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

New Trends in Physics Teaching IV
8 to O2 the other body is heated from O2 to 8 ;(< 8 ).
In general a combination of heat engine and heat pump are required to achieve the reversing
process: heat conduction between the two bodies works only in special cases.
(b) The process Y differs from a only in that the surroundings are at a temperature 8 2' , which
is greater than 02, i.e. by a heat conduction process between 8; and 02. Therefore the energy
degradation produced by a! is equal to the energy degradation due to 'd and this heat conduction
process; this means that it is greater than that of 'd alone.
The qualitative concept of energy degradation is based on a consideration of processes.
Although, in most of the examples considered, our interest has been focused either on the
energy emission (e.g. the cooling of hot water) or on the energy absorption of a system (e.g.
the melting of ice in a warm room), the degradation always referred to the full process of energy
exchange between at least two systems. If one is interested in making more precise statements
(e.g. about the behaviour of just one system involved in the process) the concept of degradation
already developedis too crude. This is the price we pay for doing without thermodynamic notions
and reasoning; for, in other words, simplicity.
The quantitative concept has to be able to describe the behaviour corresponding to degradation
of energy within a part of a process (e.g. the emission of energy only) and express it in terms of
one of the systems involved as well as the properties already described by the qualitative concept.
As already indicated, this is achieved by introducing the entropy change AS'.
In the usual phenomenological definition
Qrev
AS=- T
where Qrev is the reversibly exchanged heat and T the absolute temperature (see e.g. Zemansky
191 ).
It can be seen that entropy change is proportional to the energy exchange and inversely
proportional to the Kelvin temperature.
The following examples wil show how the energy degradation of a process can be further
subdivided in terms of entropy and be related to single systems.
Some examples
The cooling of hot water
First, we wil consider the simple example of hot water cooling in colder surroundings.
This process is associated with energy degradation and in terms of (8) we have
AS20 (9)
(This is the quantitative version of the principle (1) of spontaneous energy degradation; it
is the Second Law of Thermodynamics. The equality sign holds for the ideal limiting case in
1. This is not the only possibility; but it does permit a connection with standard thermodynamics. A more direct quantitative
development such as an application of the concept of degradation to systems would lead to a somewhat different quantity, which
would assign a value proportional to the exchanged quantity even if only mechanical and electrical energy were involved.
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