More about batteries

Batteries come in many sizes, and we use many different voltage levels. How did this come about?

The standard flashlight battery is about 1.5 V. This voltage is established by the choice of chemical reaction that is used. Batteries having this voltage have a very simple "single cell" internal construction -- the bottom and sides are one conducting piece and reactions of one kind take place at the inside surface; the top and a central post inside are another conducting piece where a different chemical reaction takes place; and the region between these is filled with a paste of chemicals that participate in the chemical reactions or keep the two reaction regions separate. The different kinds of batteries (Alkaline, Nicad, mercury, lithium, zinc-air, ...) have different reactions and slightly different voltages.

A battery is a portable source of energy. The amount of stored energy depends on the chemical reaction that is involved and on how much of the reacting chemicals are in the battery. The difference between AAA batteries, AA batteries, C batteries, and D batteries is that they store different amounts of energy. For example, one brand of batteries claims that its batteries perform as follows:
A D cell (139 g) will deliver about 63,000 J
A C cell (69 g) will deliver about 33,000 J
An AA cell (24 g) will deliver about 11,000 J
An AAA cell (11 g) will deliver about 5,000 J

Single-cell batteries with higher voltages do not exist because they would contain dangerously reactive materials. The 9 V batteries used in portable radios and the 12 V batteries used in automobiles are not single cells -- they are many cells connected in series. Making a higher voltage this way does not increase the amount of energy that is stored, but makes it possible to get the energy out faster (i.e. it is useful where high power is needed).

Around the house we sometimes use low voltages (6 V or 12 V for doorbell circuits or some kinds of exterior lighting), especially where electrical safety is a concern. The standard house circuit is 110 V to 120 V, but with a simple rewiring can provide 220 V to run large airconditioners, stoves, and other kinds of heaters. The distribution lines within a city use several 1000 V, while power is moved from the power plant to the city by transmission lines that are 100,000 V or more.

More about capacitors

The diagram for a capacitor tries to describe what is inside. Some capacitors actually look like this: a pair of conducting plates with a small separation between them. The "plates" can be large pieces of metal foil, separated by insulating paper, and then rolled up to make it compact. However, it would take many square miles of foil to make one farad this way; the typical value for a capacitor of this type is 1/1,000,000 farad.

The section on the sources of electrical energy