1
Lecture 29
AC circuits Phasors
AC circuits. Phasors.
Driven RLC circuit
Analogy between circuits and SHM:
Circuits
Mechanical
Oscillations.
LC
Si/
(
S
H
M
No energy dissipation.
Spring/mass (SHM)
Oscillations with decreasing
amplitude.
Energy dissipation.
RLC
Spring/mass with
damping
Driven oscillations.
Energy dissipation and supply
RLC with AC
generator
Spring/mass with
damping and driving
force
Preview
–
Source
:
produces an oscillating voltage (supplies whatever
current the rest of the circuit “requires”)
–
Resistor
:
causes a voltage drop when a current flows
through
As soon as the voltage changes so does the
~
through.
As soon as the voltage changes, so does the
current
always
in phase
.
–
Capacitor
:
resists change in charge
Q
resists change in
voltage. Voltage across capacitor lags
behind the current by 90°.
–
Inductor
:
resists change in magnetic flux
resists change
in current.
Voltage across inductor is
ahead of the current by 90
˚
.
AC generator
Generic problem: we are given an AC voltage source that “drives” a circuit
Supplies a given emf that
depends on time (usually as a
sine of cosine
cos
Et
sine of cosine)
Goal: Determine current that flows:
cos
iI
t
•
Current must be the same through all elements if they
are in series.
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 Spring '07
 Johnson
 Physics, Energy, Mass

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