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Chapter 25
Current, Resistance, and
Electromotive Force
PowerPoint® Lectures for
University Physics, Twelfth Edition
– Hugh D. Young and Roger A. Freedman
Lectures by James Pazun
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Goals for Chapter 25
• To consider current and current density
• To study the intrinsic property of resistivity
• To use Ohm’s Law and study resistance and resistors
• To connect circuits (mentally, virtually, or with
actual parts) and find emf
• To examine circuits and determine the energy and
power in them
• To describe the conduction of metals
microscopically, on an atomic scale
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Introduction
• Electrons leave one terminal
of a battery, pass through wire
of low resistance, reach a light
bulb with a special calibrated
resistor sealed in a bulb of
inert gas, and then return to
the opposite terminal of the
battery.
• The electron’s journey has
been interrupted by our special
resistor because we had a
nefarious ulterior motive. We
wanted light!
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The direction of current flow
• In the absence of an external field, electrons move
randomly in a conductor. If a field exists near the
conductor, its force on the electron imposes a drift.
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Current flowing
•
Positive charges would move with the electric field, electrons move in
opposition.
•
The motion of electrons in a wire is analogous to water coursing
through a river. This fits the metaphor used earlier.
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Current flow requires conductors throughout
•
In Figure 25.4, a negative terminal of a
battery extends through wire to a bare
post inside the open tube. Another
open tube next to the first one also
contains a bare post with wire running
back to entry of a light bulb resistor.
The exit of the light bulb resistor
continues through wire back to the
positive terminal on the battery.
•
If the tubes are immersed in a
conducting fluid, the bulb will light. If
the fluid is nonconducting, the light
will remain off.
•
Consider Example 25.1.
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Resistivity is intrinsic to a metal sample (like density is)
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Resistivity and temperature
• Resistivity rises with increasing
temperature. The electronic motion is
analogous to shopping on quiet days
(lower T) or busy days (higher T). See
Figure 25.6.
• Table 25.2 tabulates resistivities.
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Resistance is an extensive property (like mass)
• Copper is a good conductor, but it’s still possible to add
magnitudes of resistance with copper because it takes more mass.
• Figure 25.7 illustrates the model.
• Figure 25.8 shows an unfortunate example of the heat generated
when current and resistance are unmatched.
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Resistors are color-coded for assembly work
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Current–voltage relationships
• Ohm’s Law is linear, but current flow through other devices may
not be.
• Follow Example 25.2.
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Calculating resistances
• Refer to Example 25.3 to see the effects of changing temperature.
• Refer to Example 25.4 to calculate the resistance of a hollow tube
(unlike a normal wire). Figure 25.11 (below) illustrates this
example.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Electromotive force and circuits
• You’ve probably
already thought
“water doesn’t
flow through a
pipe without a
pump; why should
electrons flow
through a wire?”
• If those were your
daydreams, you’re
right. See Figures
25.12 and 25.13 at
right.
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Ideal diagrams of “open” and “complete” circuits
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Internal resistance
• We generalize at the
outset, but the truth of
a battery is that you
only get 12 V when a
12 V battery isn’t
connected.
• Making a connection
allows electrons to
flow, but internal
resistance within the
battery actually
delivers incrementally
less than 12 V.
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Symbols for circuit diagrams
• Shorthand symbols are in use for all wiring components. See
below.
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Source in an open circuit I
• Consider Conceptual Example 25.5.
• This example is illustrated in Figure 25.17.
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Source in an open circuit II
• Follow Example 25.6.
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Voltmeters and ammeters
• Follow Conceptual Example 25.7.
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A source with a short circuit
• Follow Example 25.8.
• Figure 25.20 (below) illustrates this example.
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Potential changes around a circuit
• The net change in potential energy must be zero for the entire
circuit.
• Local differences in potential and emf do occur. See Figure 25.21
below.
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Energy conversion and power input to a source
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Power and energy in circuits
•
Consider Problem-Solving Strategy 25.1.
•
Refer to Example 25.9, illustrated by Figure 25.25 below.
•
Refer to Example 25.10.
•
Refer to Example 25.11, illustrated by Figure 25.26 below.
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A microscopic look at conduction
• Consider Figure 25.27.
• Consider Figure 25.28.
• Follow Example 25.12.
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