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By Root 1567 0

doesn't happen all by itself. We can't just wire up any old bunch of stuff and expect some electricity to happen. We need something to precipitate the movement of electrons around the circuit. Looking back at our diagram of the no-frills flashlight, we can safely assume that the thing that begins the movement of electricity is not the wires and not the lightbulb, so it's probably the batteries.

Almost everybody knows a few things about the types of batteries used in flashlights:

They're tubular in shape and come in different sizes, such as D, C, A, AA, and AAA.

Regardless of the battery's size, they're all labeled "1.5 volts."

One end of the battery is flat and is labeled with a minus sign (–); the other end has a little protrusion and is labeled with a plus sign (+).

If you want your appliance to work right, it's a good idea to install the batteries correctly with the plus signs facing the right way.

Batteries wear out eventually. Sometimes they can be recharged, sometimes not.

And finally, we suspect that in some weird way, batteries produce electricity.

In all batteries, chemical reactions take place, which means that some molecules break down into other molecules, or molecules combine to form new molecules. The chemicals in batteries are chosen so that the reactions between them generate spare electrons on the side of the battery marked with a minus sign (called the negative terminal, or anode) and demand extra electrons on the other side of the battery (the positive terminal, or cathode). In this way, chemical energy is converted to electrical energy.

The chemical reaction can't proceed unless there's some way that the extra electrons can be taken away from the negative terminal of the battery and delivered back to the positive terminal. So if the battery isn't connected to anything, nothing much happens. (Actually the chemical reactions still take place, but very slowly.) The reactions take place only if an electrical circuit is present to take electrons away from the negative side and supply electrons to the positive side. The electrons travel around this circuit in a counterclockwise direction:

In this book, the color red is used to indicate that electricity is flowing through the wires.

Electrons from the chemicals in the batteries might not so freely mingle with the electrons in the copper wires if not for a simple fact: All electrons, wherever they're found, are identical. There's nothing that distinguishes a copper electron from any other electron.

Notice that both batteries are facing the same direction. The positive end of the bottom battery takes electrons from the negative end of the top battery. It's as if the two batteries have been combined into one bigger battery with a positive terminal at one end and a negative terminal at the other end. The combined battery is 3 volts rather than 1.5 volts.

If we turn one of the batteries upside down, the circuit won't work:

The two positive ends of the battery need electrons for the chemical reactions, but there's no way electrons can get to them because they're attached to each other. If the two positive ends of the battery are connected, the two negative ends should be also:

This works. The batteries are said to be connected in parallel rather than in series as shown earlier. The combined voltage is 1.5 volts, which is the same as the voltage of each of the batteries. The light will probably still glow, but not as brightly as with two batteries in series. But the batteries will last twice as long.

We normally like to think of a battery as providing electricity to a circuit. But we've seen that we can also think of a circuit as providing a way for a battery's chemical reactions to take place. The circuit takes electrons away from the negative end of the battery and delivers them to the positive end of the battery. The reactions in the battery proceed until all the chemicals are exhausted, at which time you throw away the battery or recharge it.

From the negative end of the battery to the positive end of the battery, the electrons