Creating a Circuit
In Investigation 1, which uses one bulb, electrons come out of the negative end of the left battery to begin their journey through the circuit. They travel through clips 2 and 1, the wire, clips 5 and 6, the bulb, clips 7 and 8, the paper clip, clips 4 and 3, and back into the positive end of the right battery. This path is called a complete electrical circuit.
Batteries are rated in volts, and volts are a measure of the energy that electrons have when they leave the battery. As electrons travel through the complete electrical circuit, we’ll assume that the only place they lose energy is in the bulb, where their energy is transformed to the light and heat given off by the bulb. The process of energy transformation resists the electron flow through the bulb, and the bulb is said to have electrical resistance. It is very important to realize that while an electron loses energy on its trip around the circuit, the electron itself doesn’t leak out of the circuit or disappear. For every electron leaving the battery to enter the circuit, another enters the battery from the circuit.
Connecting Bulbs in Series
In Investigation 2, three bulbs are connected in a way that forces electrons to pass through all the bulbs to get back to the battery. This is referred to as a series connection. When three bulbs are connected in series, each bulb glows less brightly than one bulb alone. This is because three bulbs provide more total resistance than one bulb. This decreases the total flow of electrons, called the current, and results in a smaller amount of energy being transformed to light and heat. Additionally, each bulb only gets one-third of this reduced amount of energy.
When you remove one bulb in a series circuit, the others go out. It doesn’t matter which one you remove. When you create a gap in the circuit the electrons can no longer flow.
Changing the Voltage
When you move one of the battery clips to the nail between the batteries in Investigation 3, the circuit is powered by one battery instead of two. The total amount of energy available is cut in half, so the bulb gets dimmer.
Connecting Bulbs in Parallel
In Investigation 4, the configuration is changed so that the electrons arriving at clip 1 now have two alternative paths to get to clip 6. Bulbs connected in this fashion are said to be connected in parallel. If the two lightbulbs are identical, half the electrons will go through one bulb and half through the other. The electrons will then recombine into one flow at clip 6 and return to the battery.
When two or more bulbs are connected in parallel, all the bulbs will be as bright as a single bulb. This is because each additional bulb provides another path for electron flow, and each electron only moves through and loses energy in one bulb. The alternative paths actually reduce the total resistance in the circuit. In fact, the resistance of two identical bulbs in parallel is half the resistance of a single bulb, allowing twice as much current to flow in the circuit. When one of the bulbs in the parallel circuit is removed, there is still a closed loop, so current flows through the other bulb and it stays lit.
From these experiments, you’ll be able to see that removing any one of the bulbs in a series circuit makes all the bulbs go out, but removing one or more of the bulbs in a parallel circuit has no effect on the remaining bulbs. You’ll also discover that the more bulbs you place in series, the dimmer each one gets, whereas adding more bulbs in parallel has no effect on the brightness of any of the bulbs.