Physics and Physicists: Measuring The Speed Of Light - Using Chocolates!

Physics and Physicists: Measuring The Speed Of Light - Using Chocolates!

Physics and Physicists: The Physics Of Ponytails

Physics and Physicists: The Physics Of Ponytails

The physics of floating pyramids

An unexpected result is reported here.

Results just in from an experiment that levitated open-bottomed paper pyramids on gusts of air reveal a curious phenomenon: When it comes to drifting through the air, top-heavy designs are more stable than bottom-heavy ones. The finding may lead to robots that fly not like insects or birds but like jellyfish.
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The researchers placed hollow paper pyramids inside the cylinder. The objects were about 1 to 5 centimeters high and were made of tissue paper or letter paper on carbon fiber supports, like tiny homemade kites. Physicist Bin Liu led the experiments, attaching a beadlike weight to a post running down the center of the pyramid and changing the height of the bead to give the object a different center of mass. Common sense says that the pyramid should be most stable when the bead is at the bottom of the post, like ballast in the hold of a ship. But when the team released the pyramids over the subwoofer, the opposite was true: the bottom-heavy pyramids were likely to flip over and fall, whereas the top-heavy ones remained upright and continued to hover (see first video), the group reports in an upcoming issue of Physical Review Letters.
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The team suspected that the effect was due to swirls of air that develop along the pyramid's sides. To see the swirls in action, Zhang's group examined a two-dimensional version of the pyramid experiment in water. They placed upside-down V shapes into a pan of water and rocked it to create currents. As the water ran past the V, it created tiny whirlpools at the ends of the V's two legs (see second video). These swirls pushed away from the upside-down V, moving downward, which exerted an upward force on the V-the same mechanism that creates lift in the pyramids.

If the V was tilted, however, the swirls went in different directions: Those on the higher leg shoved it sideways, while the lower leg got a weaker upward push. This would straighten the upside-down V. Team member Leif Ristroph showed that the same sorts of swirls roll off the sides of the pyramids: They push the pyramid upright as long as the center of mass is above the tilted-up side, much in the same way that you can balance a vertical stick on the end of your finger by moving the bottom of the stick in the direction of the tilt, Zhang says. For bottom-heavy pyramids, this same mechanism causes them to flip over-it's like moving the top of the stick in the direction of the tilt, encouraging it to fall.

Paring with spin fluctuations

A review of an interesting work observing the paring mechanism of an exotic superconductor.

Hattori et al. are able to correlate this field-angle-dependence of the magnetic fluctuations with another striking property of UCoGe, which is that its superconductivity is exceptionally sensitive to the direction of an applied magnetic field. When the magnetic field is perpendicular to the c axis the superconductivity is very robust, surviving to around 10 tesla; however, as the field direction is rotated towards the c axis, the critical field for destruction of superconductivity falls precipitously. An obvious interpretation of this behavior would be that the component of the applied field that is parallel to the c axis induces a large magnetic polarization, and the large internal field thus generated disrupts the paired electrons either through coupling to their spins or their orbital motion. This sort of physics is very well understood (indeed this is why ordinary superconductors don’t like magnetic fields) so it can be modeled quite accurately and, surprisingly, it doesn’t fit the measurements in UCoGe. Rather, Hattori et al. argue that their results are better explained if the magnetic field is disrupting not the pairs directly, but rather the underlying pairing mechanism. This, in particular, explains the striking parallel in the suppression of the magnetic fluctuations and the suppression of the superconductivity as the magnetic field is rotated towards the c axis. It is strong evidence that magnetic fluctuations are the ones doing the pairing.

Work in China looks like ......

http://www.nature.com/nature/journal/v481/n7382/full/nj7382-535a.html?WT.ec_id=NATURE-20120126

The Chinese work ethic often makes an impression on foreign researchers. “It's humbling to see people working so hard,” says Tsun. He says that half the Pasteur Institute is sometimes still in the lab after 7 or 8 p.m. — something that Tsun rarely saw during his PhD research at the University of Oxford, UK.

But hard work doesn't always translate into creativity. Many of the students “haven't been trained so much in using their knowledge to generate new ideas and find new solutions”, says Danielsen. “They work extremely hard and very long hours, but I am not sure whether they are able to step back a bit and reflect on the results.” Wickham says that the science is often highly managed by professors, and researchers are not encouraged to take risks or learn from their mistakes.

A planet orbiting a pair of suns !!

http://www.nature.com/nature/journal/v481/n7382/full/nature10818.html?WT.ec_id=NATURE-20120126

Array of Graphene dots absorbs light perfectly

http://physics.aps.org/articles/v5/12

http://prl.aps.org/abstract/PRL/v108/i4/e047401

The results may be used in e.g. solar cells ! How to broaden the absorption spectrum?

A talk by P A Lee on SC and FM coexisting in oxide interface

http://videochannel.ust.hk/Watch.aspx?Section=Channels&Channel=2&SubType=All&View=Icon&Sort=Date&Page=1&Current=3&Mode=Play

Physics and Physicists: The Physics of Wind-Blown Sand and Dust

Physics and Physicists: The Physics of Wind-Blown Sand and Dust

Physics and Physicists: The Science of Color

Physics and Physicists: The Science of Color

Physics and Physicists: What Is The Scientific Method?

Physics and Physicists: What Is The Scientific Method?

A video on A.Einstein

http://www.youtube.com/watch?v=RVJyaJ5TNpc

http://www.youtube.com/watch?v=uKrRocH8M5M&feature=fvwrel

http://www.youtube.com/watch?v=i6XWNUzEZkY&feature=related

http://www.youtube.com/watch?v=mHM0SYyGfcw&feature=related

http://www.youtube.com/watch?v=dB6_0pcUfBc&feature=related

http://www.youtube.com/watch?v=Dtk_gChLchw&feature=related

Separation device

A newly proposed separation devices has come out (http://physics.aps.org/articles/v5/6).

Rubí and Peter Hänggi of the University of Augsburg, Germany, led a team that has developed a new approach to these ratchet sorters. They start with a mathematical framework in which the entropy of the system is treated like potential energy, with entropy “barriers” that repel particles. These are regions where particles are restricted to a small space, which reduces the number of states (locations and velocities) that a particle can occupy. Fewer states means lower entropy. Like balls rolling down a hill, particles tend to move away from these low entropy spots.

The team applies this formalism to a tube with walls that periodically ramp from a narrow diameter to a wide diameter and back, with an asymmetric or “sawtooth” profile. This shape forms distinct but still connected chambers, or segments, each of which is a few microns long. Entropic barriers inhibit travel between segments; however, the barriers are steeper going to the left, so the net motion of the particles is to the right.

In order to clearly see the entropic effect in their computer simulation and analytical calculations, the researchers apply an oscillating force that essentially shakes the particles back and forth inside the tube. In a real experiment, this force could be an oscillating electric field.

Resistance becomes lower under pressure

The property of matter often changes drastically as external knobs such as temperature (goes lower) and pressure (gets raised) are tuned (http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.108.026403).

Wustite (FeO) is a prototype for the iron-bearing minerals found in the Earth. Though FeO is insulating at ambient conditions, in the late 1980s researchers observed it undergo a transition to a metallic state when compressed by shock waves. The nature of this transition has, however, been unclear.

In a paper in Physical Review Letters, Kenji Ohta of Osaka University, Japan, and colleagues report their combined theoretical and experimental attack on the problem. The research team measured high-temperature resistivity and structural x-ray diffraction patterns of FeO in a diamond anvil cell to simulate conditions in Earth’s mantle. At a temperature of 1900 kelvin and pressure of 70 gigapascals, Ohta et al. were able to watch as FeO in a rocksalt atomic structure became metallic without any structural changes.

To understand these findings, Ohta et al. performed density-functional calculations of electrical conductivity as a function of temperature and pressure. The results suggest that their observations are consistent with a new kind of insulator-metal transition involving fluctuations between a high-spin state to a low-spin state in the FeO. For geophysicists, this makes the picture of conductivity deep in the Earth richer: both insulating and metallic phases must be added to the phase diagram, with potential implications for thermal and electrical conductivity, and in turn models of the planetary magnetic field. –David Voss