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Chapter 22
Current and Resistance
Topics:
• Current
• Conservation of current
• Batteries
• Resistance and resistivity
• Simple circuits
Sample question:
How can the measurement of an electric current passed through a
person’s body allow a determination of the percentage body fat?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-1
Reading Quiz
1. The charge carriers in metals are
A. electrons
B. positrons
C. protons
D. a mix of protons and electrons
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Slide 22-2
Answer
1. The charge carriers in metals are
A. electrons
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Slide 22-3
Reading Quiz
2. A battery is connected to a resistor. Increasing the resistance of
the resistor will
A. increase the current in the circuit.
B. decrease the current in the circuit.
C. not affect the current in the circuit.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-4
Answer
2. A battery is connected to a resistor. Increasing the resistance of
the resistor will
B. decrease the current in the circuit.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-5
Reading Quiz
3. A battery is connected to a resistor. As charge flows, the
chemical energy of the battery is dissipated as
A. current
B. voltage
C. charge
D. thermal energy
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-6
Answer
3. A battery is connected to a resistor. As charge flows, the
chemical energy of the battery is dissipated as
D. thermal energy
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Slide 22-7
Batteries
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
W
chem
∆Vbat = ____
=
q
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-12
Definition of a Current
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Slide 22-9
Student Current Demo
1. What happens as conductor gets longer
2. What happens as conductor gets wider
3. What happens as conductor gets hotter
Potential, Charge, and Field Circuit Model
Charge and Current (need to load before class)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-3
Conservation of Current
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Slide 22-10
Rank the bulbs in the following circuit according to their brightness,
from brightest to dimmest.
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Slide 22-11
Simple Circuits
The current is determined by
the potential difference and
the resistance of the wire:
∆V
_____
chem
I = R
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-13
Resistivity
The resistance of a wire
depends on its dimensions
and the resistivity of its
material:
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-14
Checking Understanding
A battery is connected to a wire, and makes a current in the wire.
i.
Which of the following changes would increase the
current?
ii. Which would decrease the current?
A.
B.
C.
D.
E.
iii. Which would cause no change?
Increasing the length of the wire
Keeping the wire the same length, but making it thicker
Using a battery with a higher rated voltage
Making the wire into a coil, but keeping its dimensions the
same
Increasing the temperature of the wire
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Slide 22-15
Power in Circuits
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Slide 22-19
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 now traded for a 6.0 V battery.
The power dissipated by the resistor is now
A. 1.0 W
B. 2.0 W
C. 3.0 W
D. 4.0 W
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-20
Answer
A resistor is connected to a 3.0 V battery; the power dissipated in
the resistor is 1.0 W. The battery is now traded for a 6.0 V battery.
The power dissipated by the resistor is now
D. 4.0 W
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-21
Electric Blankets
1. 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?
2. For the electric blanket of the above 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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-22
Additional Clicker 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
Rated voltage
10 V
8V
12 V
6V
3V
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Rated power
1W
1W
2W
2W
3W
Slide 22-23
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
Rated voltage
10 V
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Rated power
1W
Slide 22-24
Kirchhoff’s Laws
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Slide 23-11