Chapter 22
Current and Resistance
1. The charge carriers in metals are
A. electrons
B. positrons
C. protons
D. a mix of protons and electrons
Topics:
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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?
Slide 22-1
Slide 22-2
1. The charge carriers in metals are
A. electrons
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.
Slide 22-3
Slide 22-4
2. A battery is connected to a resistor. Increasing the resistance of
the resistor will
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
B. decrease the current in the circuit.
Slide 22-5
Slide 22-6
Properties of a Current
3. A battery is connected to a resistor. As charge flows, the
chemical energy of the battery is dissipated as
D. thermal energy
Slide 22-7
Slide 22-8
Definition of a Current
Conservation of Current
Slide 22-9
Slide 22-10
Batteries
Rank the bulbs in the following circuit according to their brightness,
from brightest to dimmest.
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
Wchem
∆Vbat = ____
=ε
q
The wires below carry currents as noted. Rate the currents IA, IB
and IC:
A. I A &gt; I B &gt; I C
B. I B &gt; I A &gt; I C
C. I C &gt; I A &gt; I B
D. I A &gt; I C &gt; I B
E. I C &gt; I B &gt; I A
Slide 22-11
Slide 22-12
Simple Circuits
Resistivity
The current is determined by
the potential difference and
the resistance of the wire:
I =
The resistance of a wire
depends on its dimensions
and the resistivity of its
material:
∆V
_____
chem
R
Slide 22-13
Slide 22-14
Checking Understanding
Measuring Light Intensity
A battery is connected to a wire, and makes a current in the wire.
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 roo
oom lights are turned off, so less light
falls on the photoresistor. How does this affect the current in the
circuit?
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
Slide 22-15
A. The current increases.
B. The current decreases.
C. The current is not affected.
Slide 22-16
Measuring Body Fat
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 roo
oom lights are turned off, so less light
falls on the photoresistor. How does this affect the current in the
circuit?
The resistivity of the body is a good measure of its overall composition. A
measure of the resistance of the upperr arm
a is a good way to estimate the percent
fat in a person’s body. Let’s model a pers
erson’s upper arm as a cylinder of diameter
8.0 cm and length 20 cm. We can model
el 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.
B. The current decreases.
A. Assume that the arm has 30% fatt a
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 off the
th arm is made by applying a voltage and
measuring a current. Too much currentt can
c 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?
Slide 22-17
Power in Circuits
Slide 22-18
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
Slide 22-19
Slide 22-20
Electric Blankets
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
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
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
a
example, as the temperature
of the wire increases, what hap
appens to the resistance of the wire?
How does this affect the current in the wire? The dissipated
power?
D. 4.0 W
Slide 22-21
Slide 22-22
1. A set of lightbulbs have different rated voltage and power, as
in the table below. Which one has the highest resistance?
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
Rated power
1W
1W
2W
2W
3W
Bulb
A
Slide 22-23
Rated voltage
10 V
Rated power
1W
Slide 22-24
2. A set of lightbulbs have different rated voltage and power, as
in the table below. Which one has lowest resistance?
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
Rated power
1W
1W
2W
2W
3W
Slide 22-25
Rated voltage
Rated power
E
3V
3W
Slide 22-26
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.
Bulb
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.
Slide 22-27
Slide 22-28
4. In Trial 1, a battery is connected to a single lightbulb and the
brightness noted. Now, in Trial
ial 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?
4. In Trial 1, a battery is connected to a single lightbulb and the
brightness noted. Now, in Trial
ial 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.