-
7/27/2019 Is the 2nd Law Always True
1/4
Is the Second Law of
Thermodynamics alwaystrue?
Trn L Quang Vinh (VP2003)
L Quang Nguyn
2002
News reported breathlessly: "One of the mostimportant principles
of physics, that disorder, orentropy, always increases, has been
shown to beuntrue."Scientists at the Australian National
University(ANU) have carried out an experiment involvinglasers and
microscopic latex beads that disobeys
the Second Law of Thermodynamics.
Maxwells Demon 1
Consider a box filled with a gas at sometemperature.
Maxwells demon allows only the faster moleculesto move to the
left side and the slower moleculesto move to the right side.
Maxwells Demon 2
Thus eventually the boxis hot on the left and cold
on the right.A heat engine can thenrun between the twosides to
generate work without any change tothe surroundings!
Heatengine
-
7/27/2019 Is the 2nd Law Always True
2/4
Maxwells Demon 3
This imaginary situation seemed to contradict thesecond law of
thermodynamics.To explain the paradox scientists point out that
torealize such a possibility the demon would stillneed to use
energy to observe the molecules (inthe form of photons for
example).
Jeansmiracle
Put a water pot on a cooker. Thermal energytransmits from the
cooker to the water pot andwater will certainly be boiled.James
Jeans, a British physicist andmathematician, wondered whether the
reverseprocess is possible: thermal energy transmits fromthe water
pot to the cooker and water will freeze!
Jeansmiracle (cont.)
Jeans calculated that the probability for thatmiracle to occur
is one over ten to the power of thousand (10 1000 )!So we can be
sure that in reality the second law ishardly violated.
Boltzmans argument
Consider two insulatedvessels communicated by a
small tube.Each vessel contains onemolecule.There will be a
period of time during which onemolecule moves from onevessel to the
other.
-
7/27/2019 Is the 2nd Law Always True
3/4
Boltzmans argument (cont.)
So the entropy of the system decreases. Thisclearly violates the
second law.However, for vessels containing a large number of
molecules the probability of this process isextremely small.Again
we can be sure that in reality the second lawis hardly
violated.
The ANU experiment
By measuring the motion of the latex bead that issuspended in
water, and calculating the minusculeforces on it,the researchers
were able to show that the bead wassometimes kicked by the water
molecules in such away that energy was transferred from the water
tothe bead.
In effect, thermal energy was extracted from thereservoir and
used to do work (helping to move thebead) in apparent violation of
the second law.
Explanation
Entropy is just a statistical average and realsystems will
fluctuate about this average.These fluctuations are very small for
the largenumber of molecules in common objects, but thefact remains
that entropy will fluctuate up anddown.About the only surprise in
these new results is thatviolations can be found in a system as
large asmicron-sized beads in water.
Fluctuation Theorem (FT)
The FT determines the probability that entropy willflow in a
direction opposite to that dictated by thesecond law of
thermodynamic.The FT is more general than the second law. It canbe
applied to both microscopic and macroscopicsystems.When applied to
macroscopic systems, the FTverifies the second law of
thermodynamics.
-
7/27/2019 Is the 2nd Law Always True
4/4
Consequences
The experiment suggests that nanoscale machines
may have to deal with phenomena that are morebizarre than most
engineers realize.Such tiny devices may even end up
runningbackwards for brief periods due to the counter-intuitive
energy flow.
Consequences (continued)
The research may also be important to biologistsbecause many of
the cells and microbes they studycomprise systems comparable in
size to the bead-and-water experiment.
Conclusion
The Second Law is always true for large-scalesystems.However,
for micro-scale systems there is apossibility, determined by the
FluctuationTheorem, that they would run against the directionof
increased entropy.
