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DC Circuits Lecture
Direct Current .
Other sources of direct current are solar panels, thermal electric generators, and space age fuel cells. Of course you probably heard of DC generators. Older cars used them and cities used them over a hundred years ago. When a coil is spun in a magnetic field an alternating sinusoidal voltage is generated. This alternating current can be rectified to produce pulsating DC. This pulsating DC can be filtered to produce a steady DC. Rectification and filtration are topics for later lectures. The commutator on a DC generator serves as a rectifier.
Direct Current versus Alternating Current.
In 1882 Edison built the first DC power plant In downtown Manhattan, NY. The Pearl street station power plant provided 110 volts direct current for lighting. Low voltage meant that lots of current was required to deliver power. Power = I (in amperes) times V (in Volts). Each sixty watt bulb required just under a half ampere of current. This meant it took a lot of copper cable to carry the power required to illuminate a city district. The distance between power plant and user was limited to one mile.
Nikola Tesla developed and patented key features of an AC distribution system. Together with George Westinghouse they founded the Westinghouse Electric Company. The Westinghouse company built a large AC distribution system . Electricity at high voltage can be transmitted for long distance with low power loss. High voltage electricity over several thousand volts can be generated by both DC and AC generators, but only high voltage AC electricity could be stepped down by transformers to a safe 110 volts. The Edison light bulbs were rated at 110 volts. Transformers do not work on direct current. The AC system made it possible for a few power plants built on the waterfront, to service the entire Island of Manhattan. By 1895 hydroelectric AC power from Niagara Falls was transmitted using high tension transmission lines from Niagara to New York City. This provided huge quantities of hydroelectric energy for the city. Hydroelectric and large coal burning plants electrified most of the United States by 1939.
Today a direct current transmission line is being used to transmit power over a thousand kilometers from Oregon to California. Modern Electronics makes the stepping up and down of DC voltages possible. DC High Voltage transmission does not generate stray electromagnetic radiation (commonly called static). This static or 60 Hertz Hum is commonly heard on AM radios in the vicinity of high voltage transmission lines.
Direct Current also can be transmitted more efficiently through underground or underwater cables, because the changing E field of the alternating voltage heats the cable insulation. Today, solar collectors produce direct current. The batteries of battery powered cars are charged with direct current. Read the Winter edition of the Energy Matrix to learn more about DC power grids.
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