New trends in physics teaching, v.4; The ... - unesdoc - Unesco

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heat is interpreted as the result of the degradation of mechanical energy. The particles of the
system move in a disordered way; and the degraded part of the mechanical energy only serves
to increase the disorder or agitation of the particles. Work and heat are thus seen to be physical
quantities of the same kind.
This idea is supplemented by the demonstration that, conversely, heat can be transformed into
work.
At the stage when the pupil is being introduced to these concepts, it is essential to begin to
familiarize him with the concept of temperature which is closely linked with that of heat.
Temperature should be seen as a characteristic of particle agitation and hence to be a parameter
whereby the state of a system can be described. It should be made clear that heat and temperature
are two separate concepts, and that heat should be considered as a form of energy while
temperature is a state variable (or co-ordinate). In order to complete the study of the concept
of heat energy, the effects of heat (combustion, chemical reactions, etc.) should be indicated and
examples of heat sources should be cited, taking into account the question of heat transfer and
the concept of thermal equilibrium.
All this should lead to that part of the course relating to thermodynamics, i.e. the part relating
to the study of transformations and equilibrium states of systems defined by position parameters
and temperatures.
Thermodynamic (or absolute) temperature is defined later at a more advanced level. The
temperature is a measure of the degree of particle agitation and is defined as a quantity T proportional
to the kinetic energy of a particle of mass m, moving with velocity of translation
v : %mv2 = ?2kT, k being a constant and the coefficient % being chosen for reasons of convenience
which will be justified by the calculations to be made on the basis of thermodynamic models.
In this part of the course, dealing with thermodynamics, the concept of internal energy is
tackled. The internal energy of a system is the sum of the kinetic energies and potential energies
of ail the particles making up the system and attention has already been drawn to the difficulties
involved in calculating the mechanical energy of systems containing a very large number of
particles acting upon one another.
At this stage, which might be said to be an initiation to the concept of energy, the teaching is
confined to the case of systems which are at rest from the macroscopic point of view, and in
thermodynamic equilibrium. The macroscopic variables (temperature, pressure, etc.) conserve
the same values in time and the system conserves its own energy; at equilibrium the energy of the
system is a constant and is thus said to be a function of state.
It wil be necessary to help the pupil to grasp the theoretical, not to say utopian, character of
this definition in the light of the difficulties relating to the microscopic scale. The example can,
however, be cited of perfect gases for which a model may be adopted in which each atom is
represented by a dot and there is no need to consider any remote interaction between the atoms.
Hence, there is no potential energy. In the general case, the variations in internal energy can be
determined with the help of state variables, which are macroscopic. Here Joule’s experiments
provide experimental support and make it possible, among other things, to explain the equivalence
of heat and work.
Exchanges of energy between the system considered and its surroundings may take place in the
form of heat or work (with sign conventions according to whether the system receives or gives up
energy to its surroundings).
By means of Joule’s experiment it can be shown that a system may undergo an infinite number
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