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Physicists at Lyon’s
Institut de Physique Nucléaire (CNRS/IN2P3(1)
and the University of Claude-Bernard Lyon I), working in collaboration
with the Institut für Lonenphysik of Innsbruck (Austria), have experimentally
established, for the first time in a direct manner, the existence of a
negative calorific capacity. The system, a finite set of 15 hydrogen molecules,
suddenly lost heat during its liquid/gas transition, despite the contribution
of external energy.
The phenomenon is commonplace. The temperature of water rises when it
is heated. But as soon as the water begins to boil, its temperature ceases
to rise until all the water has been turned into vapor. This behavior
is typical of macroscopic mediums. Exposing a system to additional energy
will increase the temperature of the system until it begins to change
phase (i.e. evaporation or fusion); even though it is constantly heated,
it will maintain a constant temperature until the evaporation (or fusion)
is complete. In physics, this characteristic is referred to as the positive
calorific capacity(2) of a system.
But, are there systems with a negative calorific capacity – that
is, systems which lose temperature when heated? As early as 1970, the
theoretical physicist Hans Thirring postulated such was the case with
stars, whose core temperatures increase as they gradually lose energy
via radiation. At the other end of the scale, the nuclei of atoms also
drop in temperature just when they are vaporized.
A team from Lyon’s Nuclear Physics Institute, in collaboration with
the Institut für Lonenphysik of the University of Innsbruck, has
just demonstrated, for the first time in a direct manner, the existence
of a negative calorific capacity in hydrogen aggregates, small units made
up of about 15 molecules. Their experiments consisted of studying a number
of collisions between a hydrogen aggregate and a fixed helium atom and
detecting the resulting fragments of each collision. The collisions are
of interest because they made it possible to deposit a significant amount
of energy on the aggregate for a sufficiently short enough period to prevent
the system from reacting. As the size of the resulting fragments changed
with the amount of energy deposited on the aggregate, the researchers
were able to verify a drop in temperature during the vaporization of the
aggregates.
Between a gas and a liquid
In contrast to macroscopic systems, where the two phases (gaseous and
liquid) can coexist, there can be no clear distinction between the two
phases in a system made up of such a small number of particles: it is
thus either gaseous or liquid. As soon as it is found in an intermediate
state, which by nature is unstable, it seeks to return to a gaseous state
“as soon as possible,” even if that means a temperature loss
similar to that previously observed. In the macroscopic world, where a
large number of molecules are involved, a sudden drop in temperature of
all the system’s molecules is not possible; thus the two phases are
observed coexisting at the same temperature while only a small fraction
of molecules, caught between the liquid/gas interface, are affected by
the transition phase.
Somewhere between macroscopic and microscopic
But one question remains unanswered. If such is the case, exactly where
lies the fine line that divides the microscopic from the macroscopic world?
The question is not merely academic. It has acquired importance in light
of the current development of nanotechnologies, where the miniaturization
of devices depends on electronic switches that may be composed of just
a few atoms.
(1) Institut National de Physique Nucléaire
et de Physique des Particules
(2) A system's calorific capacity is the energy it requires
to increase its temperature by one degree Kelvin
Reference:
Physical Review Letters, Vol. 89,183403, October 28, 2002
What is
temperature?
Determining the temperature of microscopic systems is far from trivial.
In this case, it is measured by the statistical distribution of the size
of the aggregates detected after a high-speed collision. This measurement
implies the conventional definition of the word “temperature”
according to statistical thermodynamics: temperature is a scale that represents
the average kinetic energy of a molecule with a system. Once again, the
question of the fine line between the macro and the microscopic world
is posed: to what extent can we still refer to a statistical distribution
of kinetic energy?
Researcher
contacts:
Bernadette Farizon
Tel : +33 4 72 44 83 89
e-mail : bfarizon@ipnl.in2p3.fr
Michel Farizon
Tel : +33 4 72 44 84 01
e-mail : mfarizon@ipnl.in2p3.fr
IN2P3 contact:
Dominique Armand
Tel : +33 1 44 96 47 51
e-mail : darmand@admin.in2p3.fr
Press contact
:
Martine Hasler
Tel : +33 1 44 96 46 35
e-mail : martine.hasler@cnrs-dir.fr
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