Number 690, June 30, 2004
by Phil Schewe and 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
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 Update
475.)
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
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 NASA
website.)
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