Direct current is electrical current that flows steadily in one direction between two ends of a circuit. In an electrical circuit, one side has a negative charge (missing protons) and the other has a positive charge (missing electrons). The electrons in the circuit’s wire will consistently flow toward the positively charged side, moving to replenish the missing electrons. Direct current (DC) is steady, and its voltage is not easily changed.
Direct current was popularized in the United States by inventor and electrician Thomas Edison.
Direct current is one of two types of electrical current; the other is alternating. Unlike direct current, alternating current‘s flow changes direction as the sides of an electromagnetic circuit are continuously flipped. As the negative and positive poles on a rotating electromagnet change location, the flow of charge follows. Alternating current (AC) is steady even though it changes direction; it’s typically pictured as a sinusoidal wave. The flow of current reverses regularly.
Alternating current is typically chosen to transmit electrical signals across long distances (such as the cables on telephone poles that you see while driving). It’s ideal for reducing energy loss in cables; very high voltages in wires tend to have lowered current, so wires are less likely to overheat. In the late 1800s and early 1900s, Thomas Edison, George Westinghouse, and Nikola Tesla debated and experimented with direct current (Edison) and alternating current (Westinghouse) to determine which were more useful for nationwide electricity. In the end, years later, both became common for different purposes.
Direct current is also sometimes used for long-distance electrical transmission, but it’s not as widespread as AC usage. AC is popular for long-distance electrical signals because it can easily be transformed into different voltages. Long-distance transmission requires very high voltage, and the homes and businesses that receive that electrical power need much lower voltage. AC wires and systems use transformers, which contain circuits that step down or step up (increase or decrease) AC voltage before it enters a building or leaves one. DC is typically much harder to transform than AC.
Because alternating current is what runs between buildings and initially powers electricity for home devices, computers and other such devices must use a power supply unit to convert AC to DC. The PSU contains a rectifier, which funnels current into only one direction and is responsible for the AC-to-DC conversion. Computers absolutely must use DC. They need the electrons to always travel in one direction to keep the hardware supplied with constant electrical power. Because AC changes constantly, fluctuating in levels of power, it wouldn’t be able to keep a computer running. The power supply in a desktop computer takes the AC that comes through the wall socket and converts it to DC that the computer can use.