Text 799, 70 rader
Skriven 2006-07-26 05:47:52 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 786
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 786 July 25, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu
                
CHEMICAL TRANSISTOR.  A new device, the chemical equivalent of a
transistor, might make possible ultrasensitive bio-medical
single-antigen detection.  The things we associate with transistors,
the closing or opening of a switch or the amplification of a signal,
are normally carried out by injecting a tiny electric signal into a
gate electrode which then changes the environment of a nearby
channel region.  This allows a current to be shut off or to be
amplified.   In an experiment carried out by physicists at the
University of California at Irvine, the same things are done through
chemical reactions.   Philip Collins (collinsp@uci.edu,
949-824-9961) and his colleagues use carbon nanotubes as the central
working substance of their device. The nanotubes, immersed in a
liquid, can be switched from a conducting state to an insulating
state by oxidizing them---that is, by chemically removing the free
electrons.  The chemical reactions are triggered by an electrical
potential applied across the interaction area (figure at
www.aip.org/png/2006/262.htm ).   What the Irvine researchers
(Philip Collins, collinsp@uci.edu, 949-824-9961) show is that this
process can be performed reversibly and over short periods of time,
as fast as 10 microseconds.  This is quite slow by today's
transistor standards; the more important promise for prospective
chemical field effect transistors (or ChemFETs) is the potentially
large amplifications.  It looks as if only a few electrons' worth of
oxidation can be used to switch currents as large as microamps.  In
a future bio-detector the switching would be provided not by an
applied electrochemical signal but by the trace presence of antigens
docking with antibodies attached to the nanotubes.  In previous
detectors, chemical actuation has required the presence of tens of
antigens; here, a single antigen might be enough to change the state
of the nanotube.  (Mannik et al., Physical Review Letters, 7 July
2006; lab website www.physics.uci.edu/~collinsp/)

ASTEROID ENCOUNTER ENHANCEMENT.  Just what you wanted to know: that
the likelihood of an encounter between the earth and an asteroid is
enhanced by the gravitational pull between the two bodies.  The
formula for the enhancement is worked out in a new paper by
University of Iowa physicist James Van Allen, the same man who, a
half century ago, predicted the existence (later confirmed) of
plasma-particle radiation belts around the earth.  The enhancement
of the collision cross section, which Van Allen admits leaves out
additional forces exerted by the sun and other planets in the solar
system, equals 1 plus the square of the ratio of the escape velocity
for the planet (Van Allen worked out the case Earth, Mars, Jupiter,
and Saturn) to the approach velocity of the asteroid (starting far
away).  (American Journal of Physics, August 2006)

ASIAN STORMS PUSH THE EARTH AROUND.  Earth's axis of rotation
undergoes several gyrations, such as the precession of the
equinoxes, which takes about 26,000 years. Recently two of the most
important axis gyrations inadvertently cancelled each other,
allowing geophysicists to measure other, subtler gyrations that
would normally be difficult to detect.  The two larger wobbles are
the 433- day-cycle Chandler Wobble (whose origin is not very well
known) and the wobble caused by annual weather oscillations.  Their
combined effect is normally to cause the rotational axis to migrate
by as much as 10 meters.  But from December 2005 to February 2006
their mutual nullification reduced the axis excursion to less than 1
meter.  This allowed Belgian scientists to study fainter, lesser
forces whose exertions could briefly be measured.  The scientists
saw signs of what they believe to be an influence on Earth's wobble
day by day triggered by storms over Asia and northern Europe.
(Geophysical Research Letters, July 2006)       

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 * Origin: Big Bang (1:106/2000.7)