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11/16/2011
Chapter 22
Current and Resistance
Properties of a Current
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?
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Slide 22-1
Definition of a Current
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Slide 22-8
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Conservation of Current
Slide 22-9
Slide 22-10
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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 > 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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-11
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Slide 22-12
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11/16/2011
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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-13
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Slide 22-14
Checking Understanding
Measuring Body Fat
A battery is connected to a wire, and makes a current in the wire.
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.
i.
Which of the following changes would increase the
current?
ii. Which would decrease the current?
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
E. Increasing the temperature of the wire
A.
B.
C.
D.
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Slide 22-15
Power in Circuits
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
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 22-19
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Slide 22-22
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