Recall now what you learned from the section of this blog titled “Current”.
Current is defined as the flow of electrons from a point of negative charge
(meaning a point of higher density of electrons) to a point of lesser density of
electrons. The lesser density point need not be “positively” charged, only less
negative than the other point. A current, or flow of electrons, will occur
because the difference in charges is seeking to even itself out, to obtain a
charge differential of zero.
If the transfer or flow of electrons occurs without interruption in time, and
without reversal of direction, it is termed direct current or dc or DC. The flow
may occur at any voltage greater than zero. There needs to be a voltage
differential greater than zero else the current cannot flow. The current section
below describes the amount of charge flow required in one second to constitute a
single amp of current. That current may be measured in units, tens, hundreds,
thousands, or more, or in tenths, hundredths, or thousandths, millionths of an
amp. Amp is the shortened name for Ampere. If the measurement involves
thousandths of an amp, we term the measurement is in milliamps, abbreviated ma.
If millionths of an amp, we use the term microamps and the abbreviation ua or
µa. There are one thousand microamps in a single milliamp. Likewise, there are
one thousand milliamps in a single amp.
If the DC current flow is interrupted, particularly at a regularly timed rate,
it is termed pulsed DC. The pulses may vary at a relatively slow rate, maybe
once or twice per second, or at a very fast rate, perhaps hundreds, thousands,
or millions of times per second. As long as the pulses vary their voltage from
zero to a positive or negative value, the current stream is considered to be
pulsed DC. If the current varies from a positive value to a negative value, and
back to a positive value, it is considered to be alternating, not pulsed. The
nuances here will become important later on.
DC current was the second type of electricity to be generated on demand, the
first was static electricity generated by rubbing materials with fur or fabric.
DC was created within devices we now call batteries using chemical reactions
between dissimilar materials and compounds. The first individual to do that was
Alessandro Volta in 1800. Those early devices were named after him and termed
voltaic piles.
The ability to generate electricity on demand using voltaic piles greatly
stimulated the study of electricity and its properties. Throughout the early and
mid 1800’s, physicists, chemists, and scientists were able to replicate and
verify each others discoveries about the properties of electricity.
Today, most electronic devices utilize DC in some particular way. Useful
voltages ranging from about 1.5 volts produced by simple carbon or alkaline
batteries through 3.3 volts used by modern electronics in cell phones to 5 volts
used by most computers to 12 volts used by most modern cars all rely on DC. Some
railroads and most subways utilize very high DC currents to drive their motors
at voltages from 750 to 3000 volts. The first practical incandescent electric
lights developed by Thomas Edison in the late 1800’s also utilized DC. DC
however is difficult to transmit over long distances. The primary advisary of
Edison was Nikola Tesla who was a proponent of alternating current, AC, and
participated in bitter arguments with Edison and his DC electricity
distribution. Edison, known for the invention of many things including the
phonograph and moving pictures, was wrong about the practicality of wide-scale
distribution of DC for electric lighting purposes. Tesla’s AC eventually
dominated and replaced Edison’s DC as a more efficient alternative. Today, we
have the benefit of using both and can easily convert the AC transmitted from
far away generators to DC to power our electronic devices.
73…W3SEH