Electric current is defined as the flow of charged particles, such as electrons or ions, through space or an electrical conductor, such as the copper used in wires. The way the particles move through the conductor is what determines is the current is alternating or direct.
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In alternating current (AC), the voltage and direction of the electric current periodically change direction. Many sources of electricity, such as electromechanical generators, produce AC current with voltages that alternate in polarity, reversing between positive and negative over time.
For example, it may start from zero, grows to a maximum, then decrease back to zero, reverse, reach a maximum in the opposite direction, then return again to the original value, and repeat this cycle indefinitely.
If the voltage in an AC circuit is plotted over time, you are likely to see several different waveforms such as a sine wave, square wave (when it stays in the peak for a few seconds before going back to zero), triangle wave (when it has a constant slew rate), sawtooth wave (asymmetric triangle waves), etc. However, sine is the most common waveform and the AC in most mains-wired buildings have an oscillating voltage in the sine waveform.
The interval of time between the same value on two successive cycles is called the period, which is measured in seconds. The number of cycles (or periods) per second is called the frequency, and it’s measured in Hertz. The maximum value in either direction is the amplitude of the alternating current and is measured in amps or volts.
The discovery of alternating current can’t be attributed to a single person. There are many people who contributed to the discovery and development of AC electricity.
English scientist Michael Faraday (1791-1867) proposed the principle of electromagnetic induction that would later be used by French instrument maker Hippolyte Pixii (1808-1835) to develop the first practical dynamo alternating current generator in 1832. Pixii's alternator used a magnet rotated by a hand crank to produce AC.
More people worked in the development of AC generators after this. In 1878, the Ganz Company began working with single-phase AC power systems in Budapest, Austria-Hungary. The following year, Walter Bailey, in London, developed a prototype for a weak AC motor by making a copper disc rotate using alternating current.
This was followed in 1882 by the work of Sabastian Ferranti and Lord Kelvin (William Thompson) in London to develop early AC power technology, including an early transformer. Two years later, Lucien Gaulard developed transformers and constructed a power transmission system from Lanzo to Turino, in Italy. The demonstration of AC power included a 25-mile (40 km) trolley with step-down transformers that allowed the path to be lit with low-power Edison incandescent lights and arc lamps.
Other AC technology included the development of motors and power transmission systems by people such as Galileo Ferraris, William Stanley, and Nikola Tesla, who ended up being the protagonists of the War of the Currents in the late 1880s. In this period of emerging electrical facilities, there was a commercial competition of AC vs DC . Or rather, between George Westinghouse and Nikola Tesla (who supported the use of alternating current) and Thomas Alva Edison (who supported the use of direct current).
In direct current (DC) the electrons move only in one direction with no variations over time. The voltage is constant, and so is the polarity. As the direct current flows, the electrons flow from the point of low potential to the point of high potential, from the negative terminal to the positive terminal, and the resulting current is in the opposite direction (from positive to negative).
The waveform of direct current can be a pure sine wave that is either positive or negative. It is also represented with a straight line in coordinate axes when it’s constant, but when it has variable tension, it’s called pulsating direct current and it’s represented with square or rectangular waves. Pulsating direct current (PDC) is often obtained by adding rectifiers to the alternating current.
While the alternating current won the War of the Currents and is still the most-used type of current in power distribution to the present day, DC is still used in many other applications nowadays. But to understand them, we have to take a look back at the history of the direct current.
The history of direct current is related to the history of alternating current. Both types of current were investigated almost simultaneously within the study of electricity itself. Technically, the elucidation of direct current came first, with the invention of the voltaic pile, an electrical battery created by Italian physicist Alessandro Volta in 1880. This use of direct current came before the development of the first electric generators which produced alternating current.
While the discovery of direct current can be attributed to Volta, it was French physicist André-Marie Ampère who described DC in its most basic forms, while attempting to figure out how electrical flows worked.
In the late 1870s and early 1880s, power plants were developed to generate both alternating current and direct current, to be used to illuminate the streets with arc lamps. American inventor Thomas Alva Edison , the founder of the electric utility Edison Illuminating Company, firmly defended the use of direct current for lighting purposes, but it was much easier to control the voltage of alternating current using transformers. Higher voltages allowed the electricity to be transmitted for longer distances, which is why alternating current was, and still is, preferred for use by power distribution companies worldwide.
For general electrical usage, direct current was not effectively used until the mid-1950s when the high-voltage direct current (HVDC) electric power transmission system was developed for bulk transmission of electrical power.
Due to its cost-effectiveness, most high voltage power transmissions use alternating currents. This means that all the lights and home appliances that we have in our residences are powered by alternating current.
Each country in the world has established a residential voltage and frequency of AC, ranging from 110V to 240V and 50Hz to 60Hz.
Alternating current can also be found in AC motors, a kind of electric motor that relies on alternating current to turn the movement of electrons into mechanical energy. The most common uses of AC motors are in hydraulic pumps, fans, air conditioners, and conveyor systems.
Direct current can also be used to power electric motors. DC motors are often used in elevators, vacuums, sewing machines, tools, electric vehicles, flashlights, and small appliances. If the device needs continuous power, direct current is the best option.
As stated above, direct current started with the first electric battery back in 1880 and it’s still used for that purpose. Today, direct current's importance is largely in the power it provides to modern electronic devices that use batteries, such as cellphones and laptops, whose chargers are designed to convert the AC from the socket into DC to charge the batteries. DC is also used in LEDs and solar cells.
We use alternating current mainly due to the limits of direct current that were found in the 1880s. At that time, technology did not exist to easily and efficiently manipulate the voltage of direct current, whereas transformers could be used to step up and down the voltages of AC.
Transformers were the most convenient option because they allowed high voltages to travel through the electrical grids. High voltages are essential to overcome the resistance that comes with electricity transmission. This way, lesser amounts of energy were lost midway, which made the system more efficient and cost-effective. This meant that electricity was at a lower, less dangerous voltage when it reached homes and other buildings.
Direct current couldn’t compete with that. The lack of practical methods to convert it to high voltages made it necessary to reduce the transmission range of the energy. This forced the companies that bet on DC to build local power plants, just around one mile (1.6 km) away from the end-user at most, which meant higher costs and that rural areas were left out.
Knowing this, it’s no surprise that we ended up using AC in our houses and offices. But it’s important to note that technology has advanced a lot since those times, and we’ve found new advantages and applications for the direct current since then.
The high voltage direct current-based power transmission systems (HVDC) became feasible with the development and economization of conversion technologies throughout the 20th century.
Mercury-arc valves, thyristors, and other rectifiers made it possible to convert high voltage AC to DC and then back to AC in the step-down process. This is a more effective method to carry electricity to longer distances without significant losses, which makes HVDC systems more efficient and eco-friendly. They also allow the interconnection of electric grids, something that amplifies and stabilizes the networks, even if they operate on different voltages and frequencies. Furthermore, the HVDC systems can increase the capacity of the power grids.
These are the main reasons why the AC vs DC battle has been somewhat revived in the second half of the 20th century, with the HVDC systems being used for international connections, such as NorNed undersea cable (which goes from Norway to The Netherlands) and Morley interconnector (North Ireland-Scotland); or simply to connect more than 400 miles long (640 km) distances, such as the Pacific DC Intertie, an 846 mile long (1360 km) HVDC line that goes from the Pacific Northwest to Southern California in the US.
While many experts agree on the fact that the HVDC system is more convenient than the HVAC system, it is more expensive in terms of the equipment, so it’s limited to these applications only.
To sum it up, we can say that the main differences between alternating current and direct current are:
In conclusion, it’s impossible to determine a specific answer for which kind of current is better, as it will always depend on what you need that current for.
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