Explaining the Mundane: Holiday Special

Merry Christmas!‘Tis the season to celebrate life with one another by giving gifts, and of course, pondering mundane scientific phenomena. For this weeks’ post, it’s another installment of “Explaining the Mundane” but with a slight seasonal theme.

Why do all of the lights on a chain of Christmas lights go out when only one bulb is burnt out?

Just like Clark Griswold, many holiday decoration enthusiasts have experienced a fair share of tangled lights, broken bulbs, and lack of juice to illuminate a fantastic outdoor display that would rival ballpark stadium lights. One common issue that Christmas decorators around bound to run into eventually is the dreaded burnt-out bulb – one bulb somewhere causing the entire chain (or sometimes just part of the chain) to die out like the Christmas spirit after seeing next month’s credit card bill. You can search along the whole strand of lights for that single little light, or you can just throw it out and get a new frustration-inducing mess of wires and filaments. But what’s actually going on when this one little light goes out? Well, science can help explain it!

Analogies between electricity and fire could be easily made; humans discovered that both were incredibly dangerous and difficult to control (probably through injurious trial-and-error), but once a basic mastery was achieved, it was obvious that these tools were great for lighting dark places and keeping things warm. Early scientists sought to understand electricity, and through their experiments, they discovered another good basic element analogy for electricity: water. Flowing water can describe many aspects of basic electricity; most relevant to this topic is current. (Of course, there are limitations to this analogy, as described here)

If you think about a mountain river that starts high above sea level, it’s easy to imagine which way the current of the river is going to flow. Rain that collects on top of the mountain will start flowing down the hill, pulled in the direction that gravity takes it. This is similar to how current flows in a circuit. Electricity starts at a high point (although unlike gravity, it’s not actually physically high; it just would rather be at a different position energetically) and flows to a lower point. When the stream narrows, the river slows down. When a circuit experiences a resistance, a similar effect occurs for electricity flow. You can split the river into multiple streams, and you can have multiple points in the river where it narrows. The same situations can occur in electrical circuits.

simplecircuit

Let’s consider a simple circuit with a light bulb. The light bulb can act like a narrowing point in the river; the current will decrease when the light bulb is lit. If you hook up two wires to a light bulb and hook those wires up to a battery, like shown in the picture, you get a complete circuit – a Christmas light, just like that. Now, how do you connect a chain of Christmas lights together? You could connect each one to the battery, like shown in the next image. Or you could connect them in a row, like shown in the image that follows.

inparallel

inseries

There are tradeoffs for doing it each way. In the first case, you would have to engineer a way to manage each wire going to the bulb. However, if a bulb burnt out and opened the circuit (shown below), you could still get electricity flowing through the entire system and the rest of the bulbs would be unaffected. If you built your Christmas lights like the second image, it would be cheap and easy, but all of the lights would go out if one light went out. (The red X represents an open circuit and thus blocked current in both pictures.) Manufacturers like things cheap and easy, and with a few modifications, have mostly stuck with the second version: Christmas lights are typically wired in a row.

inparallelburnt inseriesburnt

There are some interesting engineering tricks that have been implemented to try to avoid burning out an entire chain of lights just for one tiny light bulb. One of these tricks is to put in a shunt wire. This is a tiny wire that can complete the circuit when a bulb burns out. So instead of losing all of your lights or having to search for that single burnt out bulb, the shunt wire fills in where the circuit has been opened and keeps subsequent lights lit. The shunt wire doesn’t appear to be very robust, and so problems with your Christmas lights may continue to persist.

Are no two snowflakes alike?

Unique,_snow_flake

We are like snowflakes, different in our own beautiful way – so the adage goes. But are we?? I’ll leave that to psychologists and philosophers. Instead, I want to pick at the premise of the quote: snowflakes are unique. Before I go into that, I think it’s interesting to mention that you can be any sort of scientist that you want to be. There’s a small niche of people who call themselves bubble scientists. In Teas, there is the Columbia Scientific Balloon Facility for unmanned balloon scientists. And there are a few passionate explorers trying to understand snowflakes.

The classic microscopic image of a snowflake that comes to mind for most people is probably the 6-pointed shape with small branches sticking out on each point, like the leaves of a fern emerging from a central hexagon. It’s not a far stretch to imagine that the formation of the points and the dendritic features that extend off each point must form random size, shape, and number distributions due to environmental factors.

For example, a snowflake forms in the atmosphere when water vapor in a cloud drops below the freezing point and crystallizes. There is a tiny speck of dust or another small crystal that acts as a “nucleation point”. The water vapor can latch onto this particle and start forming a solid crystal. But the shape and structure of the crystal is highly dependent on the exact temperature and how much water is available to solidify. At just around freezing, the classically-shaped snowflake will form. If there isn’t enough water available, a solid plate might form instead. At lower temperatures though, columns and needles are more likely to form. Then, as the temperature drops more, the snowflakes start to look more like plates again. And the kicker: snowflake researchers have very little idea about why this happens, but they’re working hard to figure it out. It might sound crazy that snowflake research is justified, but other endeavors is similar to this has been incredibly important. For example, tin changes its arrangement when the temperature is lowered, making it structurally inadequate – important to know for scientists and consumers alike.

So are snowflakes unique? Well, it’s semantic. “Snowflake” and “unique” can have different definitions. Typically, snowflake scientists would say that a snowflake is an ice crystal grown from water vapor. It’s easy to say that on a molecular level, snowflakes are unique. The individual molecules (maybe billions or more) are moving around, constantly changing neighbors and shape and position. Snowflakes are definitely unique in this picture. As you zoom out farther and farther, it’s difficult to make distinctions. Even under a microscope, some snowflakes might look exactly alike. But when you get down to the stuff they’re made of, each snowflake can claim to be unique – and maybe that’s true for people, too.

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