304 DIY Science: Illustrated Guide to Home Chemistry Experiments
SBSTITUTIU oNS ANd modIfICATIoNS- If you do not have patch cables with alligator clips, you
may substitute lengths of ordinary copper wire (such
as from a discarded lamp or appliance cord or from a
discarded electronic device). Use wooden clothespins
to clamp the wires to the electrodes, as shown in
Figure 16-5. - You may substitute another citrus fruit, such as
oranges or limes, for the lemons. - You may substitute electrodes of different metals
for the copper and/or magnesium electrodes. If you
substitute different metals, choose two metals with
standard reduction potentials that are as far apart as
possible. (Aluminum is a reasonable substitute for
magnesium, and much easier to find.)
In everyday life, the words “cell” and “battery”
are often used interchangeably. For example,
most people refer to AA cells as “AA batteries.”
But in chemistry the terms “cell” and “battery”
have specific meanings. In the preceding
experiments, we’ve been working with
cells, which are self-contained or separated
containers that use an electrochemical
reaction to produce electricity. A battery is an
interconnected group of two or more cells.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ digital multimeter (dmm)£ patch cables with alligator clips (see Substitutions
and modifications)£ sharp knife£ steel wool or sandpaper£ lemon (3)A battery may be constructed by connecting cells in series, £ electrodes, copper and magnesium (3 each)
which is to say, connecting the anode of one cell to the cathode
of the next cell in a daisy chain. In a series battery, the voltages
are additive. For example, if you construct three cells, each of
which provides 1.2V at a current of 50 mA, and connect those
three cells in series, the resulting battery provides 3.6V (1.2V · 3)
at a current of 50 mA. The advantage of a series battery is that it
provides higher voltage than an individual cell can provide.
Conversely, a battery may be constructed by connecting cells
in parallel, which is to say, connecting the anodes of all of the
member cells together and the cathodes of all of the member
cells together. A battery connected in parallel has the same
voltage as any of the individual cells, but the current is additive.
For example, if you construct three cells, each of which provides
1.2V at a current of 50 mA, and connect those three cells in
parallel, the resulting battery provides 1.2V at a current of 150
mA (50 mA · 3). The advantage of a parallel battery is that it
provides higher current than an individual cell can provide.
In theory, it’s also possible to construct a hybrid series/parallel
battery that contains a group of series batteries connected
in parallel (or, another way of looking at it, a group of parallel
batteries connected in series). Such hybrid batteries provide
both higher voltage and higher current than the individual cells
provide. In practice, though, when both higher voltage and
higher current are needed, it’s more common to construct a
series battery (to boost voltage) whose electrodes are physically
large with high surface areas and can therefore provide higher
current directly. A 12V car battery is an excellent example of
such a battery.
Another important aspect of a battery is its internal resistance.
Voltage, current, and resistance are interrelated, as stated by
Ohm’s Law: E = I · R
LABORATORY 1 6.6:
BUILd A BATTERy
where E is voltage in volts (V), I is current in amps (A), and R
is resistance in ohms (Ω). At a particular voltage, the amount
of current in a circuit is determined by the resistance of the
circuit, including the internal resistance of the battery. Higher
resistance means lower current, and vice versa. For that
reason, battery designers take great pains to minimize internal
resistance.In this laboratory session, we’ll examine the voltage, resistance,
and current of a single cell. We’ll then use multiple cells to
construct batteries in series and parallel, and examine the
characteristics of those batteries.