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Science and Society

Blackout Reveals Pollution

n August 2003, problems in a huge electri- 70 percent fewer light-scattering particles 2. Are there factors on the ground
Ical grid resulted in power plants shutting from the air in the same area than when the that might have contributed to the
down across the Ohio Valley and a massive power plants were running. The scientists findings?
electric power blackout that affected some stated that the result could come from an 3. Do the results mean that power
50 million people. Scientists from the Uni- underestimation of emissions from power plants contribute that much
versity of Maryland took advantage of the plants or from unknown chemical reactions pollution, or what else should be
event to measure different levels of atmo- in the atmosphere. considered?
spheric air pollution while the fossil-fueled 4. How would you conduct a pollution-
power plants were shut down. Scooping many QUESTIONS TO DISCUSS measuring experiment that would
air samples with a small airplane 24 hours leave no room for doubt about the
after the blackout, they found 90 percent 1. Are there atmospheric factors that results?
less sulfur dioxide, 50 percent less ozone, and might have contributed to the findings?

voltages. Thus, with three cells connected in a series circuit, the connected in series, the resistance (R) of the combination

voltage of the circuit is 4.5 volts. A 9 volt battery is made up of is equal to the sum of the resistances of each component. In
six 1.5 volt cells connected in a series circuit. You can see the symbols, this is written as
smaller cells of a 9 volt battery by removing the metal jacket. A . . .
R = R + R + R +
12 volt automobile battery works with different chemistry than total 1 2 3

the flashlight battery, and each cell in the automobile battery equation 6.10
produces 2 volts. Each cell has its own water cap, and six cells
are connected in a series circuit to produce a total of 12 volts.
Cells with all positive terminals connected and all negative EXAMPLE 6.11
terminals connected (see Figure 6.36B) are in a parallel circuit.
Three resistors with resistances of 12 ohms (Ω), 8 ohms, and 24 ohms

A parallel circuit has a resultant voltage determined by the largest
are in a series circuit with a 12 volt battery. (A) What is the total resis-
cell in the circuit. If the largest cell in the circuit is 1.5 volts, then
tance of the resistors? (B) How much current can move through the

the potential difference of the circuit is 1.5 volts. The purpose of
circuit? (C) What is the current through each resistor?
connecting cells in parallel is to make a greater amount of electri-

cal energy available. The electrical energy that can be furnished
by dry cells in parallel is the sum of the energy that the individual SOLUTION
cells can provide. A lantern battery, for example, is made up of
four 1.5 volt dry cells in a parallel circuit. The total voltage of the A.

battery is 1.5 volts. The four 1.5 volt cells in a parallel circuit will
R = R + R + R
total
1
3
2
last much longer than they would if used individually.
= 12 Ω + 8 Ω + 24 Ω
= 44 Ω
RESISTANCES IN CIRCUITS
B. Using Ohm’s law,
Electrical resistances, such as lightbulbs, can also be wired into
V _
a circuit in series (Figure 6.37). When several resistances are I =
R
12 V
Lamp Lamp Lamp = _
44 Ω
= 0.27 A
C. Since the same current runs through one after the other, the cur-
rent is the same in each.
Electron current
+ –
Dry In a parallel circuit, more than one path is available for
cell the current, which divides and passes through each resistance

independently (Figure 6.38). This lowers the overall resistance
FIGURE 6.37 A series electric circuit. for the circuit, and the total resistance is less than any single
166 CHAPTER 6 Electricity 6-28