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Alternating current Back-to-basics: What is alternating current?

| Author / Editor: Luke James / Erika Granath

Alternating current (AC) is an electric current that periodically reverses its direction, in contrast to direct current (DC) which only flows in a single direction which cannot change sporadically.

Alternating current (AC) is an electric current which periodically reverses direction, in contrast to direct current (DC) which flows only in one direction.
Alternating current (AC) is an electric current which periodically reverses direction, in contrast to direct current (DC) which flows only in one direction.
(Source: Public Domain / Unsplash )

Most students of electrical engineering and related subjects begin their studies by learning about direct current (DC). This is because most of the digital electronics these students will build will use DC. However, it is important to understand alternating currents (AC) and its concepts too, because it has many useful properties and use cases.

How alternating currents is produced

Although DC, the unidirectional flow of an electric charge, is perhaps one of the simplest of electrical engineering concepts, it is not the only “type” of electricity in use. Both AC and DC describe types of current that flow in a circuit.

Alternating current (green curve). The horizontal axis measures time; the vertical, current or voltage.
Alternating current (green curve). The horizontal axis measures time; the vertical, current or voltage.
(Source: Public Domain )

Many sources of electricity, most notably electromechanical generators, produce AC current with voltages that alternate in polarity, reversing between positive and negative over time. An alternator can also be used to purposely generate AC current.

In an alternator, a loop of wire is spun rapidly inside of a magnetic field. This produces an electric current along the wire. As the wire spins and periodically enters a different magnetic polarity, the voltage and current alternate on the wire. This current can change direction periodically, and the voltage in an AC circuit also periodically reverses because the current changes direction.

AC comes in several forms, as long as the voltage and current are alternating. If an AC circuit is hooked up to an oscilloscope and its voltage is plotted over time, you are likely to see several different waveforms such as sine, square, and triangle – sine is the most common waveform and the AC in most mains-wired buildings have an oscillating voltage in the sine wave form.

Alternating currents applications

AC is most commonly found in mains-wired buildings such as homes and offices. This is because generating and transporting an AC current across long distances is relatively easy. At high voltages of over 110kV, less energy is lost in power transmission. At higher voltages, lower currents are produced, and lower currents generate less heat in the power line due to a lower level of resistance. This therefore means less energy lost as heat. AC currents can be converted to and from high voltages with ease by using transformers.

Alternating currents can be converted to and from high voltages with ease by using transformers.
Alternating currents can be converted to and from high voltages with ease by using transformers.
(Source: Science ABC )

AC is also great for use in electric motors because motors and generators are the same device. The only difference between a generator and a motor is that a motor converts electrical energy into mechanical energy. These motors are used in all kinds of appliances like refrigerators, washing machines, and dishwashers.

Although generators and motors are great, AC’s most useful application is perhaps transformers.

An effect of electromagnetism (known as “mutual induction”), where two or more coils of wire are placed so that the changing magnetic field in one coil induces a voltage in the other, can be used to make a device called a transformer. If there are two mutually inductive coils and one is energized with AC, an AC voltage will be created in the other coil.

This is where AC becomes very useful

The fundamental use of a transformer is stepping voltage up or down from the powered coil to the unpowered coil. This provides AC an advantaged well above DC in the realm of power distribution because, as mentioned above, transmitting electrical power over long distances is a lot more efficient with higher, stepped-up voltages and smaller, stepped-down currents. Before reaching power outlets, voltage is stepped back down and current is stepped back up.

This type of transformer technology has made long-range electric power distribution efficient and practical. Without transformers, it would be far too costly to construct power systems in their current long-distance form. And, because mutual inductance relies on changing magnetic fields, transformers only work with AC.

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