What Is AC & DC Electricity?

Today's scientists understand electricity to be one of the most fundamental phenomena in nature. Electrical impulses constantly course throughout our bodies, and even the very matter of our world is held together by electric charges. Despite this, electricity still had to be discovered, and there is some controversy regarding who was the first to do this.

The discoverer may have been the English physician William Gilbert, who was the first to use the word "electricus" in the year 1600. It may have also been English scientist Thomas Browne, who coined the word "electricity" a few years later.

Americans like to believe it was the inventor Benjamin Franklin, who proved that lightning was electricity in 1752. There is even evidence to show that the ancient Greeks and Persians knew about electricity. Whoever gets the prize, it's a sure bet they discovered DC electricity (direct current). AC electricity (alternating current) didn't come along until the 19th century.

Scientists visualize electricity as the flow of negatively charged particles called electrons. They are the same particles that orbit the nuclei of all the atoms that constitute matter.

The two fundamental laws of electricity are that opposites attract and like repels like. Consequently, electrons will flow toward a positive terminal and away from a negative one. The flow occurs only in one direction, and the strength of the flow, or current, depends on the difference in charge between the two terminals. This difference is the voltage between the terminals.

In the absence of external input, the electrons will accumulate on the positive terminal and reduce the potential difference between the two terminals, and eventually the flow will stop.

Perhaps the best-known example of DC current flow is a lightning strike. Proving that lightning is an electrical phenomenon was Benjamin Franklin's real accomplishment. Franklin flew a kite in a thunderstorm and attached a key to the kite string. When the key became electrically charged and gave him a mild shock, he was elated. He had proven that electrical charge builds up in the clouds, and that lightning is a discharge of this electrical energy in a momentary flash of DC current.

A battery is another common source of DC electricity. It consists of a pair of oppositely charged terminals, and when you connect the terminals with a conductor, electricity flows from the negative terminal (the cathode) to the positive one (the anode).

The charge difference in a battery is typically provided by a chemical process in its core, and this process can continue only for a limited time. If you keep drawing power from a battery, it eventually stops producing charge and goes dead.

The English physicist Michael Faraday discovered electromagnetic induction in 1831 when he found that he could generate an electric current in a coil of conducting wire by moving a magnet back and forth inside the coil.

Crucially, Faraday noted that the current changed direction whenever he changed the direction of the magnet. French instrument maker Hippolyte Pixii used this discovery to build the first alternating current generator in 1832.

AC electricity is always produced by an induction generator of the type built by Pixii, although modern generators are far more sophisticated than Pixii's machine. The generator may employ rotating magnets, or it may have a rotating coil, but there is always some type of rotation involved, and the period of the rotation defines how often the current changes direction.

Because it changes direction, AC electricity has an associated frequency, which is the number of times per second it reverses.

You don't have to look far to find examples of AC electricity. The lights in the room in which you're sitting, as well as the air conditioner, electric heater and all the appliances, run on AC power, which is generated at your local power station.

Most power stations use steam generated by fossil fuels, nuclear fission or geothermal processes to spin a turbine. The turbine generates electricity by electromagnetic induction, and the speed of rotation is carefully governed to produce electricity with a fixed frequency. In North America, the frequency is 60 Hz (cycles per second), but in most of the rest of the world, it's 50 Hz.

Windmills are renewable energy sources that also generate AC electricity, but they rely on the wind to spin their turbines instead of fossil fuels or nuclear fuel. Some wave generators also have turbines that produce AC power. When the waves compress a hydraulic system or a pocket of enclosed air, the stored energy is used to spin a turbine.

In the electrified world of the 21st century, it's hard to imagine a time when there was no electricity, but that time wasn't very long ago. At the end of the 19th century, the light bulb had been invented, but there was no way to generate power and get it into homes so people could use the new invention.

Thomas Edison, who helped develop and market light bulbs, was in favor of a network of DC generating stations, while Nikola Tesla, a Serbian inventor and former employee of Edison's, favored AC generators. Tesla won, and here are some of the reasons:

• At the voltages necessary for wide-scale electricity use, AC electricity can be transmitted further along power lines with less voltage drop. If Edison had prevailed, and DC electricity had become the standard, there would have had to be power stations within a mile of each other. Tesla, on the other hand, was able to power the entire city of Buffalo, New York, with a single induction generator positioned under Niagara Falls. 

• AC power generation is cheaper. A hydroelectric generator such as the one at Niagara Falls can create electricity from a natural process. No other input is needed. 

• The voltage of AC power can be altered with a transformer. At the time of Tesla and Edison, this wasn't possible with DC current. Today, however, transformers are available that employ internal circuitry or inverters to alter the voltage of DC current. 

Although the electricity that comes through the power lines is AC, electronics equipment often requires DC electricity. In a circuit diagram, the direct current symbol is a straight line with three dots or lines underneath it, while that for alternating current is a single wavy line. To convert AC current to DC, electronics specialists usually use a circuit component called a diode, or rectifier. It passes current in one direction only, thus creating a pulsing DC signal from an AC current source.

The tool for converting DC to AC current is called an inverter. It uses transistors, which are circuit components that can switch on and off very rapidly, to direct current along a series of circuit paths that effectively changes its direction across a pair of central terminals, which is the part of the circuit to which you attach the AC load. Inverters are used in electric vehicles. They are also used in photovoltaic systems to convert DC electricity generated by solar panels to AC current for use in the home.