Amateur Radio Manual/Sources of Current
We can make electrons move in a conductor by supplying energy. If we apply the energy equally, for instance, by heating it, the electrons will move randomly from atom to atom. This isn't very useful to us. We want to be able to control the direction and flow of the electrons. We can accomplish this by attaching a negative source to one end of our conductor and a positive source to the other end. We have just created a circuit. The electrons will flow from the negative end to the positive end.
As we stated earlier, heat is one method of causing electrons to flow. We can also do this by using friction, pressure (some crystals will create electron flow under pressure see piezoelectric effect), magnetism, photoelectric effect, and chemical reactions. Chemical reactions are perhaps the most common source of direct current, better known as a cell or battery.
A battery usually consists of two conductors, or electrodes, placed in an electrolyte or the chemicals responsible for reacting and producing current flow. There are a large number of different types of batteries. The common carbon-zinc flashlight battery is an example of a primary cell, that is a battery that cannot be recharged. The lead acid car battery is an example of a secondary cell, or a cell that can be recharged. Be cautious with alkaline batteries, some are primary and some are secondary types. Attempting to recharge a primary battery can be dangerous especially if the heat built up inside the cell causes it to explode.
|Carbon Zinc||primary cell||common flashlight battery|
|Alkaline||may be both||various sizes and uses|
|Mercury||primary||maintains rated voltage until used up|
|Nickel Cadmium||secondary||portable radios|
|Lead Acid||secondary||car battery|