Students know how to design and build simple series and parallel circuits by using components such as wires, batteries, and bulbs.

Electricity will only flow if there is a complete circuit. For safety reasons, students experiment with electricity using batteries as the power source rather than wall AC electricity. Simple circuits generally involve one or more batteries, wires to make connections, and light bulbs. Sometimes a simple circuit will also include a switch to make or break the connection.

The bulb is actually part of the circuit. An electric light bulb has a thin wire inside it (the filament) that becomes very hot when electricity passes through it. The glowing hot filament gives off the light that we see and use. When a light bulb burns out, the filament has broken due to wear and tear from the heat. The light bulb no longer completes the electric circuit, and that is why the light bulb is “dead,” no longer giving off light. Illustrations that show the light bulb filament as part of the electric circuit help students understand how light bulbs work and that the light bulb is part of the electric circuit.

The simplest kind of circuit is a series circuit. There is only one way to hook up a series circuit. Place the light bulbs one after the other. As the California Science Framework points out (page 57), “the series circuit is like a circular road that has no intersections; a series circuit has only a single path, and all components must carry the same current.” If one light bulb goes out, then all the light bulbs do not work because the only possible circuit has been broken.

In contrast to a series circuit, there are many ways to construct parallel circuits. If you are using light bulbs, the best way to test if you have a parallel circuit is to set it up so that all the bulbs are lit. Now unscrew one of the bulbs. If the other bulbs remain lit, then the bulbs were on parallel circuits. If another bulb goes out, then it was connected in series with the bulb that had been unscrewed.

It also helps to practice drawing diagrams of series and parallel circuits, and to trace the circular flow of the electricity. This is particularly true for parallel circuits because these can look very different from each other. With parallel circuits, you should always be able to trace a complete circle from one end of the battery through a bulb (call it Bulb A) and back to the other end of the battery without having to go through a different bulb. If you have to go through a different bulb (call it Bulb B), then Bulb A and Bulb B are connected in series with each other. The diagram below shows two setups that will give the same results. Each setup has three bulbs, two of which are connected in series with each other, while the third bulb is connected in parallel with the other two.