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Energie und Entropie

From Energy Devaluation to Exergy

Schlichting, H. Joachim; Backhaus, Udo. In: G. Marx (Ed.), Proceedings of the 6th Danube Seminar on Physics Education, Budapest 1983, S. 228 – 242

In school, energy is mostly introduced as a conserved quantity: pupils are taught that it can neither be created nor destroyed. Critical pupils and students feel that this contradicts their experience, according to which
– fuel tanks of cars and trucks become empty,
– their parents have to pay for electrical energy, and gas as each month goes by,
– the depletion of (conventional) energy resources has taken energy into the focus of social-political interest. All this indicates that energy may be consumed.
In order to avoid a discrepancy between what the pupil perceives as reality and the scientific description of reality, it must be stressed that ‚conservation‘ and ‚consumption‘ of energy are not contradictory concepts. They express two complementary aspects of common sense experience. Although the ‚consumption‘ aspect has been conceptualized in physics by entropy, exergy, etc., these quantities are normally not treated in school.
This is at least partially a consequence of the way in which entropy is introduced into the conceptual scheme of physics: in fact, this offers little chance to perceive any relationship between entropy and the experiences of energy consumption and to realize the importance of entropy within physics. The notion of exergy seems to be nearly unknown to physics teachers.

In this paper we shall sketch an introduction procedure of entropy resp. of the closely related concept of exergy. First of all, the experiences of energy consumption are conceptualized by the notion of ‚energy devaluation‘ or ‚degradation‘, which already leads to a far-reaching qualitative understanding. It offers a realistic opportunity to discuss the above mentioned problems in undergraduate physic courses.

The quantitative concept ‘energy devaluation’, finally, is specified by the quantity of ‘exergy’, which takes better into account our intention underlying the experiences of devaluation than entropy does.

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