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Chapter 22
Current and
Resistance
© 2010 Pearson Education, Inc.
PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
22 Current and Resistance
© 2010 Pearson Education, Inc.
Slide 22-2
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Slide 22-3
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Slide 22-4
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Slide 22-5
Properties of a Current
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Slide 22-12
Definition of a Current
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Slide 22-13
Example Problem
The discharge of the electric eel can transfer a charge of
2.0 mC in a time of 2.0 ms. What current, in A, does this
correspond to?
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Slide 22-14
Conservation of Current
å Iin = å Iout
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Slide 22-15
Checking Understanding
Rank the bulbs in the following circuit according to their
brightness, from brightest to dimmest.
A.
B.
C.
D.
ABCD
ABCD
ADBC
BCAD
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Slide 22-16
Answer
Rank the bulbs in the following circuit according to their
brightness, from brightest to dimmest.
A.
B.
C.
D.
ABCD
ABCD
ADBC
BCAD
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Slide 22-17
Checking Understanding
The wires below carry currents as noted. Rate the currents IA, IB
and IC:
A. I A > I B > I C
B. I B > I A > I C
C. I C > I A > I B
D. I A > I C > I B
E. I C > I B > I A
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Slide 22-18
Answer
The wires below carry currents as noted. Rate the currents IA, IB
and IC:
A. I A > I B > I C
B. I B > I A > I C
C. I C > I A > I B
D. I A > I C > I B
E. I C > I B > I A
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Slide 22-19
Batteries
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
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Slide 22-20
Simple Circuits
The current is determined by
the potential difference and
the resistance of the wire:
DVwire
I=
R
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Slide 22-21
Resistivity
The resistance of a wire
depends on its dimensions
and the resistivity of its
material:
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Slide 22-22
Checking Understanding
A battery is connected to a wire, and makes a current in the wire.
Which of the following changes would increase the current?
(1) Increasing the length of the wire; (2) keeping the wire the
same length, but making it thicker; (3) using a battery with a
higher-rated voltage; (4) making the wire into a coil, but keeping
its dimensions the same; (5) increasing the temperature of the
wire.
A.
B.
C.
D.
E.
All of the above
1 and 5
1, 4, and 5
2 and 3
None of the above
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Slide 22-23
Answer
A battery is connected to a wire, and makes a current in the wire.
Which of the following changes would increase the current?
(1) Increasing the length of the wire; (2) keeping the wire the
same length, but making it thicker; (3) using a battery with a
higher-rated voltage; (4) making the wire into a coil, but keeping
its dimensions the same; (5) increasing the temperature of the
wire.
A.
B.
C.
D.
E.
All of the above
1 and 5
1, 4, and 5
2 and 3
None of the above
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Slide 22-24
Ohm’s Law
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Slide 22-25
Checking Understanding: Measuring Light
Intensity
A battery is connected to a photoresistor. When light shines on this
resistor, it increases the number of charge carriers that are free to
move in the resistor. Now, the room lights are turned off, so less
light falls on the photoresistor. How does this affect the current in
the circuit?
A. The current increases.
B. The current decreases.
C. The current is not affected.
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Slide 22-26
Answer
A battery is connected to a photoresistor. When light shines on this
resistor, it increases the number of charge carriers that are free to
move in the resistor. Now, the room lights are turned off, so less
light falls on the photoresistor. How does this affect the current in
the circuit?
A. The current increases.
B. The current decreases.
C. The current is not affected.
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Slide 22-27
Example Problem
The filament of a 100-W bulb carries a current of 0.83 A at the
normal operating voltage of 120 V.
A.
B.
What is the resistance of the filament?
If the filament is made of tungsten wire of diameter
0.035 mm, how long is the filament?
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Slide 22-28
Conceptual Example Problem
If you use wire of the same diameter operating at the same
temperature, should you increase or decrease the length of the
wire from the value calculated in the previous example in order
to make a 60 W light bulb? (Hint: The bulb is dimmer. What
does this tell us about the current?)
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Slide 22-29
Power in Circuits
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Slide 22-30
Energy and Power in Resistors
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Slide 22-31
Checking Understanding
A resistor is connected to a 3.0 V battery; the power dissipated in
the resistor is 1.0 W. The battery is traded for a 6.0 V battery. The
power dissipated by the resistor is now
A.
B.
C.
D.
1.0 W
2.0 W
3.0 W
4.0 W
© 2010 Pearson Education, Inc.
Slide 22-32
Answer
A resistor is connected to a 3.0 V battery; the power dissipated in
the resistor is 1.0 W. The battery is traded for a 6.0 V battery. The
power dissipated by the resistor is now
A.
B.
C.
D.
1.0 W
2.0 W
3.0 W
4.0 W
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Slide 22-33
Example Problem
An electric blanket has a wire that runs through the interior. A
current causes energy to be dissipated in the wire, warming the
blanket. A new, low-voltage electric blanket is rated to be used at
18 V. It dissipates a power of 82 W. What is the resistance of the
wire that runs through the blanket?
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Slide 22-34
Conceptual Example Problem: Electric Blankets
For the electric blanket of the previous example, as the
temperature of the wire increases, what happens to the resistance
of the wire? How does this affect the current in the wire? The
dissipated power?
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Slide 22-35
Example Problem
Many websites describe how to add wires to your clothing to
keep you warm while riding your motorcycle. The wires are
added to the clothing; a current from the 12-V battery of the
motorcycle passes through the wires, warming them. One
recipe for a vest calls for 10 m of 0.25-mm-diameter copper
wire. How much power will this vest provide to warm the wearer?
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Slide 22-36
Example Problem
Standard electric outlets in the United States run at 120 V; in
England, outlets are 230 V. An electric kettle has a coiled wire
inside that dissipates power when it carries a current, warming
the water in the kettle. A kettle designed for use in England
carries 13 A when connected to a 230 V outlet.
A. What is the resistance of the wire?
B. What power is dissipated when the kettle is running?
C. The kettle can hold 1.7 L of water. Assume that all
power goes to heating the water. How long will it take
for the kettle to heat the water from 20ºC to 100ºC?
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Slide 22-37
Summary
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Slide 22-38
Additional Questions
1. A set of lightbulbs have different rated voltage and power,
as in the table below. Which one has the highest
resistance?
Bulb
A
B
C
D
E
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Rated voltage
10 V
8V
12 V
6V
3V
Rated power
1W
1W
2W
2W
3W
Slide 22-39
Answer
1. A set of lightbulbs have different rated voltage and power,
as in the table below. Which one has the highest
resistance?
Bulb
A
B
C
D
E
© 2010 Pearson Education, Inc.
Rated voltage
10 V
8V
12 V
6V
3V
Rated power
1W
1W
2W
2W
3W
Slide 22-40
Additional Questions
2. A set of lightbulbs have different rated voltage and power,
as in the table below. Which one has lowest resistance?
Bulb
A
B
C
D
E
© 2010 Pearson Education, Inc.
Rated voltage
10 V
8V
12 V
6V
3V
Rated power
1W
1W
2W
2W
3W
Slide 22-41
Answer
2. A set of lightbulbs have different rated voltage and power,
as in the table below. Which one has lowest resistance?
Bulb
A
B
C
D
E
© 2010 Pearson Education, Inc.
Rated voltage
10 V
8V
12 V
6V
3V
Rated power
1W
1W
2W
2W
3W
Slide 22-42
Additional Questions
3. A battery makes a circuit with a lightbulb as shown. Two
compasses are near the wires before and after the bulb in the
circuit. Which compass experiences a larger deflection?
A. Compass 1 experiences a larger deflection.
B. Compass 2 experiences a larger deflection.
C. Both compasses experience the same deflection.
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Slide 22-43
Answer
3. A battery makes a circuit with a lightbulb as shown. Two
compasses are near the wires before and after the bulb in the
circuit. Which compass experiences a larger deflection?
A. Compass 1 experiences a larger deflection.
B. Compass 2 experiences a larger deflection.
C. Both compasses experience the same deflection.
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Slide 22-44
Additional Questions
4. In Trial 1, a battery is connected to a single lightbulb and the
brightness noted. Now, in Trial 2, a second, identical, lightbulb
is added. How does the brightness of these two bulbs compare
to the brightness of the single bulb in Trial 1?
A. The brightness is greater.
B. The brightness is the same.
C. The brightness is less.
© 2010 Pearson Education, Inc.
Slide 22-45
Answer
4. In Trial 1, a battery is connected to a single lightbulb and the
brightness noted. Now, in Trial 2, a second, identical, lightbulb
is added. How does the brightness of these two bulbs compare
to the brightness of the single bulb in Trial 1?
A. The brightness is greater.
B. The brightness is the same.
C. The brightness is less.
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Slide 22-46
Additional Example Problem
How much time does it take for 1.0 C to flow through each of the
following circuit elements?
A.
B.
A 60 W reading light connected to 120 V.
A 60 W automobile headlamp connected to 12 V.
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Slide 22-47