PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 819 April 11, 2007 by Phillip F. Schewe, Ben Stein
www.aip.org/pnu
NEWTON*S SECOND LAW OF MOTION, that pillar of classical physics, the
formula that says the force on an object is proportional to
acceleration, has now been tested, and found to be valid, at the
level of 5 x 10^-14 m/s^2. This is a thousandfold improvement in
precision over the best previous test, one carried out 21 years ago
(Physical Review D, vol 34, p 3240, 1986). The new test was
performed by physicists at the University of Washington using a
swiveling torsion pendulum, a special kind of pendulum in which the
restoring force is not gravity (as you would have in a hanging
pendulum) but is provided by a very thin torsion fiber. One
implication of Newton*s law is that the pendulum*s frequency (its
tick-tock rate) should be independent of
the amplitude of its swiveling (as long as the oscillation is
small). Looking for a slight departure from this expected
independence the Washington researchers watched the pendulum at
very small amplitudes; in fact the observed swivel was kept so small
that the Brownian excitation of the pendulum was a considerable
factor in interpreting the results. Newton*s second law is expected
to break down for subatomic size scales, where quantum uncertainty
frustrates any precise
definition of velocity. But for this experiment, where the pendulum
has a mass of 70 g and consists of 10^24 atoms, quantum
considerations were not important. According to one of the
scientists involved, Jens Gundlach (206-616-3012,
[EMAIL PROTECTED]), this new affirmation
that force is proportional to acceleration (at least for
non-relativistic speeds), might influence further discussion of two
anomalies: (1) oddities in the rotation curves for
galaxies---characterizing the velocity of stars as a function of
their radii from the galactic center---suggest either that extra
gravitational pull in the form of the presence of as-yet-undetected
dark matter is at
work or that some new form of Newton*s Second Law could be
operating (referred to as Modified Newtonian Dynamics, or MOND); and
(2) the ongoing mystery surrounding the unaccounted-for
accelerations apparently characterizing the trajectory of the
Pioneer spacecraft
(seehttp://www.aip.org/pnu/1998/split/pnu391-1.htm). (Gundlach et
al., Physical Review Letters, upcoming article).
PLASMON-ASSISTED SOLAR CELLS. Because of its ubiquity in
electronics, silicon is the favorite semiconductor used in solar
photovoltaic cells. Still, one would like to reduce the amount of
Si needed for large-area devices. Furthermore, silicon is a poor
light emitter and absorber, and therefore solar cell efficiencies
have generally been poor. The efficiency of thin-film Si cells is
even poorer than for wafer-thick Si cells. How to make the cells
cheap (using thin films) but also nicely absorptive is an important
goal. Scientists at the University of New South Wales in Australia
have now enhanced the absorption of sunlight using surface
plasmons. When light strikes a metal sample it can initiate
electrical disturbances in the surface, either as localized
excitations called surface plasmons or as moving waves called
surface plasmon polaritons. The plasmons can be considered as a
sort of proxy for the light, except at a shorter wavelength. If,
moreover, the plasmon energy can be efficiently collected and
transferred to an underlying waveguide as part of a solar cell, then
the cells* yield can grow. This what the New South Wales
researchers do. They use silver nanoparticles to ex
cite surface
plasmons, which enhances light trapping. For 1.25-micron-thick
thin-film cells, the enhancement was by a factor of 16 for light
with a wavelength of 1050 nm. For wafers, the enhancement was by a
factor of 7 for light with a wavelength of 1200 nm. Silicon normally
absorbs light only weakly in this part of the spectrum, so the
enhancement is significant. Across all wavelengths, the
photocurrent enhancement for the 1.25-micron film and the wafer
samples was, respectively, 33% and 19%. According to Supriya Pillai
([EMAIL PROTECTED]), optimizing the nanoparticle size
should bring additional improvements. (Pillai et al., Journal of
Applied Physics, upcoming article)
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