Clas Blomberg - Physics of life-Elsevier Science (2007)

66 Part II. The physics basis
for most of what we say in this book; there are possibilities to define temperatures that vary
in a room or inside some object under study (which can be a living cell).
We illustrate here the ideas by the special process that once started all this and also today
is important for developing the main ideas, the Carnot process, works as follows:
Carnot process:
1. A gas (steam, air) is expanded at high pressure and high temperature. It expands, pushes
a piston and thereby performs considerable work. The process is most efficient if it
takes place at constant temperature, thus using the high temperature effect as much as possible.
This means that the gas is heated during this stage. (Otherwise the temperature
would decrease.)
2. At a next step, the temperature is decreased by a more rapid expansion. In this step, no
heat is transferred to the gas. The system then attains a low temperature.
3. The processes are reversed at the low temperature. In this step, the gas is compressed at
the low temperature, which should be kept constant for maximum efficiency.
4. The system is further compressed rapidly without heat transfer (compare step 2). The
temperature increases and the system gets to the starting position of step (1).
The work that is performed at step (1) at high temperature is larger than what is required at
step (4). In both these steps, there is mainly an exchange of heat and work, and the energy
changes very little. The steps (2) and (4) are, most efficiently, reversed versions where energy
changes by an equal amount in both steps. There should not be any heat transfer.
It is important to go through all the steps, also the restoration steps (3) and (4) to provide
a continuous (cyclic) process.
Thus, there is a net flow of heat to the system and this is what has been converted to performed
work: as we get back to the initial state, there is no overall change of energy and heat
and work compensate each other. The steps (2) and (4) change the temperatures, and when
no heat is involved ideally, compensate each other. We very much refer here to an ideal
machine, which all the time passes through equilibrium steps. Such a machine is very relevant,
as it provides the limit performance of a machine working between two temperatures and
which produces work.
We shall also mention another energy concept, the enthalpy, denoted H. This is particularly
useful for gas changes and pressure effects; in particular steam engines as a change of
enthalpy includes a work by or against a constant pressure. In technical or chemical thermodynamics
where processes in gases are very important, it is usually treated as more relevant
than the energy U. When pressure effects play a minor role, as is in most cases in this book,
the distinction between energy and enthalpy is less important.
The analysis of the heat machine can be used as a starting point for the very important
definitions of the entropy concept. If the steps proceed at equilibrium (which corresponds to
an ideal process), then they are also limits between what can be attained in a real machine
and what cannot. A limit between what is allowed and what is not allowed. The entropy is
introduced to quantify the limit. We can think about the process as composed by limit
processes, but also that the four end steps are well-defined equilibrium steps, that is the
original high-temperature compressed state, the high-temperature expanded state, the lowtemperature
expanded state and the low-temperature compressed state as four equilibrium