Download 22_LectureOutlines

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?
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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.
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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.
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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
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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
Properties of a Current
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Slide 22-8
Definition of a Current
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Slide 22-9
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.
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 22-11
Batteries
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
W
chem
∆Vbat = ____
=
q
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Slide 22-12
Simple Circuits
The current is determined by
the potential difference and
the resistance of the wire:
∆V
_____
chem
I = R
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Slide 22-13
Resistivity
The resistance of a wire
depends on its dimensions
and the resistivity of its
material:
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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
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 22-16
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?
B. The current decreases.
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Slide 22-17
Measuring Body Fat
The resistivity of the body is a good measure of its overall composition. A
measure of the resistance of the upper arm is a good way to estimate the percent
fat in a person’s body. Let’s model a person’s upper arm as a cylinder of diameter
8.0 cm and length 20 cm. We can model the composition of the arm by assuming
that the muscle, far, and nonconductive portions (the bone) form simple regions.
This simple model actually works quite well. For a typical adult, the bone has a
cross-sectional area of 1.0 cm2; to a good approximation, the balance of the arm
is fatty tissue or muscle.
A. Assume that the arm has 30% fat and 70% muscle. What is the resistance of
the arm?
B. Now, assume that the arm has only 10% muscle. What is the resistance
now?
The measurement of the resistance of the arm is made by applying a voltage and
measuring a current. Too much current can be uncomfortable and, as we will see,
can be dangerous. Suppose we wish to limit the current to 1.0 mA. For each of
the above cases, what is the maximum voltage that could be employed?
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Slide 22-18
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
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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
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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?
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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
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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
Additional Clicker 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
Rated voltage
10 V
8V
12 V
6V
3V
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Rated power
1W
1W
2W
2W
3W
Slide 22-25
Answer
2. A set of lightbulbs have different rated voltage and power, as
in the table below. Which one has lowest resistance?
Bulb
Rated voltage
Rated power
E
3V
3W
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Slide 22-26
Additional Clicker 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 22-27
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?
C. Both compasses experience the same deflection.
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Slide 22-28
Additional Clicker 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.
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Slide 22-29
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?
C. The brightness is less.
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Slide 22-30
Additional Examples
How much time does it take for 1.0 C to flow through each of the
following circuit elements?
A. A 60 W reading light connected to 120 V.
B. A 60 W automobile headlamp connected to 12 V.
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Slide 22-31