Swans on Tea

Creating Hell in a Pop Bottle

Some outstanding slow-motion combustion, thermodynamics, and safety advice. Half a gram of water turns into about a liter of Hydrogen + Oxygen. Then, boom!

[W]hen H2 and O2 burn, there is actually a reduction in the number of molecules of gas, which would, if all other conditions were the same actually produce a reduction in pressure, however the temperature of the exhaust gas is not the same, it goes from about 300K to 3000K which in a confined system would increase the pressure from about 1 to 10 atmospheres. This is getting close to the failure threshold of these bottles, and also represents a significant rate of release of energy.- caution is required, and this really isn’t something you should be trying unless you really know what you are doing.

There’s a neat effect just after the 2:00 point of this video: the pilot does a barrel-roll, and the beverage in his cup does not spill. Then, he pours iced tea into his cup while doing the maneuver. The beverage in the cup remains pretty much parallel to the support the whole time.

The physics here is the same as with a swinging bucket; one must realize that the plane isn’t simply rotating along its axis — it’s following a circular path, and there is always lift (i.e. a force) going from the bottom of the plane to the top. I recreated this (to an extent) with a clear container and some Romulan Ale (I only use it for medicinal purposes). The first frame is where I was holding the bottle, so it’s at rest. The liquid is clearly at an angle to the container, and is parallel with the floor.

And the second is while the bottle is a freely swinging pendulum, and you can see the liquid is now level with the bottom of the container.

Blah, blah, blah. Oh, balls. I was working on this a while ago and now find that Rhett has a post up about it, though not following the same path I was going to take. Pouring tea in a plane – upside down, where he’s worked out all of the physics, with diagrams and pictures with circles and arrows and a paragraph under each one explaining what it is. So I’ve abandoned my v/2 (half-fast) explanation in favor of a link to his.

On properly heating your pan

How to keep food from sticking in a stainless-steel pan. The first explanation about pores biting into the food sounds hokey, but then we get to the Liedenfrost effect, which is demonstrated in the video.

The water “hovering” over the stainless steel pan like mercury happens due to the phenomenon known as the Leidenfrost effect. You can read more about it on wikipedia, but the basic idea is this: at a certain temperature known as the Leidenfrost point (roughly around 320˚F for water, but varying with surface and pressure), when the water droplet hits the hot pan, the bottom part of the water vaporizes immediately on contact. The resulting gas actually suspends the water above it and creates a pocket of water vapor that slows further heat transfer between the pan and the water. Thus it evaporates more slowly than it would at lower temperatures. At the proper temperature, a similar effect happens with the food you place in the pan, preventing the food from sticking.

Optical Trapping and the Momentum of Light

The explanation talks about the wave-particle duality, but I think that’s a distraction. This is a dipole force phenomenon; the beam’s intensity is greatest at the center, and where the focal point occurs, as shown in the drawing at the end of the post. This gives rise to a gradient in the electric field. If you put a dielectric particle in this region, it will feel a force in the direction of the field maximum, or toward the highest intensity of light.

Dot Physics has been assimilated by the Borg, but (unsurprisingly) Rhett, through his blood, sweat and tears, continues to post cool stuff. This time, it’s an angular momentum demo and explanation. I’ll post the video here as a teaser; go to the post for the explanation

I agree with a couple of commenters there — I also like the demonstration where you spin the wheel and sit on a rotatable stool, and then reorient the wheel.

Making light rays in the classroom

Laser level as the source, and lenses using gelatin as the medium, to demonstrate Snell’s law.

The Boy Who Harnessed the Wind: Persistence, Jury-Rigging, and Ingenuity Against All Odds

A Malawian youth, whose family could not afford his school tuition, learns some physics and builds a windmill to generate electricity for his village.

William scoured trash bins and junkyards for materials he could use to build his windmill. With only a couple of wrenches at his disposal, and unable to afford even nuts and bolts, he collected things that most people would consider garbage-slime-clogged plastic pipes, a broken bicycle, a discarded tractor fan-and assembled them into a wind-powered dynamo. For a soldering iron, he used a stiff piece of wire heated in a fire. A bent bicycle spoke served as a size adapter for his wrenches.

William now has a blog

There’s a fairly well-known science question which asks

How does the amount of energy per gram of TNT compare with the energy per gram of a chocolate chip cookie?

I’ve discussed before why I think the answer should be, “About the same,” if you’re doing a first-order approximation, and depending on what options you give for an answer.

We’ve also visited the energy content of a candy bar. So along that vein (or clogged artery) we have

Recrystallized Rocketry

which tests sugar as an ingredient in rocket propellant, in the form of pixy sticks, creme from an oreo cookie, and a Snickers bar.

Creme filling from one Double Stuff Oreo weighed out at 5.7 grams. Mixed with 11.4 grams finely powdered KNO3 (FireFox) with mortar and pestle.

Result: Burns very sluggishly, requires some assistance from torch flame

Adjustment: Added 0.2 grams red iron oxide (Fe2O3) to catalyze the burn. Works much better.

Throwies are simple LED circuits — the LED and a battery, with an optional magnet so they will stick to ferromagnetic materials.

Evil Mad Scientist Laboratories does a pretty exhaustive analysis on the circuits, looking at battery life and potential danger of these simple circuits. Some thoughts on throwies

This data shows a couple of interesting things. First is that the power-law model seems to hold fairly well. Second, the power function that pops out is not very different from that of the data from only the first half hour– integrating both out to 24 hours gives two answers– 150 mAh and 186 mAh –that differ by only 25%. The estimate based on the long data record (150 mAh) is the more accurate one, but this does suggest that we should be able to use the data from the first half hour alone to get a fairly good “factor of two” estimate of the performance over 24 hours.

Melt and Blow CD Bubbles

Learn how to blow bubbles with your old plastic CDs in this edition of ‘It’s Effin Science.’

Physics Buzz: How to Build a Spectrometer with Just Three Household Items

How to build a cereal box spectrometer.