3 items
VERIFYING OHM’S LAW BY DIRECTLY OBSERVING ELECTRONS
Chapters 1,2
Posted on Capt 7th February 2006
Physicists can now determine the drift speed of electrons in a current carrying
conductor and have verified Ohm’s Law by directly observing electrons.
Electrons in a conductor can be thought of as electrically charged particles.
They will have random velocities, and with no applied voltage, there is no net
movement along the conductor and thus no electric current. When a voltage
V is applied across the conductor, all the electrons acquire a very very small
additional speed along the conductor - the so-called drift speed v.
The technique to measure their drift speed uses magnetic resonance imaging
(MRI) technology. Like all MRI systems, it detects particles by monitoring their
spin. In a magnetic field, the spin axis of each electron rotates, or precesses,
around the field, just as a tilted, spinning gyroscope or top precesses around
the vertical gravitational field. The precessing electrons generate radio waves
whose frequency depends on the magnetic field strength.
To measure electron velocity, a measurement with no current flowing through
the sample is taken. A short pulse of microwaves is used to start all the
electrons precessing. Although they started out synchronised, electrons in
different places soon get out of step because of their different local magnetic
fields. After a short while, a second pulse forces all the spins to jump ahead in
their cycles by different amounts, reversing their order, with the slowestprecessing spin in the lead and the fastest behind. After the same period of
time, this re-ordering caused the spins to line up again, just where they
started, and generate a radio “echo”. [An analogy for this would be as though
runners on a racetrack suddenly stopped five seconds into the race and then
ran backwards for five seconds, putting them all back at the start
simultaneously.]
When the same experiment is performed with an electric current flowing, the
electrons are now subjected to a whole range of magnetic field strengths and
thus precession speeds, as they move through the sample. So the second
pulse does not re-synchronize all of the spins, and analysis of the radio echo
showed how fast they moved along the conductor - ie their drift speed. A
major technical problem is that the electron current gives rise to its own
magnetic field. This can be cancelled it out by subtracting echoes from two
runs with different applied field arrangements. The experiments show that
electron drift velocity depends on current in just the way classical theory
predicts if the material used obeyed Ohm’s Law. I = nAqv. For a given
conductor, n, A and q are constant, so I  v.
Ohm’s Law requires that the ratio R = V/I is a constant. Thus for Ohm’s Law
to hold I  V, ie v  V, which is what the technique showed.
Ref: Physical Review Letters. 27th January 2006.
MICROSCOPIC ANNIVERSARIES
Chapters 2, 5
Posted on Capt 2nd March 2006
The Scanning Tunnelling Microscope (STM) was first described 25 years ago
in an internal IBM document in March 1981. One of its drawbacks was that it
only works with materials that conduct electricity and can thus form part of an
electrical circuit with the quantum mechanical tunnelling current between the
tip and the surface itself.
This restriction was overcome 20 years ago this week with the announcement
of the AFM or Atomic Force Microscope. This measures the minute force
between the tip and the sample via the deflection of an extremely thin
cantilever. The deflection is monitored by the deflection of a laser beam from
the surface of the cantilever as it deflects. It can resolve down to single
atoms.
Incidentally, another 20th anniversary from the same laboratory was the
announcement of the discovery of ‘high temperature superconductivity’ at 35
K in an oxide of lanthanum, copper and barium. Although other materials with
significantly higher transition temperatures have been discovered, this
‘breakthrough’ has yet to see widespread commercial applications. The AFM
however, has revolutionised the study of biological (no vacuum is needed)
and non-conducting materials, and has fuelled the explosion of interest in
designer materials nanotechnology.
Ref: Nature 2 March 2006 p2, pp14-15
COMPUTING SCIENCE & SCIENTIFIC COMPUTING
interalia Chapters 1,2,3
Posted on Capt 28th March 2006
Planned for 2010, the Large Synoptic Survey Telescope’s 3 billion pixel
sensor array will start streaming 30 terabytes of data each night. Given clear
night skies, it will survey the whole night sky in just one week. Each week’s
survey will be analysed for transient changes building up a data base of
billions of objects. Such automation will change the way scientists generate
theories and test them. Scientists will have to understand computer science in
much the same way as they now have to understand mathematics as a tool
with which to do their jobs.
For another measure of the phenomenal changes taking place on the way
science is being done, consider the following. In the 1990s it took the
international human genome project a decade to sequence a single human
genome. At the current rate of technological innovation in gene sequencing,
by 2050 it will be feasible to determine the complete genome of every human
being on the planet.
As you read this a range of techniques from mathematics, statistics, and
computer science are being used to create scientific models from empirical
data in an automated way. Machine learning techniques are being used to
generate hypotheses from data, make predictions and test them.
Ref: Nature 23 March 2006 p383, 409.