Alternating Current (AC)
Circuits
AC Voltages and Phasors
Resistors, Inductors and Capacitors in AC
Circuits
RLC Circuits
Power and Resonance
AC Sources and Phasors
Consider a sinusoidal voltage source:
This source can be represented graphically as a vector called a
phasor
:
t
time
T
V
max

V
max
V
max
V(t)
b
c
b
c
d
e
d
a,e
a
Great web page:
http://www.phys.unsw.edu.au/%7Ejw/AC.html
Resistors in AC Circuits
Since both have the
same arguments in
the sine term,
i
R
and
v
R
are in line, they
are
in phase
.
Root Mean Square (RMS) Current
and Voltage
Announcements
Cell phones on silent
Check mid

term grades.
Inductors in AC Circuits
Inductive
Reactance
For a sinusoidal voltage,
the current in the inductor
always
lags
the voltage
across it by 90
°
.
Inductors in AC Circuits
Filter for woofer
Low frequencies
pass through
High frequencies
get blocked.
V
R
Frequency
Woofer
Capacitors in AC Circuits
Capacitive
Reactance
For a sinusoidal voltage,
the current in the
capacitor always
leads
the voltage across it by
90
°
.
Inductors in AC Circuits
Filter for tweeter
High frequencies
pass through
Low frequencies
get blocked.
V
R
Frequency
Tweeters
RLC Series Circuit
Using Phasors in a RLC Circuit
Impedance
R = 425
W
L = 1.25 H
C = 3.5
m
F
= 377 s

1
V
max
= 150 V
Example 33.5
Power in an AC Circuit
For a purely resistive load,
=0
No power loss occurs in
an ideal capacitor or
inductor.
Resonance in Series RLC Circuits
A circuit is in resonance when its current is maximum.
I
rms
=max
when
X
L

X
C
=0
Resonance Frequency
Tuner for radio!
www.barlowwadley.it
Inductor
Variable
Capacitors
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