Swans on Tea

A lot of articles compare micron-ish thicknesses to that of a human hair, which makes me wonder if NIST has a standard human hair used for calibration.

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This is how you fill a water bottle with fire

This is a quick experiment that you can do at home. By that, I mean it is physically possible to do at home, not that it is actually a good idea to do at home. Yes, even if you do promise not to sue.

Sounds like something to do in the back yard at the next family gathering.

Physics demonstrations: vortex cannon!

So how does this “vortex cannon” work, and what does it demonstrate? There is a surprisingly amount of physics and history behind such vortices, and they can be a lot more powerful!

I have an airzooka air cannon as part of my office armament, and it does an admirable job of shooting projectiles of air. However, one of my colleagues had expressed an interest in upping the ante, so we took it upon ourselves to make one using a 5-gallon bucket.

We followed the general path of the steps outlined in an instructable, though we substituted light-duty bungee cords for the elastic.

Cutting out the end of the bucket is a tad messy with all the plastic shavings. In cleaning up my clothes I employed a version of a trick we used in the navy when getting ready for an inspection — a few windings of tape on your hand, sticky-side out, does a good job at grabbing lint. Or plastic shavings.

Another deviation from the basic instructions was that since we used bungee cord, we drilled holes in the bucket for the anchor end, and zip-tied the other ends together, having removed the hooks. That also allowed us to use 3 lines.

The handle is temporarily an optics post. The bucket is just strong enough for it, so we need to add a second layer of something to shore that up and put both a proper handle on the body and a proper grip on the elastic. No metal, though, since a failure of some sort could be very bad news if this became a slingshot. We should be ready for our picnic next week.

DIY Magnetic Putty

By adding a ferrous component to an already wacky toy we can keep all characteristics of the original putty, but now have the additional dimension of magnetism! I’ve seen magnetic thinking putty for sale on other websites, but I’ll show you how you can make your own for a fraction of the price and in about 20 minutes.

These “Buckeyball” magnets look like fun (but IMO a tad pricey for a good-sized collection). The towers are just for structural integrity; it’s the ones on the battery itself that are responsible for the motor action.

Spaces of Banana Control

In addition to precise temperature control, the ripening process also depends on atmospheric design. Over a 24-hour period, each roomful of bananas is gassed with ethylene, a plant hormone that accelerates ripening (and is also, curiously, the most produced organic compound in the world).

I learned about the role of ethylene a few years back, after wanting to ripen some bananas quickly and being told that putting them in a paper bag would do the trick. I did a test with one banana in the bag and one outside as a control, and the one in the bag did indeed ripen faster. And the internet told me that it was the ethylene that did the ripening, which is why “one bad apple ruins the lot” — as fruit like bananas and apples ripen, they give off ethylene, and that accelerates through the process, so one ripe piece cause the others to ripen pretty quickly. The paper bag concentrates the gas relative to it being out in the open and causes more ripening. Curiously, a plastic bag did not work — this could be because humidity inhibits ripening, and it’s possible the permeability of the paper bag allows water vapor to escape but still retain some ethylene to do its magic chemistry.

You can’t wow the family for Thanksgiving, but maybe the gang is still around, or you can store these away for a future family gathering.

From 10 quirky science stunts

One step better would be an explanation for the inevitable “How does that work?” (or similar) question that arises, so here’s a quick explanation of the physics-y ones. The ping-pong ball tends to stay in the column of air owing to Bernoulli’s principle — pressure decreases the faster the air moves, so there is lateral pressure gradient which gives you a restoring force. The coin and card trick works because the force on the card is probably larger than the force on the coin, and even if they are comparable, card is less massive so it undergoes a much larger acceleration. Consequently, it moves away much faster than the coin, so the coin doesn’t get displaced very far.

Jabbing a sharp pencil through a bag tends to form a hole that conforms to the pencil, and water seals small gaps at low pressure owing to the surface tension. Straws are stronger along their length and covering then end with your thumb means air can’t escape; the increased pressure from the potato being forced into it makes it even stronger against collapsing. Matches burning heat up air and make it expand, so the density is lower than outside. When the matches go out the air cools, lowering the pressure. The egg provides a good seal, and the outside pressure forces the egg inside.

The others have elements of chemistry biology/physiology. So I will pass on those explanations.

There’s a DIY explanation at Cute video of kids DIYing their own penny battery, where I saw the video, but I found a higher-resolution copy, along with some other DIY battery projects, in this pdf

The how-to-build article, which includes a link to a paper describing the math and physics behind the phenomenon.

I’ve been growing salt crystals. Unlike a pet, salt crystals won’t tear up the furniture, do their business on the carpet or need to be walked in the rain. Really I was testing to see if I could do this as a time-lapse project and wanted to test how long it would take. And it has fulfilled Hofstadter’s law: it always takes longer than you expect, even when you take into account Hofstadter’s law.

I took the standard approach of heating some water and dissolving a bunch of (uniodized) salt in it, letting it cool and pouring it into a beaker. And I waited. And waited. Finally, after a few weeks:

One thing I should have anticipated is how long it takes. There was some salt left in the pot when I poured the solution into the beaker, so I thought the crystallizing would begin quickly, but it didn’t. Plus, the evaporation was slow. I knew that boiling point elevation and freezing point depression are colligative properties (they depend on the number of dissolved atoms) so I reasoned that evaporation rate should be as well. And it is — there is Raoult’s law

The vapour pressure of an ideal solution is dependent on the vapour pressure of each chemical component and the mole fraction of the component present in the solution.

The vapor pressure of salt is very low, so as its concentration rises the total vapor pressure of the solution drops, and so does the evaporation rate. Rather than evaporating fully, one would expect it to reach an equilibrium with the atmosphere which would depend on the humidity. In fact, if the salt concentration were high enough, one might expect it to dehumidify the air, which is precisely what some people do. Salt concentrations are used in dehumidifiers — you expose the solution to the air and let is “grab” some water, then heat it up (often solar, for a completely passive system) to let the excess water evaporate, and cool it again in a cycle. Or you can have a solution with some solute left in the container, and as you “grab” the water, you dissolve more of the salt, so it can continue doing its job as long as there is more salt that can dissolve.

Another unexpected event in all of this is that I was getting salt crystallizing on the surface of the water. A small “raft” would float there until it grew massive enough that it would sink (or someone poked it). I had thought the crystallization would just build on any crystal that started up, but there are lots of small cubes rather than just a few large ones. Still, the biggest cubes are perhaps 10-20x larger on a side than the original grains.

Cool Things You Can Do With a Blue Laser: Reflection vs. Fluorescence

[W]hat is going on here? This isn’t just reflection, this is something else. How do I know? If it were just reflection, the only color would be green (same as the incident light). This is an example of fluorescence. Basically, in fluorescence, the light doesn’t just oscillate the electrons. The light excites the electrons to a higher energy level.