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Alternating Current and Phasors
AC voltage from a wall outlet is a
sinusoid with an amplitude of about
170 volts and a frequency of 60 hertz
(377rad/s). The electrons are not
actually traveling in a loop through
the transmission lines, but instead are
simply vibrating back and forth in the wire. The voltage in an outlet is normally given as
120V. This is the
rootmeansquared
voltage, a kind of average where the square
r
oot of
the integrated
m
ean of the voltage
s
quared is taken. (A simple time average would be
zero.) For sine waves, the
RMS
value is always
V
RMS
=V
max
/
√
2
. In this case,
170/
√
2
≈
120
.
A
phasor
is a mathematical construct similar to a vector. Using phasors to solve AC
circuit problems is much easier than trying to process all the calculus and trigonometry
involved with straightforward sinusoidal values and circuitelement behavior. A phasor
consists of a magnitude and a phase angle that are both straightforward to determine.
Suppose you are asked to convert a sinusoidal voltage like
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This note was uploaded on 04/05/2012 for the course PHYS 131 taught by Professor Tibbets during the Spring '11 term at Cuyamaca College.
 Spring '11
 Tibbets
 Current

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