Engineering Thermodynamics
Introduction to Classical Thermodynamics
Thermodynamics is the study of energies. More specifically, introductory
thermodynamics is the study of energy transfer in systems. Classical
thermodynamics consists of methods and constructs that are used to �account� for
macroscopic energy transfer. In fact, energy accounting is an appropriate
synonym for classical thermodynamics. In much the same way that accountants
balance money in and money out of a bank account, rocket scientists simply
balance the energy in and out of a rocket engine. Of course just as a bank
account�s balance is obfuscated by arcane devices such as interest rates and
currency exchange, so too is thermodynamics clouded with seemingly difficult
concepts such as irreversibility and enthalpy. But, also just like accounting, a
careful review of the rules suggests a coherent strategy for maintaining tabs on
a particular account.
If a statement about the simplicity of thermodynamics failed to convert
would-be students, they may be captured with a few words on the importance of
understanding energy transfer in our society. Up until about 150 years ago or
so, the earth�s economy was primarily fueled by carbohydrates. That is to say,
humans got stuff done by converting food, through a biological process, to fuel
we could spend to do work (e.g. raise barns). This was a hindrance to getting
things accomplished because, as it turned out, most individuals had to use the
brunt of that energy to grow and cultivate more carbohydrates (e.g. crops and
livestock). We won�t even talk about how much food the horses ate!
Today, we have the luxury, primarily through an understanding of energy, to
concentrate our energy production into efficient low maintenance operations.
Massive power plants transfer energy to power tools for raising barns. Extremely
efficient rocket engines tame and direct massive amounts of energy to blast TV
satellites into orbit. This improvement in energy mastery frees humanity�s time
to engage in more worthwhile activities such as watching cable TV. Although most
are content to blissfully ignore the intricacies that command their way of life,
I challenge you to embrace the contrary.
By no means is the energy battle over. Understanding energy transfer and
energy systems is the second step to destroying the limits to what humanity can
next accomplish. The first step is commanding an interest in doing so from an
inclined portion of the population. Given the reader (and editor) has read this
far through this aggrandizing rhetoric, I welcome your interest and hope to see
it continue until the end.
The Main Macroscopic Forms of Energy
It will be in the best interest of the reader to have defined energy before
it is discussed further. There are three primary forms of energy that are
discussed in macroscopic thermodynamics. Several other forms of energy exist,
but they generally exist on a microscopic level and should be deferred to more
advanced study.
Kinetic Energy
The first form (probably most easily understood idea of energy) is defined by
the motion of an object. Kinetic energy is the energy of a moving mass. For
instance, a moving car will have more kinetic energy than a stationary car. The
same car traveling at 60 km/h has more kinetic energy than it does traveling at
30 km/h.
Kinetic Energy = (1/2) x (mass) x (velocity)2
Ratio of v602 / v302 =((60)(1,000
meters/sec)(3600 sec/hour))2 / ((30)(1,000 meters/sec)(3,600
sec/hour))2
So, once the algebra is completed properly we find the vehicle traveling at
60 km/h has four times the kinetic energy as when it is traveling at 30 km/h,
while the vehicle has zero kinetic energy when it is stationary because the
velocity = zero results in 1/2mv2 being zero.
Potential Energy
The second type of energy is called potential energy. Gravitational potential
energy describes the energy due to elevation. A car at a height of 50 m has more
potential energy than a car at a height of 25 m. This may be understood more
easily if the car is allowed to drop from its height. On impact with the earth
at 0 m, the car that initially rested at 50 m will have more kinetic energy
because it was moving faster (allowed more time to accelerate). The idea that
potential energy can convert to kinetic energy is the first idea of energy
transfer. Transfers between kinetic and potential energy represent one type of
account balance rocket scientists need be aware of.
Internal Energy of Matter
The third and most important concept of energy is reflected by temperature.
The internal energy of matter is measured by its temperature. Hot water has more
internal energy than the same amount of cold water. Internal energy is a measure
of kinetic energy of the molecules and atoms that make up the substance. Since
each atom or molecule is acting on its own accord, this internal energy is
different from the bulk kinetic energy associated with the movement of the
entire solid. The internal energy of matter is exhibited by molecular motion.
The molecules of a gas at high temperature zip around their container constantly
colliding with walls and other molecules. The molecules of a high temperature
solid also move around a lot; however, since they are stuck together with other
molecules, the most they can do is vibrate in place.
In a nutshell, the above forms of energy are studied in classical
thermodynamics. Those forms of energy are allowed to transfer among each other
as well as in to or out of a system. Thermodynamics essentially provides some
definitions for interpreting thermodynamic systems. It then goes on to define an
important rule about fairly balancing energy and one rule about the quality of
energy. (some energy is more valuable) Understanding the framework and the few
rules that govern macroscopic thermodynamics proves to be an incredibly powerful
set of tools for analyzing a myriad of not only engineering problems, but issues
of practical concern. CONTRIBUTION BY CHANGES
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