Chapter 18
The Battery
There are many ways of creating an electrical potential; one of the simplest is thebattery, in which an
electrical potential is created by a chemical reaction. A battery consists of two strips of dissimilar metal
(calledelectrodes) placed in solution called anelectrolyte. The electrolyte will preferentially dissolve one of
the electrodes, leaving an electric charge on one of the electrodes and the opposite charge on the other. As
an example, consider the common zinc-carbon battery. Two electrodes—one of zinc and one of carbon—are
placed in an electrolyte of sulfuric acid. The acid dissolves a little of the zinc electrode, placing Zn^2 Cions in
solution and leaving extra electrons behind on the zinc electrode, so that it becomes negatively charged. If the
battery is not connected to anything, then the system reaches an equilibrium condition: as the zinc electrode
becomes negatively charged, it will tend to attract the Zn^2 Cions back to it are restore the zinc again. If the
battery is connected to something, the zinc ions will continue to be produced, and will start to pull electrons
from the carbon electrode, which will become positively charged. As the battery continues to be used, the
electrodes will become more and more dissolved, until one of the electrodes is used up and the battery dies.
The amount of potential difference between the two electrodes (theterminalsof the battery) depends on
the chemistry, and in particular on the two metals present. In the case of a zinc-carbon battery, the potential
between the terminals is 1.5 V. Other types of batteries will have other potential differences, as shown in
Table 18-1.
Table 18-1. Common battery types.Battery Type CTerminal Terminal Potential
Zn-C C Zn 1.5 V
Alkaline MnO 2 Zn 1.5 V
Silver oxide Ag 2 O Zn 1.55 V
Lead acid PbO 2 Pb 2.1 V
Ni-Cd NiOOH Cd 1.2 V
Ni-Zn NiOOH Zn 1.65 V
NiMH NiOOH metal alloy 1.2 V
Lithium ion Li compound Li compound 3.6 VWhen batteries are connected inseries(end to end), their voltages add. This is what you’re doing when
you put several batteries into a device like a calculator or flashlight: theCterminal of one battery is connected
to theterminal of the next. For example, you may put four size AAA alkaline batteries into a calculator,
which provides a total potential of 4 1:5VD 6 V. A 9 V battery actually consists of six individual batteries
connected in series in a single casing. A car battery consists of six lead-acid batteries connected in series, for