316 Week 9: Alternating Current Circuits
It is also clearly undesirable, as the heat that appears in the iron core islostand hence reduces
the available power (voltage and current alike) on the secondary compared to what comes in through
the primary. To minimize eddy currents, the iron core is usually made oflaminatedstrips of iron
separated by insulating resin or out of insulatedwiresof iron. The small cross-sectional area of
the individual conductors thus minimizes flux, voltage and current,and thereby losses to heating
through eddy currents.
Now, high voltage is dangerous. Dielectric breakdown can easily occur of the voltage is high
enough – power can simply leap through the air in an electrical arc andfry whatever it passes
through on its way to ground. Nevertheless, we find it very usefulto use high voltage totransmit
electrical power long distancesby using the fact that current goesdownas the voltage goesupfor
any given power being delivered.
Power Transmission
When electricity was first introduced into society on a grand scale (largely by Thomas Edison, to
use in his recently invented light bulbs) Edison wished to power the world with direct current (DC)
lines from his generating stations directly into your home, at a very low (and thereby safe) voltage.
Edison had a number of patents on various aspects of DC power generation, storage, and metering,
and had a vested interest in all of this technology. However, Edisonwas no mathematician, and
did notunderstandelectricity or Maxwell’s equations (indeed, at the time Maxwell’s equations were
only about 20 years old and there weren’t a lot of people who weren’t mathematicians or physicists
whodidunderstand them).
There is just one problem. Atlowvoltages, delivering power across miles of wire to households
can easily be shown to wastealmost allof that energy heating the wires that carry it, and leave
almost nonefor the households at the end!
At the same time, a young man named Nikola Tesla^89 , whowasa competent mathematician
and who had worked for Edison for a while before he resigned claiming (correctly) that he was
undervalued by Edison, who cheated him out of a promised payment of $50,000. He realized that
the secret to the economical transmission of power was the use ofhigh voltages (and correspondingly
low currents) in the transmission process, something that is only possible if one uses alternating
current (AC) and transformers like the one schematized above. Tesla quit working for Edison (and
General Electric) and ultimately went to work for Westinghouse, that gradually prospered on the
basis of his new scheme. This was the so-calledWar of the Currents^90.
Edison lost (although Tesla never really benefitted much from his victory, being cursed with bad
luck that seemed to guarantee that he would never become rich from his cornucopia of enormously
valuable inventions. Tesla was dead on correct – Edison’s solution wasno solution at all and
could never have supported the centralized generation and distribution of electrical power that is
the fundamental basis of modern civilization with its vast and distributed productivity and its
unprecedented degree of personal comfort and information access, all enabled by mass-produced
electrical power distributed by Tesla’s solution. We absolutely need to learn, and understand, this
solution as it is of paramount importancetoday, some 130 years later, as we struggle to convert
(^89) Wikipedia: http://www.wikipedia.org/wiki/Nikola Tesla. Tesla was the original “mad scientist” – he is the
original inventor of the radio (and was cheated of the patent), he worked for Edison redesigning Edison’s DC generators
(and was cheated of the promised payment), invented the Tesla coil, polyphase generators and motors, invented the
X-ray tube and photographed the bones of his own hand before Roentgen (but failed to publish or patent and lost
the technical descriptions in a fatal fire that destroyed much of his work prematurely), he purportedly invented a
“death ray”, but destroyed it after a single apocryphal demonstration of its effects. He had a photographic memory
and reportedly experienced direct insight into problems hewas working on, bypassing all normal routes to invention
or design. He is basically an enormously interesting personI a strongly recommend reading at least the wikipedia
article on him.
(^90) Wikipedia: http://www.wikipedia.org/wiki/War of Currents.