Alternating current (AC) is a type of electric current that periodically reverses its direction of flow. Its polarity changes over time, and its magnitude fluctuates sinusoidally, typically following a waveform like a sine wave. This variation results in the current alternating between positive and negative values as it cycles through different phases, characterized by its frequency. This behavior is opposite to a direct current (DC), where the waveform remains steady over time.
Alternating current (AC) was developed and popularized by Serbian-American inventor and engineer Nikola Tesla in the late 19th century.
An alternator is a type of generator that creates alternating current (AC). It works by having magnets, called the rotor, spin around near a group of wires that are wrapped in coils on a metal core called the stator. As the magnets rotate, they cause an electric current to flow in the wires, which produces AC voltage. This process repeats as the rotor continues to spin.
An alternating current can be represented graphically, showing its periodic nature and how its direction and magnitude continuously change. This waveform illustrates how AC alternates between positive and negative values, corresponding to the changing polarity of the current. The curve smoothly oscillates, peaking at maximum positive and negative values, with zero crossings where the current momentarily equals zero.
Each complete oscillation from one peak to the next represents one cycle, with the time taken for one cycle being the period of the waveform. The frequency, measured in hertz (Hz), defines how many cycles occur per second, and the amplitude of the wave corresponds to the maximum current or voltage. The wavelength of an AC signal is the distance over which the signal''s shape repeats.
The symbol for alternating current (AC) is typically represented as a sine wave, reflecting the sinusoidal nature of AC''s voltage and current changes over time.
Generating and transmitting AC over long distances is efficient due to the use of high voltages, which are easily converted with transformers. Large generators produce power and then pass through transformers to increase the voltage for long-distance transmission. Once the electricity reaches a substation, the voltage is lowered and distributed through smaller power lines. Local transformers further reduce the voltage for home use. After passing through a household meter, the current enters a service panel with breakers to prevent overloads and is then distributed to outlets and switches throughout the home.
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Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. The abbreviations AC and DC are often used to mean simply alternating and direct, respectively, as when they modify current or voltage.[1][2]
This means that when transmitting a fixed power on a given wire, if the current is halved (i.e. the voltage is doubled), the power loss due to the wire''s resistance will be reduced to one quarter.
The power transmitted is equal to the product of the current and the voltage (assuming no phase difference); that is,
Consequently, power transmitted at a higher voltage requires less loss-producing current than for the same power at a lower voltage. Power is often transmitted at hundreds of kilovolts on pylons, and transformed down to tens of kilovolts to be transmitted on lower level lines, and finally transformed down to 100 V – 240 V for domestic use.
High-voltage direct-current (HVDC) electric power transmission systems have become more viable as technology has provided efficient means of changing the voltage of DC power. Transmission with high voltage direct current was not feasible in the early days of electric power transmission, as there was then no economically viable way to step the voltage of DC down for end user applications such as lighting incandescent bulbs.
The frequency of the electrical system varies by country and sometimes within a country; most electric power is generated at either 50 or 60 Hertz. Some countries have a mixture of 50 Hz and 60 Hz supplies, notably electricity power transmission in Japan.
A low frequency eases the design of electric motors, particularly for hoisting, crushing and rolling applications, and commutator-type traction motors for applications such as railways. However, low frequency also causes noticeable flicker in arc lamps and incandescent light bulbs. The use of lower frequencies also provided the advantage of lower transmission losses, which are proportional to frequency.
The original Niagara Falls generators were built to produce 25 Hz power, as a compromise between low frequency for traction and heavy induction motors, while still allowing incandescent lighting to operate (although with noticeable flicker). Most of the 25 Hz residential and commercial customers for Niagara Falls power were converted to 60 Hz by the late 1950s, although some[which?] 25 Hz industrial customers still existed as of the start of the 21st century. 16.7 Hz power (formerly 16 2/3 Hz) is still used in some European rail systems, such as in Austria, Germany, Norway, Sweden and Switzerland.
Off-shore, military, textile industry, marine, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds. Computer mainframe systems were often powered by 400 Hz or 415 Hz for benefits of ripple reduction while using smaller internal AC to DC conversion units.[citation needed]
For low to medium frequencies, conductors can be divided into stranded wires, each insulated from the others, with the relative positions of individual strands specially arranged within the conductor bundle. Wire constructed using this technique is called Litz wire. This measure helps to partially mitigate skin effect by forcing more equal current throughout the total cross section of the stranded conductors. Litz wire is used for making high-Q inductors, reducing losses in flexible conductors carrying very high currents at lower frequencies, and in the windings of devices carrying higher radio frequency current (up to hundreds of kilohertz), such as switch-mode power supplies and radio frequency transformers.
As written above, an alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated is lost. Depending on the frequency, different techniques are used to minimize the loss due to radiation.
At frequencies up to about 1 GHz, pairs of wires are twisted together in a cable, forming a twisted pair. This reduces losses from electromagnetic radiation and inductive coupling. A twisted pair must be used with a balanced signaling system so that the two wires carry equal but opposite currents. Each wire in a twisted pair radiates a signal, but it is effectively canceled by radiation from the other wire, resulting in almost no radiation loss.
At frequencies greater than 200 GHz, waveguide dimensions become impractically small, and the ohmic losses in the waveguide walls become large. Instead, fiber optics, which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.[citation needed]
Alternating currents are accompanied (or caused) by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation:
Below an AC waveform (with no DC component) is assumed.
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