General studies of alternating current and direct current
Alternating Current
Alternating Current (AC) Alternating current describes the flow of charge that changes direction periodically. As a result, the voltage level also reverses along with the current. AC is used to deliver power to houses, office buildings, etc.
AC can be produced using a device called an alternator. This device is a special type of electrical generator designed to produce alternating current.
A loop of wire is spun inside of a magnetic field, which induces a current along the wire. The rotation of the wire can come from any number of means: a wind turbine, a steam turbine, flowing water, and so on. Because the wire spins and enters a different magnetic polarity periodically, the voltage and current alternates on the wire.
Waveforms
AC can come in a number of forms, as long as the voltage and current are alternating. If we hook up an oscilloscope to a circuit with AC and plot its voltage over time, we might see a number of different waveforms. The most common type of AC is the sine wave. The AC in most homes and offices have an oscillating voltage that produces a sine wave.
Applications
Home and office outlets are almost always AC. This is because generating and transporting AC across long distances is relatively easy. At high voltages (over 110kV), less energy is lost in electrical power transmission. Higher voltages mean lower currents, and lower currents mean less heat generated in the power line due to resistance. AC can be converted to and from high voltages easily using transformers. 
AC is also capable of powering electric motors. Motors and generators are the exact same device, but motors convert electrical energy into mechanical energy (if the shaft on a motor is spun, a voltage is generated at the terminals!). This is useful for many large appliances like dishwashers, refrigerators, and so on, which run on AC.
DC (direct current)
DC (direct current) is the unidirectional flow or movement of electric charge carriers (which are usually electrons). The intensity of the current can vary with time, but the general direction of movement stays the same at all times. As an adjective, the term DC is used in reference to voltage whose polarity never reverses.
In a DC circuit, electrons emerge from the negative, or minus, pole and move towards the positive, or plus, pole. Nevertheless, physicists define DC as traveling from plus to minus.
Direct current is produced by electrochemical and photovoltaic cells and batteries. In contrast, the electricity available from utility mains in most countries is AC (alternating current). Utility AC can be converted to DC by means of a power supply consisting of a transformer, a rectifier (which prevents the flow of current from reversing), and a filter (which eliminates current pulsations in the output of the rectifier).
Virtually all electronic and computer hardware needs DC to function. Most solid-state equipment requires between 1.5 and 13.5 volts. Current demands can range from practically zero for an electronic wristwatch to more than 100 amperes for a radio communications power amplifier. Equipment using vacuum tubes, such as a high-power radio or television broadcast transmitter or a CRT (cathode-ray tube) display, require from about 150 volts to several thousand volts DC.
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