Thermodynamics is a branch of physics which deals with the energy
and work of a system. It was born in the 19th century as scientists
were first discovering how to build and operate steam engines.
Thermodynamics deals only with the large scale response of a system
which we can observe and measure in experiments. Small scale gas
interactions are described by the kinetic
theory of gasses which is a complement to thermodynamics.
The basic ideas of thermodynamics are taught in high school physics classes,
so the Wright brothers knew and used these concepts, particularly in their
There are three principal laws of thermodynamics which are
described on separate slides. Each law leads to the definition of
thermodynamic properties which help us to
understand and predict the operation of a physical system. We will
present some simple examples of these laws and properties for a
variety of physical systems, although, as aerodynamicists, we are
most interested in thermodynamics in the study of
Fortunately, many of the classical examples involve gas
dynamics. Unfortunately, the numbering system for the three laws of
thermodynamics is a bit confusing. We begin with the zeroth law.
The zeroth law of thermodynamics
involves some simple definitions of thermodynamic equilibrium.
Thermodynamic equilibrium leads to the large scale definition of
temperature, as opposed to the small scale
definition related to the kinetic energy of the molecules. The
first law of thermodynamics relates the
various forms of energy in a system (kinetic and potential) to the
work which a system can perform and to the
transfer of heat. This law is sometimes taken
as the definition of internal energy, and introduces an additional
state variable, enthalpy.
The first law
of thermodynamics allows for many possible states of a system to
exist. But experience indicates that only certain states occur. This
leads to the
of thermodynamics and the definition of
another state variable called
The second law stipulates that the total entropy of a system plus its
environment can not decrease; it can remain constant for a reversible
process but must always increase for an irreversible process.
It also leads to the important relations between the pressure and temperature
of a gas when it is compressed or expanded.
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