Chapter 21
7.
Picture the Problem
: A battery for a pacemaker is rated for a certain number of amperehours.
Strategy:
Convert the number of amperehours to coulombs of charge, then use equation 211 to
determine the time required for the specified average current to deliver that much charge.
Solution:
1. (a)
Find
from equation 211:
2.
(b)
Solve equation 211 for
:
Insight:
The long life span of the battery is helpful in order to minimize the frequency of invasive
surgery required to replace the batteries! The major risk, however, is the toxicity of the chemicals used
to make the battery.
11.
Picture the Problem
: A copper wire of known dimensions has an intrinsic resistance.
Strategy:
Use table 211 together with equation 213 to determine the resistance of the wire.
Solution:
Apply equation 213 , using
from table 211 and
:
Insight:
This is a large resistance, and copper wires are often used for electric power and
communication lines. However, the choice of a larger diameter can make a huge difference. For
instance, if the diameter of the wire were
3.0 mm, the resistance would only be 23 Ω, a reduction by a factor of 30!
12.
Picture the Problem
: The section of copper wire between a bird’s feet has an intrinsic resistance. If
any current is flowing in the wire, there is a small potential difference across the two ends of the wire
section.
Strategy:
Ohm’s Law (equation 212) gives the relationship between potential difference, current,
and resistance for any circuit element. In this problem we first apply equation 213 and the data from
table 211 to find the resistance of the wire section, and then apply Ohm’s Law to find the potential
difference.
Solution:
1. (a)
Combine
equations 212 and 213 to
find
V
:
2.
(b)
Since
V
is directly proportional to the separation
L
of the bird’s feet, the answer to part (a) will
increase if
L
increases.
Insight:
Landing on a high voltage wire will not kill a bird because the potential difference between its
feet is small. However, if another part of the bird’s body gets too close to a grounded conductor, a
spark could leap from the bird’s body to the grounded conductor and kill the bird.
13.
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 Spring '08
 Dr.G
 Physics, Charge, Resistor, $1.00, 3.0 mm, 2.0 hours

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