When I was thinking about what to do for the holiday special, I started thinking about my Christmas lights and thought it would be a great topic of conversation, with quite a bit of physics and electronics to boot. I also found an awesome article from energy.gov which you can dig into HERE.
You can think of your string of christmas lights as a continuous line, which starts from one prong of the plug, goes straight through all the bulbs and ends at the other prong. Its possible to think of each bulb in the string as having an input and output. The current runs in, and through a very thin piece of metal called a filament. As the electrons pass through this thin piece of metal, it emits photons, allowing the christmas light to glow.
It is possible to wire up the string of lights in two different ways. The plug can run into the input on light #1, the output of #1 connects to the input of #2, and so on. This is referred to "daisy chaining" or a circuit "in series". The problem with this configuration is that if one filament burns out, the entire circuit goes open and none of the lights will light. When the light is replaced, the entire circuit begins functioning again.
It is possible to connect the lights a different way. Imagine the positive terminal and negative terminal of the plug running as the long legs of the ladder. Each light's input and output terminals will connect to the positive and negative as "rungs" in the ladder. The downside of this is that it will take a lot more wire and effort to connect. The upside is that if a single bulb goes out, it is the only light to go out.
When I was hanging my lights this year, I removed a few lights from the beginning of the strand and when we finally plugged it in, roughly the first quarter of the strand was out. Once I replaced the bulbs, the first quarter came back into commission. The reason that only the first quarter goes out is because modern strands are a hybrid of series and parallel, which makes only portions of the lights fail.
Anyway, this is a small introduction to series and parallel circuits by way of holiday cheer. For everyone celebrating Christmas, Merry Christmas. For everyone else, Happy Holidays and we'll see you soon!
