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Chapter 31
Alternating Current
PowerPoint® Lectures for
University Physics, Twelfth Edition
– Hugh D. Young and Roger A. Freedman
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Goals for Chapter 31
• To introduce phasors and alternating current
• To study voltage, current, and phase angle
• To consider resistance and reactance
• To introduce impedance and its application to the
L-R-C series circuit
• To consider power in AC circuits
• To see resistance in AC circuits
• To apply AC knowledge to transformers
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Introduction
• After Thomas Edison found a
filament that worked for his new
invention, the light bulb, he tried
to design a system to send power
from one place to another across
large distances. He quickly found
that I2R power loss in the cables
was frustrating, essentially a
collection of resistance heaters.
• Another fortuitous application of
AC current in long metal
conductors was the creation of
electromagnetic waves.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
AC current
•
Notice the new electronic symbol at the top of page 1062.
•
Consider phasor diagrams.
•
Review Figure 31.1.
•
Explain Figure 31.2. (Until now, only Captain Kirk came to mind when
someone said “phasor.”)
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A full-wave rectifier circuit
• Refer to Figure 31.3 at right
and 31.4 below.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Current in the device I’m using right now
• A desktop PC draws current from a plug to the wall, but what are the
details?
• Smile at Figure 31.5. (It’s what we’re doing.)
• Follow Example 31.1, illustrated by Figure 31.6.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Resistors in an AC circuit
• Ohm’s Law applied in oscillatory fashion.
• Consider Figure 31.7.
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Inductors in an AC circuit
• Replace the resistor in the previous slide with an inductor.
• Consider Figure 31.8.
• Follow Example 31.2.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Capacitance in an AC circuit
•
Because this is a series circuit, the
current is the same through the
capacitor as through the resistor just
considered.
•
Consider Figure 31.9 at right.
•
Follow Example 31.3.
•
Figure 31.10 (below) illustrates
Example 31.3.
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Comparing AC circuit elements
•
Table 31.1 (bottom) provides a valuable summary/comparison for circuit
elements.
•
Figure 31.11 (below) shows items as a function of the angular
frequency.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
The loudspeaker, a useful application
• The woofer (low tones) and
the tweeter (high tones)
connect in parallel through a
“crossover.”
• Consider Figure 31.12.
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The L-R-C circuit
• Refer to Figure 31.13 at the bottom of
the page.
• Read Problem-Solving Strategy 31.1.
• Follow Example 31.4.
• Figure 31.14 (at right) shows a fun
example!
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
An L-R-C circuit II
• Follow Example 31.5 below.
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Power in an inductor
• Consider current, voltage, and power as functions of time.
• Consider Figure 31.16 below.
• Follow Examples 31.6 and 31.7.
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Circuit behavior at resonance
• Look at the maximum I when the impedance is a minimum.
• Refer to Figure 31.18.
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Tuning a radio
• Follow Example 31.8.
• Figure 31.20 illustrates the example.
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Transformers
•
We drive by them, carry them, and sit
in front of them.
•
Notice the new circuit symbol at the
bottom of page 1080.
•
Consider Figure 31.21 below and
Figures 31.22 and 31.23 at right.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Transformers II
• Consider Figure 31.24.
• Follow Example 31.9.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley