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The Second Law of Thermodynamics 23
Carnot cycle. The curves represent two isotherms and two adiabats.
Fig. 3.1
The upper isotherm is the higher temperature.
Carnot cycle running in a clockwise direction, transferring heat from
Fig. 3.2
the high temperature reservoir to the lower.
The work done by the system on the surrounding must be represented
by −w because by our convention w represents the work done on the
system.
Note that in the complete Carnot cycle, ∆U =0 (why?) and since
∆U = q + w (q = q 2 + q 1), it follows that q + w =0. Thus,
ξ =(q 2 + q 1 )/q 2 =1 + q 1 /q 2 (3.8)
