Transcript Figure 2.4 shows the unusual path of a confused football player. He

QUICK QUIZ 27.1
You are driving along a dark country
road at night when you start
experiencing car trouble. You pull
over, open the hood and try to figure
out what is wrong with the car. You
don’t have a flashlight in the car but
you do find a regular household light
bulb in the trunk. The bulb is labeled
100 Watts. You manage to find some
wires and connect the light bulb to the
12-volt car battery. The power
delivered to the light bulb will be
approximately: a) 1 Watt, b) 10 Watts,
c) 100 Watts, d) 1000 Watts, or e)
10000 Watts.
QUICK QUIZ 27.1 ANSWER
(a). A 100 Watt household light bulb is designed to operate at
100 Watts with 120 volts normal house voltage. If you reduce
the voltage by a factor of 10, the power is reduced by a factor of
100 according to the equation P = DV2/R, since the resistance of
the light bulb remains roughly constant.
QUICK QUIZ 27.2
You have the opportunity of using one of two power supplies for a
certain application. Power supply A has a maximum voltage
capacity of 1000 volts and a maximum current capacity of 0.1
amps. Power supply B has a maximum voltage capacity of 10 volts
and a maximum current capacity of 10 amps. Either power supply
would be suitable for your application but you would like to choose
the power supply that is the safest. Your lab partner tells you that it
is the current that kills you, not the voltage and suggests you pick
the one with the lower current capacity. In the interest of safety
you should, a) pick power supply A, b) pick power supply B, or c)
pick either power supply - they are equally safe.
QUICK QUIZ 27.2 ANSWER
(b). It is true that it is the current that kills you, but a large
current cannot travel through your body if the voltage across
your body is small. You may think of yourself as a giant
resistor. Typical human resistances can be in the
neighborhood of 1 000 to 100 000 ohms, depending on the
situation. A current of 0.1 A is generally lethal. For power
supply B, with a maximum voltage across you of 10 volts and
a moderate resistance of 10 000 ohms, the current through you
would be: I = DV/R = 10/10 000 = 0.001 A, quite safe. For
power supply A, with 1000 volts across you and a resistance
of 10 000 ohms, the current would be 0.1 A – generally lethal.
QUICK QUIZ 27.3
Three identical resistors (A, B, and C) are shown with the
voltage on each end of each resistor indicated. The relationship
between the magnitude of the currents through the three resistors is
a) IA > IB > IC, b) IC > IB > IA, c) IA > IC > IB, d) IB > IA > IC, or e) IB
> IC > IA .
QUICK QUIZ 27.3 ANSWER
(e). Recall that DV in Ohm’s law, I = DV/R, refers to the voltage
difference between the two ends of the resistor. This difference
is greatest for resistor B and least for resistor A.
QUICK QUIZ 27.4
Of the materials listed
in Table 27.1, the
material that would
make the most
sensitive thermometer
would be a) silver, b)
quartz, c) nichrome, or
d) silicon.
QUICK QUIZ 27.4 ANSWER
(d). Sensitivity refers to the ability of a device or instrument to
detect small changes in some physical parameter. The most
sensitive thermometers are made from materials that have some
physical property that changes dramatically with small changes in
temperature. The temperature coefficient of resistivity determines
how dramatically the resistivity changes with temperature. From
the materials in the chart, this coefficient has the greatest
magnitude for silicon.
QUICK QUIZ 27.5
You perform a simple experiment on a light bulb. You steadily increase the
voltage across the light bulb as you measure the current through the bulb. In addition,
as you increase voltage, you note that the temperature of the bulb increases. Which
curve below would best represent your actual data?
QUICK QUIZ 27.5 ANSWER
(c). As you increase voltage, the temperature of the bulb increases
and the resistance of the bulb increases. Since the slope of the
graph is I/DV = 1/R, the slope of the graph will decrease as the
resistance increases. Graph c best illustrates this dependence.