Page 30 - Advanced thermodynamics for engineers

CHAPTER
2
THE SECOND LAW AND

EQUILIBRIUM

The First Law of Thermodynamics states that energy cannot be created or destroyed: it can simply be
transformed from one form of energy to another. This means that work can be transformed completely
into heat. The Second Law of Thermodynamics relates to the reverse transformation of heat into work,
and it will be shown that it is not possible to completely transform heat to work. In its simplest form for
engineers, the Second Law is useful for considering the thermal efficiency of heat engines. In a more
general form, the Second Law introduces the concept of the ‘quality’ of energy.
Some of the ideas introduced in Chapter 2 are discussed in greater depth in the web version of this
book available on http://booksite.elsevier.com/9780444633736.

2.1 THERMAL EFFICIENCY
The definition of mechanical efficiency is

Work output
Efficiency; h ¼ (2.1)
Work input
It was not possible to derive a similar relationship for thermal efficiency until Joule showed the
equivalence between heat and work in 1848. Then thermal efficiency could be stated as

Useful work output
Thermal efficiency; h ¼ (2.2)
th
Thermal energy input; for a heat engine operating in a cycle

2.2 HEAT ENGINE
A heat engine is a continuously operating thermodynamic system across the boundaries of which only
heat and work flow: it is a closed system. The term continuously operating means that the heat engine
operates in a cycle, and that a number of integral cycles is/are performed.

2.3 SECOND LAW OF THERMODYNAMICS
Two statements of the Second Law will be given here. The first is the Kelvin–Planck statement which
says
It is impossible to construct an engine which will operate in a complete cycle and produce no other
effects than to transfer heat with a single reservoir and raise a weight (i.e. do work).
Advanced Thermodynamics for Engineers. http://dx.doi.org/10.1016/B978-0-444-63373-6.00002-2 13
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