Category Archives: Science

Experiment Sunday update: Electric Circuits

[Note: Each week my geeklet and I have "experiment sunday", a brief and casual exploration of hands-on science and engineering.]

This week’s experiment was a great success. That isn’t to say that it went off without a hitch; the hitches made for more valuable learning than the experiment itself. We set out to make a simple electric circuit, but what we ended up doing was troubleshooting and learning about unstated constraints.

The experiment was a series of steps from The Science Book Of Electricity (Gulliver Books, 1991). The book has a few simple experiments like this one, described in general terms with lots of photos. There’s no theory and not much in the way of details, but the descriptions were enticing enough to catch my eight-year-old geeklet’s interest without any pushing from Dad.

The first step was to make a simple circuit using a light-bulb holder, bulb, two lengths of wire, and a battery. Trivial stuff, we lacked some of the necessary materials. The hardware store had everything we needed, but our parts were lost among all the variations of bulbs and wiring available. (Lesson: a simple parts list contains a lot of assumptions.) We eventually picked out a basic bulb holder, a set of 4W night light bulbs, and three feet of coated copper wire.

On returning home, we set to wiring up the circuit. Stripping the wire was easy with a pair of scissors, but the wire itself was so thick as to be unwieldy. (Lesson: there’s wire, and there’s wire.) With some work, we were able to screw two lengths of wire onto the bulb holder. The geeklet taped one wire end to the negative end of a battery, and I touched the other wire to the positive end. Let there be light? Well, no. (Lesson: it probably won’t work the first time.)

Now came the fun part: troubleshooting! How did our setup differ from the book’s description? The geeklet noticed a caveat in the book: “The battery must be the same voltage as the bulb.” What did that mean, though? The battery’s voltage was printed helpfully on the side: 1.5V. The bulbs, though, were listed as 4W. (Lesson: units are important.) I talked a little bit about the difference between current and voltage, and we looked more closely at the package of bulbs. No hint of voltage listed anywhere. Hmm. A quick check with Mama (who recently had to buy bulbs for one of her projects) yielded the clue we needed: a bulb’s voltage is often listed on its base.

120V. A bit of a difference, then. What to do? Improvise! (Lesson: improvise!)

We scrounged through the tool box to see if there were any other bulbs. We found a krypton bulb for a flashlight, which was listed (now that we knew what to look for) as 3.6V. Closer, but how to make up the difference between a 1.5V battery and a 3.6V light? This gave me a chance to talk about serial vs. parallel circuits, and how batteries in series will add their voltage together. 3 batteries at 1.5V made 4.5V, which should be close enough to make the bulb light up.

The geeklet found and taped together three D cells, which made an impressive battery of batteries. We dropped the krypton bulb into the bulb holder—a loose fit, but it closed the circuit if placed carefully—and wired up the rest again. Let there be light? Yes! (Lesson: persistence pays off.)

The next step was to wire in a pair of thumbtacks, spaced slightly apart on a bit of cardboard. These allowed us to bridge the gap with pins, coins, cloth, buttons, and other things that may or may not carry electric current. A tester! (Lesson: even our improvised monster circuit met the requirements of the experiment.) Once that principle was shown, we used one of the handy current-carriers (a paperclip) to fashion a contact switch. The geeklet showed off the completed circuit and switch to Mama, and we talked about applications like telegraphs and signal lights.

Writing this up, I just now realize that we re-invented the flashlight using (essentially) flashlight parts. Oh well, the process was the important thing.

The Mpemba Effect: A Good Case For Citizen Science?

I just read an intriguing article on the Mpemba effect at Skulls in the Stars. Between the history of the effect and the continuing puzzle of what causes it, this is the best example of science-as-a-process I’ve ever seen:

Mpemba made his accidental discovery in Tanzania in 1963, when he was only 13 years old and in secondary school. In spite of widespread disdain from his classmates, he surreptitiously continued experiments on the phenomenon until he had the good fortune in high school to interact with Professor Denis Osborne of the University College Dar es Salaam. Osborne was intrigued, carried out his own experiments, and in 1969 the two published a paper in the journal Physics Education.

So what did Osborne’s research show? He placed a 100 cm³ beaker filled with 70 cm³ of water on a sheet of insulating foam in a freezer, and timed how long it took for the water to freeze. For temperatures up to 20 °C, the time was roughly proportional to the temperature above freezing, up to a maximum of 100 minutes at 20 °C. For higher temperatures, however, the time dropped dramatically, down to 40 minutes for 80 °C water!

Be sure to read the complete article for the whole story, including many attempts to characterize the Mpemba effect over the years. 50 years later there still isn’t a strong consensus about what causes the effect, and in many cases it’s supposed to be difficult to reproduce.

To me, this is crying out for a citizen-science experiment with lots of participants, similar to the way Biocurious works. The experiments themselves are dirt simple (and cheap) to implement; all they really require is water, a heater, and a freezer. The rest is a matter of documenting all the (potentially) relevant variables, including the heater and freezer used, the source of the water, the type of containers, and even the geocoordinates of the experimenter. (Hey, who knows, right?)

A second generation of citizen-science experiments could then be designed based on trends in the first-generation data. The fun thing about this step is that (as Galaxy Zoo has shown) the data often suggests results that weren’t expected before it was being collected. (That shouldn’t be surprising; this is science after all.)

The point of each subsequent generation would be to build more accurate predictions of which experimental setups would or would not produce the Mpemba effect. Eventually it should be possible to make a set of statements like, “Heating 50 ml of 20 °C tap water in a 100W microwave for 90 seconds is 90% likely to reduce the time required to freeze it in a 1 m³ freezer by 35%.”

Why the citizen-science approach? I suspect that rather than trying to control all the known factors to produce the desired result, we instead want to track as many factors as possible to characterize the space of results. This particular effect will probably require a “vast multidimensional array of experiments“* to characterize properly, so enlisting a large number of citizen scientists makes a lot of sense.

Besides, each and every one of the test participants can have fun guessing at the real causes involved. Who doesn’t love a little armchair theorizing?

* Yes, I’m ‘citing’ Wikipedia. The original article cited there is inaccessible, and the rest of the Wikipedia summary is informative stuff.

an analogy for particles with spin one-half

This may seem like an odd diversion, but John asked about it just this morning so I thought I’d share with the rest of the class.

Electrons, in their secret life as wibbly-wobbly quantum particle-wavey things, have a property called spin. To quote a handy article I just ran across:

One of the things that was clear from experiments was that electron have spin. A first naive picture of an electron – this is not an accurate picture but it’s a start – is as a tiny ball with electric charge – which is what flows when a current flows in a wire. If you spin a ball of electric charge, the electric charge goes around in a circle. You effectively have a tiny current going around, and when you have a current like that you have a magnetic field – the electron becomes a tiny magnet. The presence of that magnetic effect is pictured as the electric charge spinning around. If the electron was still, it wouldn’t have this magnetic effect.

It gets better:

Among the many counterintuitive properties of the electron is the fact that it has spin one-half. This is the mathematical way of saying that if you rotate an electron through 360 degrees, it doesn’t look like it did before you started! There is no parallel for this in our everyday world – we are accustomed to being able to turn objects through 360 degrees and get them back to where they started.

Oh, but there is a parallel in the everyday world, or at least in my slightly-twisted mind. Think of it like so:

  1. Imagine a reel-to-reel film projector. Running a short length of film through end-to-end works like you think it would.
  2. Tape one end of the film to the other; now you have a continuous loop of film that repeats itself. This would correspond to a spin of 1, because it looks the same after one loop.
  3. Now tape one end of the film to the other backwards, to make a Möbius strip. The film still loops, but now it does one loop with the frames reversed left-to-right. It doesn’t repeat itself exactly until the film has looped through twice, corresponding to a spin of 1/2.

Does this mean that electrons are actually tiny loops of film? No. It only provides an analogy for this one property, and even then it might not go very far. Still, as soon as someone says “there is no X”, I have to find a counterexample. :)

on deadlines and priority: a physical analogue

deadline vs priorityLooking at my to-do list today, I noticed for the millionth time how two key attributes of a task seem to be either redundant or in conflict: its due date and its priority.

It always seemed to me that you should only need to assign one or the other. If you have a deadline, then what does the priority affect? If the item is high enough priority, isn’t the due date ASAP?

Today, though, I had a flash of insight. The due date defines how much I have to work on the item in order to get it done in time, almost like the velocity of the task. The priority, however, defines how resistant the job is to being derailed by other tasks, more like the inertia or mass of the task.

Put that way, the two values aren’t redundant at all. In fact, you can put them together to determine the overall momentum of a project, based on the combination of the deadline-driven velocity and the priority-based mass. It might even be possible to come up with a formula for determining the outcome of a collision between two tasks, but I’ll leave that as an exercise for the project manager.

Let’s hear it for the invertebrates!

The Xerces Society has a new website. Yay, bugs!

Xerces Society

I mean, seriously, just because they lack endoskeletons and have more appendages than you do is no reason to get all squeamish.

Here, we’ll start you off slowly with some really, really important bugs: native honeybees and bumble bees (one of my favorites). And they’ve got all these cool books and guidelines.