direct current, where electrons flow constantly in one direction. First, large generators produce AC naturally, so conversion to DC would require an extra step and therefore an added cost. Secondly, and perhaps most importantly, electrical transformers must have alternating current to operate. Transformers are a crucial part of a power grid, because they perform the task of stepping up electrical voltage for long-range transmission, as well as bringing voltage down to a safe level for use in homes and businesses.

Transformers are simple, inexpensive devices, most commonly found in power substations and mounted on power poles near residences. In one type of power substation, transformers take the moderate AC voltage generated by the power plant and greatly increase the voltage for long-distance transmission. The high voltage allows the electricity to be transported much more efficiently. Other substations reduce the voltage at the end of a transmission line, and the electricity is then connected to a local grid. At this point, the voltage is decreased further, just before going into homes and other buildings for use by consumers.

Alternating current also has the advantage of being easy to convert to DC. This is important partially because many small household appliances will only run on DC. Many printers, laptop computers, and battery chargers, for example, use an AC adapter to change the household alternating current into DC. The adapters are interchangeable to some degree and are usually included with the appliance by the manufacturer. Converting DC to AC, on the other hand, is an expensive process, making AC the better choice for a default form of electricity.

Some types of circuits use alternating current primarily to convey information, rather than transmit electricity. Information circuits such as those used in telephone and radio transmission employ varying amounts of voltage, current, and frequency in order to convey accurate information. These types of AC circuits are not very efficient, but this attribute is actually well suited to information circuits, since their purpose is to transmit data, rather than electrical power.

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Discussion Comments
By anon64368 — On Feb 06, 2010

why cannot we use wireless transmission rather than using wire?

By anon42255 — On Aug 20, 2009

how can ac pass if it is alternating or it may like that after completion it will reverse the direction?

By Pramob444 — On Jun 07, 2009

My home has romex wire which contains a hot, neutral, and ground wire. I was taught that AC

constantly reverses direction. But yet when I flick a switch to turn on a light, I was also taught that current travels to the light via the hot wire and exits the house through the neutral wire; therefore the current flows in one direction. What type of alternating current is this?

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