Science > Physics > PHYSICS NEWS UPDATE -- Number 690 June 30, 2004 by Phillip F. Schewe,Ben Stein
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Science > Physics |
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"Sam Wormley" |
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30 Jun 2004 02:59:49 PM |
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PHYSICS NEWS UPDATE -- Number 690 June 30, 2004 by Phillip F. Schewe,Ben Stein |
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 690 June 30, 2004 by Phillip F. Schewe, Ben Stein
WHY DO TRANSFORMERS HUM? Scientists in Switzerland have explained,
for the first time, the microscopic cause of hum in those massive
yokes of iron which help transform AC electricity from one voltage
to another. When current reverses 60 times a second the iron core
of the transformer undergoes magnetetostriction twice during each
cycle. In other words, 120 times per second induced fields cause
the core to stretch slightly; a meter-sized transformer might
stretch or shrink by only a micron but this would be enough to set
up an audible 120-Hz hum. (Earth itself is a magnet. Because of
its own magnetic field, Earth's diameter is shrunk by about 10 cm.
Turn off the terrestrial field and the Earth would spring back; its
surface area would immediately increase by about 10 square
kilometers.)
The new experimental work probes theories, going all the way back to
Werner Heisenberg in the 1920s, about how the shrinkage arises from
the magnetic interactions (spin exchange) among pairs of atoms
(dimers), which share a common electron. The two magnetic ions want
to be closer together. For studying this effect iron itself is not
the best test material and the Swiss scientists (ETH Lab in Zurich
and the University of Bern) use another magnetic atom, manganese.
Mn is a common ingredient in the magneto-resistance data storage
systems found in most disk drives. Normally in a pure crystal, Mn
atoms would be arrayed in endless straight lines. But in this
experiment the Mn atoms are isolated, two by two, with plenty of
intervening magnesium atoms. This allows the researchers to
variably "dilute" the magnet interactions between Mn atoms. The
strength of these interactions (or to be more precise the energy
levels of the excited Mn atoms) is measured by scattering a beam of
neutrons from the sample, a process called neutron spectroscopy.
The observed microscopic magnetostriction mimics the striction at
the macroscopic level, but it does depart considerably from the
predictions of the traditional Heisenberg model. (Straessle et al.,
Physical Review Letters, June 25, 2004; contact Thierry Straessle,
Universite P&M Curie, 44-27-38-31, thierry.strassle@pmc.jussieu.fr,
33 44 27 38 81)
FIVE-PHOTON ENTANGLEMENT has been achieved by physicists at the
University of Science and Technology in China. Entanglement is
perhaps the weirdest of all aspects of quantum behavior. If several
particles are entangled, this means that they participate in a
single quantum state which can be in several unique states at the
same time. Furthermore, the measurable properties of the particles,
such as their spins, will be correlated, even if subsequently the
particles are located at great distances from each other and the
properties measured separately. Previously the greatest degree of
full quantum entanglement came in experiments involving four
particles. (For the case of four ions held in a trap, see
(http://www.aip.org/pnu/2000/split/pnu475-2.htm. ) The Chinese
researchers entangle two pairs of photons, and then entangle these
with yet another single photon. (Zhao et al., Nature, 1 July
2004.) The progress from four to five entangled particles is
significant since apparently the handling of quantum information
(such as in a quantum computer) with a built-in error correction
process would require the manipulation of five entangled particle
engineered to serve as qubits (see, for example, Laflamme et al.,
Physical Review Letters, 1 July 1996).
THE CASSINI SPACECRAFT ARRIVES TODAY AT SATURN after a
3.5-billion-km, seven year voyage from Earth. For four additional
years or longer the craft will loop around the ringed planet and its
moons making various measurements. In December 2004 it will deliver
a detachable probe, called Huygens, at the moon Titan, where it
should descend to the surface. Titan is of great interest to
scientists since it is the only moon in the Solar System with an
atmosphere of its own. In fact, Cassini will fly past Titan as soon
as July 2 and will do so dozens of times thereafter, coming as close
as 950 km.. Already Cassini has taken pictures of Titan which,
with the help of special filters, reveal bright and dark patches on
the moon's surface. (For more Cassini news, see
http://www.nasa.gov/mission_pages/cassini/main/index.html)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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| Title: Re: PHYSICS NEWS UPDATE -- Number 690 June 30, 2004 by Phillip F. Schewe, Ben Stein |
30 Jun 2004 04:36:59 PM |
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Sam Wormley <swormley1@mchsi.com> wrote:
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 690 June 30, 2004 by Phillip F. Schewe, Ben Stein
WHY DO TRANSFORMERS HUM? Scientists in Switzerland have explained,
for the first time, the microscopic cause of hum in those massive
yokes of iron which help transform AC electricity from one voltage
to another. When current reverses 60 times a second the iron core
While the work at the atomic level may be of interest, a TV repairman
from 50 years ago could tell you why transformers hum...
--
Jim Pennino
Remove -spam-sux to reply.
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