Electric current exists in only two forms, alternating (AC) and direct (DC). Experimenters discovered direct current electricity in the 1790s by tinkering with early chemical batteries. Since it flows in only one direction, DC is a simple form of electricity. Alternating current, a more complex kind of electricity, was discovered in the late 1800s. It flows back and forth in a rapid wavelike motion.
Electrons, tiny parts of every atom, carry electric charges. When these charges move in a wire, scientists call it a current. When many charges move at the same time, it’s a large current. Charges move most efficiently in a conductor, such as copper wire. Other materials, such as plastic, conduct charges poorly.
In a direct current, the charges move in the same direction. DC sources, such as batteries, have a positive (+) terminal and a negative (-) terminal. If no connection exists between the terminals, current can’t flow. When you connect a circuit to both terminals, current flows from the positive terminal, through the circuit, and into the battery’s negative terminal.
Charges move in an alternating current, but the current reverses direction in a repeating cycle. Alternating current in the United States has been standardized to change direction 60 times per second; other countries use 50 times per second or some other rate. Mechanical generators and electronic circuits produce AC. Because the current reverses direction, an AC outlet’s terminals are not marked positive or negative. Instead, electricians call one wire “hot” and the other “neutral.”
A transformer can easily increase or decrease the voltage of AC electricity. High voltages make it easier to send electricity long distances, such as from a power plant to a neighborhood. At the neighborhood level, transformers decrease the high voltage, bringing it down to a safe level you can use in your home or office. Transformers do not work with DC.
Anything that runs on batteries, such as flashlights, cell phones and toys, uses DC current. Both standard and hybrid cars have DC electrical systems. Computers, televisions and other electronic devices also use DC, even though they plug into the household AC socket. A power supply in the device converts household AC to low-voltage DC. Household lighting and large appliances use AC. Efficient, durable electric motors used in everything from desk fans to industrial machinery have been designed to run on alternating current.