DC Circuits Lecture
Text Version
Direct Current .
Batteries
are the most common source of DC voltage. The chemical reaction in a
battery produces a constant electromotive force, across the
terminals of a battery.
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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