For whatever reason, there are nine light bulbs installed in our living room ceiling (three recessed, controlled by two different switches, and six on a zigzag-shaped fixture, controlled by a single switch). I clearly think this is an excessive number, as evidenced by the fact that, a few weeks ago, I was working away happily in what I felt was a perfectly adequately lit room. The SigFig then walked in, looked up, and pointed out that most of the bulbs in the fixture had burned out. A single light bulb soldiered on amongst its burned-out brethren.
I did what any scientifically-minded person would: I started thinking about circuits.
Anything that runs on electricity contains some form of circuit. Simple circuits consist of electrical energy sources (such as batteries or home electric grids), resistors that transform that electrical energy into other forms of energy (such as heat and light), and wires to connect these elements. Circuit diagrams show how the parts of the circuit are put together.
Electric charges carry energy from the voltage source through the circuit; the flow of electric charge is called current. Current flow can only occur if there is an unbroken path from the positive terminal of the battery to the negative terminal of the battery.
In a series circuit, all the resistors are connected in a single-file path.
The resistor portion of a light bulb (the part that the current actually flows through) is generally a tightly-coiled section of wire called the filament. When a light bulb burns out, that filament literally breaks, causing a gap in the circuit. This gap prevents current from flowing; without electricity flowing to any of the bulbs, they all go out. An older, low-quality string of Christmas lights is wired in series, and that’s why the whole string goes out when one bulb burns out. This is why they used to have little devices that you could use to figure out which bulb was actually broken and in need of replacement. (While writing this post, I realized I hadn’t seen those devices in years. I did a little research and found that, while today’s Christmas lights are still wired in series, there’s usually a backup built into each bulb that keeps the rest of the strand lit in the event of bulb failure. Read more here.)
The lights over my head right now don’t all go out when one bulb burns out. They’re wired in parallel, which means that each bulb is on its own branch of the circuit.
One bulb burning out doesn’t affect the other current paths, allowing the other bulbs to stay lit.
While I was perfectly content with the single functional branch of our parallel-circuit light fixture, the SigFig went out and bought five replacement bulbs. I considered showing him the concrete math behind the increased power usage he caused, but I ultimately I decided that that was a conversation (and a post!) for another day.