Both alternating current (AC) and direct current (DC) represent forms of electrical power. Before the late 1800s, only DC existed, produced by batteries and mechanical generators. The invention of alternating current set off a fierce competition to see which was better for powering cities. While the proponents of alternating current won that battle, we still have both AC and DC today, providing power where circumstances determine that one or the other has an advantage.
Engineers define current as the flow of electric charges in a conductor, like a copper wire. A direct current results when all the charges flow steadily in the same direction. An alternating current is produced when charges flow in one direction, then reverse to flow the opposite way, switching back and forth 60 times per second.
Though some of the earliest electric power companies produced direct current, DC revealed a serious flaw. Because of the slight resistance in electric wires, electric power could be sent only a few miles, requiring the utilities to build many power plants to support a city. Alternating current’s advantage is that its voltage can be increased or decreased easily with transformers. A power company can sent high voltage electricity over many miles of wire with low losses, reducing the voltage with transformers once the wires reach neighborhoods. With AC, a city can be powered economically with one or two plants. While different countries use different AC frequencies and voltages, this system is the one the world uses today.
Batteries produce DC electricity from chemical reactions. When you connect a battery to a circuit, the current flows from the battery’s positive terminal to its negative terminal. Being a portable source of electric current means you can take electrical and electronic equipment with you; however, it must be designed to run on DC. You can run some AC appliances indirectly from batteries if you have an inverter. The inverter takes the battery’s DC current and electronically generates a 60-cycle AC current from it.
Alternating current has proven to be ideal for some types of electric motors, including those used for appliances and for heavy industry. A well-designed AC motor is simpler, less expensive and more durable than a DC motor of the same power level. All kinds of electronic equipment, from home computers and stereos to large-scale communications systems, run on DC. Complex electronic circuits need a tightly controlled current source on which to operate. In most cases, the equipment has a power supply that converts AC from the wall outlet to DC. For example, your cell phone charger converts AC to DC.
At voltages much higher than 50 volts, both AC and DC pose electric shock hazards. Of the two kinds of current, AC poses a greater danger. According to the University of Illinois, it induces more severe muscle contractions than DC. The rapidly alternating current stimulates sweating that lowers skin resistance, forcing yet more current through the body. AC’s danger comes from the speed of these effects, causing injury sooner than from DC.