Backreaction: Counting Atoms in a Sphere
An overview of the projects that are trying to redefine the kilogram to come up with a better standard.
Related: Round as a Baby’s . . . Nodule
Category Archives: Experiments
The Rogue Nano Rumba-Line Dancer
Nobody can see the one at the end of the line, and (s)he’s doing something different.
Nanophysicists find unexpected magnetic effect
In new research appearing this week in the journal Nature, physicists at Spain’s University of Alicante and at Rice University in Houston have found that single-atom contacts made of ferromagnetic metals like iron, cobalt and nickel behave very differently than do slightly larger versions that are on the order of the devices used in today’s electronic gadgets.
“We’ve found that the last atom in the line, the one out there on the very end, doesn’t want to align itself and behave like we expect it to,” said study co-author Doug Natelson, associate professor of physics and astronomy at Rice. “What this shows is that you can really alter what you think of as a defining property of these metals just by reducing their size.”
Doug tells the story of the collaboration and promises a post on the science.
Update: And has posted it
There Are 2 Kinds of People in the World …
Those who divide the people in the world into two types, and those who don’t.
Or, you can divide experiments up by classifying them as edible or inedible: Edible/Inedible Experiments Archive
What Goes Up Must Come Down
The PRL from the ArXiv paper I linked to in Follow the Bouncing Atom has been published.
Phys Rev Focus has a story on it
Fun with Magnets
I’ve done several of these, including a version of the homopolar motor. The eddy current damping is fun, too — you can make nonmagnetic metals react to magnets by inducing current flow in them. Lenz’s law.
No Grace Under Pressure
Atmospheric pressure is about 10 N/cm^2, but there are a whole lot of square centimeters on that tanker — the more familiar unit is N/m^2 (Pascals), where 1 atmosphere is 101325 Pa (or possibly even more familiar 14.7 psi).
Various sites showing this have claims about this happening after the tanker had been heated (from steam cleaning), and all the valves shut while it was still hot. One claims frozen (perhaps they mean liquid?) nitrogen being added. Plausible? Heating the tanker to 373K and then letting it cool to ambient should drop the pressure by 0.20 atmospheres. A tanker that’s 2 meters in radius and 10 m long has a surface area of 125.6 m^2, for a total force of 12.7 MegaNewtons. 20% of that is a lot. I don’t think the liquid nitrogen is strictly necessary, but would add to the effect.
The heat of vaporization of liquid nitrogen is 5.56 kJ/mol and its specific heat capacity of the gas is 29 J/mol-K. The volume of 125.6 m^3 means 5600 moles of an ideal gas, requiring 160 kJ/K to cool it down. Each liter of liquid nitrogen (29 moles) takes about 160 kJ to boil off, and then another .84 kJ per degree as the gas heats up from 77K. So ten liters of liquid nitrogen dumped into it will cool it about 20-25 degrees, depending on the starting point of the tanker. So that won’t hurt, but what’s probably more important is that the tanker was built to withstand some pressure difference and we see the catastrophic failure when its critical pressure difference is exceeded. Unlike the kind of test you can easily do with a can where you boil some water inside, seal it and watch it crumple as it cools, because it wasn’t designed to withstand and significant pressure difference.
Bacon: Is There Anything it Can't Do?
Constructing a bacon-plasma torch which can cut through steel. Ok, thin steel, but geez!
And, to balance the diet,
A cucumber makes an even better edible thermal-lance housing, since its outer rind contains the pressure of the very hot flame without burning up
Two For One
One of the things we’re investigating is pulsed laser systems, because they’re fun, but (especially for funding purposes) also because they are the basis of optical frequency combs (as I’ve mentioned). And things are pulsing along. One of the things that was noticed was that light from the pulsed system, running at 1560 nm, was showing up on a Silicon CCD camera. The Silicon response peaks at 900 nm and drops pretty sharply, petering out at 1100-1200 nm. There’s no way it should respond to a 1560 nm photon.
And it isn’t. It’s responding to pairs of 1560 nm photons. This is a pulsed system, so you have high peak power making it a lot easier to see nonlinear responses like two-photon transitions, because they scale as the square of the intensity. (more photons incident per unit time means a better chance to have two interacting at once, Having n photons means that if you look at any photon, the chance of another photon being around is n(n-1)) Two photons have enough energy for the interaction, since that’s the same as having a 780 nm photon, which is well above the “to be detected you must be this tall” energy cutoff
Here are two images. The square is a beamsplitter cube, and the white blob is the light. The top image is the pulsed laser, and the bottom one is a CW beam, both with around 10 mW average power.
The pulsed laser is saturating the heck out of the CCD, so the spot is really a lot brighter than from the CW beam, though we can’t say for sure based on this quick look. Even though the average power is about the same, though, the pulsed laser is repeating at about 10 MHz, and the pulses are less than a picosecond, so all of the light is being delivered in less than 10-5 of the time, so the peaks have powers measure in kW.
Mumbo, Meet Jumbo
Skulls in the Stars: Levitation and diamagnetism, or: LEAVE EARNSHAW ALONE!!!
It’s hard to tell if the author is being snarky or really looks upon mathematical physics as a “mumbojumbo” that impedes progress. Taking the statement at face value, it highlights an important and semi-common misunderstanding of many physics theorems, and so I thought I’d take a qualitative stab at explaining Earnshaw’s theorem and its relationship to diamagnetic materials and magnetic levitation.
Getting a Charge Out of Buckyballs
“Superionic” Buckyball Crystal
A team of researchers from Italy, Hungary, and the UK reasoned that buckyballs bonded into a crystal structure, like stacked fruit, would generate a material with big spaces in between the spheres. “To create large channels, we need large building blocks,” says team member Mauro Riccò of the University of Parma in Italy. And each buckyball can accommodate multiple negative charges, good for incorporating many positive ions. But which ion to use? Previous experiments found that sodium ions couldn’t move easily between the buckyballs.
The smaller lithium ion is a much better choice, Riccò and his colleagues report after completing a long characterization of their new compound, which consists of four lithium ions per buckyball.