FUEL CELLS—ENERGY FOR A POWER-HUNGRY WORLD - Chemistry: Notable Research and Discoveries - page 142

SeriesandParallelCircuits
Imagine a row of batteries with the positive end of one bat-
tery against the negative end of the next battery. Part A of
the figure opposite illustrates this situation, which is called
a series circuit. One way of thinking about how a circuit
works is to think of the flowing charges as “falling” from
an upper level to a lower one. As the charges “fall,” they
move around the circuit, constituting a current. Although a
charge falls naturally from a high level to a low one, it will
not climb back to the high level without some help. A bat-
tery is a device that lifts an electric charge from a lower
level, or voltage, to a higher one. Batteries are needed to
boost the charge back up to the higher level—that is, to
generate a voltage—so that charges can continue to flow
around the circuit. In a series circuit, a charge encounters
the row of batteries, each of which gives it a small “eleva-
tion.” A series of batteries therefore “raises” a charge to
the sum of their voltages.
Batteries in parallel, as diagrammed in Part B, have their
own separate path. A charge fl owing in this circuit encoun-
ters only one of the batteries. The charge receives only a
little “lift”—the voltage applied by a single battery (all of which
should have the same voltage). But because of the sepa-
rate paths, the parallel confi guration can push a lot more
charges than a series confi guration, though it is somewhat
risky—if one of the batteries in a parallel confi guration fails,
it could create a “short” that draws all the current, resulting
in a fi re hazard.
In other words, voltages in series add, but voltages in
parallel do not. This principle holds true for batteries, fuel
cells, and any other source of electricity. High voltages are
sometimes required to give charges an extra push, such as
when the charge must pass through an area of high resis-