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

New Trends in Physics Teaching IV
Thermodynamics. The well known expression’
AE=W+Q (13)
means that an energy exchange AE of a system may be realised by absorption (counted positively)
and emission (counted negatively) of mechanical (electrical etc.) energy (transmitted in the form
of work) or of heat2.
As already discussed qualitatively, (page 64) a heat engine (HE) transforms, in principle, a
quantity of heat Ql given out by a hot reservoir at temperature Ti partly into mechanical work
(energy) W and partly into heat Q, given to a cold reservoir at temperature T2 (< TI 1.
See figure 2.
The amount of heat Q2 given to the cool reservoir has to be large enough for the corresponding
degradation of the energy to compensate for the upgrading associated with the transformation
of heat into mechanical work. Speaking quantitatively3
Qi
The efficiency vHE which is defined as the ratio of the desired work -W and the heat supplied
may be determined by condition (1 5) together with (14):
From this we conclude that heat may be transformed completely into mechanical (electrical,
etc.) work only at infinitely high temperatures. Conversely, mechanical work may be conceived
of as heat transferred at infinitely high temperatures (see Boltzmann [ 61 ). The corresponding
entropy change is then AS = 0 and there would be no degradation. This is in accordance with
our qualitative estimate (page 63) that the mechanical (or electrical) energy has to be regarded
as the most valuable form .of energy. In practice the production of arbitrarily high temperatures
by means of electrical or mechanical work is restricted only by technical considerations.
1. See Editor’s note at the foot of page 67.
2. The problem of the exchange of chemical energy will be omitted here.
3. Relative to the system HE, Ql > 0 and Q2, W < 0; the entropy change a(;3 of the HE is zero because the HE is working
cyclically.
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