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Laws of Thermodynamics
Name: Linda G.
Status: other
Age: N/A
Location: N/A
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Date: N/A
Question:
I need a simple definition of entropy and the
three laws of thermodynamics. I do teach middle school but the
simpler the better.
Replies:
Thermodynamics is not an easy subject at any level, but possibly
easier to teach at the middle school level -- surprise! The reasons
are historical and pedagogical. Historical because thermodynamics
is an abstract subject that predates the general belief by
scientists of the existence of atoms and molecules.
Pedagogical, because most of us have been sold a "bill of goods"
regarding the laws of thermodynamics.
Here is what I mean. The first law concerns "energy". What we were
fed, at least in my experience, was a sort of shell game. Typically,
"energy" is defined for us unsuspecting students as "the ability to
do work". OK but what is work? Work is an applied force acting
against an object in the direction of the objects motion. And
immediately before the student has a chance to absorb this as being
so obvious, which it is not, we are off talking about blocks sliding
down planes, pulleys, and all the other garbage (sorry for the tough
term) that is flung out, and of course there is heat, with is
another "form" of energy, that can do work too. In fact, "energy" is
an abstract concept, idealized from various observations. It is as
abstract as lines and points, which in geometry are also presented
as so "obvious" how dare the student challenge the definitions. It
is only much later, if ever, that a student finds out that there are
geometries, that are equally valid that are based on very different
assumptions and definitions. Now back to thermodynamics:
First Law: There is this abstract concept "energy" which be
experience gives a number that if we do something to get from A to B
and back to A ends up having a value of zero -- more or less. It is
as abstract as electric fields which were 'invented' to explain
electricity and magnetism, even though Maxwell 'invented' the field
without having ever actually observed one. He only said if you let
me 'invent' this abstract quantity, the field, I can with four
equations explain all of classical electromagnetism! The First Law
says, if I divide up a wide variety of processes into abstract
quantities called work and heat and if I carry out this wide variety
of processes starting at A, going to B, and returning to A and
compute these abstract quantities work and heat add up to zero. That
is not an easy concept, and certainly not self-evident.
Second Law: Most simply stated -- Some things happen. Some things do
not. What do the things that happen have in common? What do the
things that do not happen, have in common? The answer is this
abstract quantity we call entropy. Forget the mathematical
scaffolding we with which the abstract quantity is shrouded.
Stripped to the core, the Second Law proposes an abstract quantity,
with formulas on how to calculate it, called entropy that says that
things that happen show a positive increase in this quantity we call
entropy, and the things that do not happen (under some specified
constraints) show a decrease in this quantity we call entropy. But
it is an abstraction, not something that is at all self-evident.
For historical reasons, scientists were not really sure atoms
and molecules existed, the history of the Second Law is distorted
and convoluted to make it valid even if atoms and molecules did not
exist. But today there is no valid reason to make what is
reasonable, very "scriptural" and "pure" void of any discussion of
the existence of atoms and molecules.
The Third Law cannot even be discussed properly without invoking
atoms and molecules, because it states, again an abstraction, that
there is a lowest possible energy (again an abstraction) for
collections of atoms and molecules that are perfectly ordered. Its
implications are sweeping, but the concept in terms of atoms and
molecules, is simple.
I hope this helps, but it is not an easy task you face.
Thermodynamics has been mangled so badly for the last century and a
half that it requires very careful analysis to avoid being caught
in a circular presentation.
Vince Calder
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Update: June 2012
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