Download Historical burdens on physics 112 Thermal energy

Historical burdens on physics
112 Thermal energy
Subject:
From a school book:
“The thermal energy is a part of the internal energy and essentially determined by the temperature. Since in many cases one can presuppose the
constancy of the other components, one often only considers the thermal
energy…
Heat tells us how much thermal energy is transferred from one system to
another.…
The following relation holds between transferred heat and energy change:
Q = ΔEthermal .”
From another school book:
“The potential and the kinetic energy of the particles taken together is called
thermal energy.”
From a third school book:
“The total energy of a thermodynamic system, which consists of thermal
energy (potential and kinetic energy of the particles), of chemical energy
and nuclear energy is the internal energy U.”
Deficiencies:
The intention of these definitions of thermal energy is clear: The authors of
the statements try to define a quantity which measures the “heat content” of
a system and which has the following properties:
1. It should be a state variable, i.e. it should have a well-defined value for a
system in a given state.
2. It should be an energetic quantity, i.e. a quantity that is measured in
Joule.
3. It should be a part of the internal energy. Another part would be the
chemical energy.
4. Differences of it should be equal to the process quantity Q, which in
physics is called heat.
The problem is that a quantity that meets these requirements does not exist
and cannot be defined. It is not possible to distinguish the potential and kinetic energy of particles from a part which might be called chemical energy.
Any temperature increase is related to electronic excitations, to oscillations,
to excitations of the spin system, to the dissociation of molecules, to a rearrangement of atoms, i.e. chemical reactions, and finally to nuclear reactions. There is no possibility to decompose the energy that is engaged in
these processes in an unambiguous way into a thermal and a chemical
component. If such a decomposition were possible, it would manifest itself
in the fact that one summand (the thermal energy) would depend only on
temperature and not on the chemical potential and another summand only
on the chemical potential and not on temperature. But such a decomposition is not possible.
Origin:
Physics, chemistry and technical thermodynamics need a measure for the
heat content of a system. Common sense suggests that it should be possible to define it, since we intuitively operate successfully with such a quantity. However, when trying to define a measure for heat in the 19th century,
a mistake was made: It was supposed that such a quantity should be an
energetic quantity. However, a definition of an energetic quantity with the
desired properties could not work. As a result several surrogates appeared,
each of which satisfies some of the requirements and others not. The quantity Q, which was called heat, is one of them. The problem is that Q is not a
physical quantity in the usual sense of the word. One says that it is a “process quantity” since it makes no sense to ask for its values for a given system in a given state. Chemists prefer to manage with another “surrogate”
quantity, the enthalpy. This quantity behaves like a heat content, but only as
long as one restricts to processes at constant pressure – for the physicist
an unacceptable restriction.
None of the quantities Q and H meets the justified expectation towards a
measure of a heat content. So, why not define a quantity that better suits to
our needs, the thermal energy?
It is interesting that the concept “thermal energy” can only be found in
school books, but not in University texts. Do we have to reproach to the
school text book authors for inventing untenable concepts, due to their ignorance of thermodynamics? Yes and no. Yes, because the definition does
not work. No, because they are not to blame for the fact that thermodynamics is so unfamiliar and so unpopular.
It is the University that is to blame. Here, what students learn about thermal
phenomena: Relations between four quantities that change their values simultaneously, interlaced partial derivatives, changes of variables, unintuitive quantities like enthalpy, free energy, and Gibbs’ free energy are the
requisites of the chamber of horror. For the simple explanation of the compression of the gas in a Diesel engine the so-called adiabatic index is used,
which is defined as the quotient of two partial derivatives, which are distinguished by the fact that in one of them one variable is kept constant in the
other another variable.
It is not to expect that the students get a non-tensed relation to thermal
phenomena in this way. But how can he or she, later as a teacher, present
thermal facts to beginners? It is understandable that the school teachers
and school book authors try to construct a thermodynamics that does not
offend common sense.
Disposal:
It is much simpler than one might believe. It is sufficient to abstain from
demanding that a measure of heat must be an energetic quantity. All the
difficulties disappear when introducing entropy as the measure for a heat
content.
Friedrich Herrmann, Karlsruhe Institute of Technology
Georg Job, University of Hamburg