Category Archives: Physics

All That Glitters

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What causes the colors of metals like gold?

If an energy level (like the 3d band) holds many more electrons (than other energy levels) then the excitation of electrons from this highly occupied level to above the Fermi level will become quite important. Gold fulfills all the requirements for an intense absorption of light with energy of 2.3 eV (from the 3d band to above the Fermi level). The color we see is yellow, as the corresponding wavelengths are re-emitted. Copper has a strong absorption at a slightly lower energy, with orange being most strongly absorbed and re-emitted. In silver, the absorption peak lies in the ultraviolet region, at about 4 eV. As a result, silver maintains high reflectivity evenly across the visible spectrum, and we see it as a pure white. The lower energies (which in this case contain energies corresponding to the entire visible spectrum of color) are equally absorbed and re-emitted.

Touch My Monkey

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The story of the Gömböc

To give it its full mathematical description, a Gömböc is a three-dimensional, convex and homogeneous object with exactly one stable point of equilibrium and one unstable point of equilibrium. Requiring it to be homogeneous amounts to saying that you’re not allowed to cheat: the material from which the Gömböc is made has to be uniform throughout, so you’re not allowed to use weights, as those found in roly-poly toys, or other irregularities to get the Gömböc to self-right. Convexity means that the Gömböc is not allowed to bulge inwards, in other words, the straight line connecting any two points on the Gömböc has to lie entirely within the Gömböc. It’s easy to create a non-convex shape with one stable and one unstable equilibrium point, hence the restriction to convexity.

I have this mental image of Dieter describing the Gömböc. I don’t know why.

Your three-dimensional, convex homogeneity has grown tiresome. Now is the time on Sprockets vhen ve dance!

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Physics Buzz presented their Friday Fermi Problem, well, last Friday:

Assuming you’re not in a big lecture hall and the professor shuts the door at the start of class, how long does it take for you and your classmates to deplete the oxygen enough to feel it?

They have presented the answer, and there is some additional commentary at Fine Structure about the problem of CO2 buildup (from Rhett of Dot Physics; which just supports my suspicion that physics bloggers are the main readers of physics blogs)

Pink and Green

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Pink, green

There is no spoon green dot. It’s an afterimage from seeing the pink.

Things to add — if you look away, you’ll see green afterimages. Also, if you stare at any of the pink dots for a while, they (or the rest) will still tend to disappear. When you stare at one point, images can fade, especially if they are dim and away from the point at which you are looking, which is a problem in astronomy — you’re looking at a dim star, and it has a tendency to just disappear. This, I’ve learned, is called Troxler’s fading (the wikipedia page uses this illusion as an example). We normally don’t experience this, because we tend to move our eyes a few times per second.

Update: The “pink, green” link is dead, so try this one : Lilac chaser

Were They Sawn if Half, Too?

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Mice Levitated in Lab

Scientists working on behalf of NASA built a device to simulate variable levels of gravity. It consists of a superconducting magnet that generates a field powerful enough to levitate the water inside living animals, with a space inside warm enough at room temperature and large enough at 2.6 inches wide (6.6 cm) for tiny creatures to float comfortably in during experiments.

A Hairy Proposition

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Teenager invents £23 solar panel that could be solution to developing world’s energy needs … made from human hair

Color me skeptical. The story, of course, is very skimpy on the science, but let’s look at this. The claim of “9 V (18 W)” is really hard to believe, because P=IV means 2 A of current flowing through the hair, though that will be divided up. Still, the diameter of a hair is thinner than AWG 32 wire (at about 200 microns), which has a current limit of less than 0.1 A for power transmission, and that’s for a good conductor. Hair? Not so much. The pictures show a grid of interconnected hair, which doesn’t have all that much area, so capturing any more than a small fraction of the few hundred W/m^2 of insolation is not in the cards. A 20 x 20 grid at less than 0.2mm per hair is just a few square mm of hair — it can only get you a fraction of a Watt.

Question: why don’t we have our hair generating electricity like this while it’s attached to our heads?

At best, somebody dropped a prefix representing several orders of magnitude somewhere. At worst it’s a scam.