In this chapter various relevant concepts and definitions are introduced, and will be used throughout the course. We recommend that you read this chapter together with Chapter 1 of Potter & Somerton, and attempt as many of their Supplementary Problems as possible. Definitely do Problems 1.28 through 1.35, which include pressure and force balance equations. All answers to the Supplementary Problema are provided by Potter & Somerton at the end of their chapter. The concept of pressure is fundamental to thermodynamics, and you need to be able to handle absolute, gage, and vacuum pressures, as well as the various units of pressure: Pa, mmHg, bar, atm, psi. Other problems in this set deal with temperature, forces, energy, acceleration due to gravity, properties, state, equilibrium and units. All of these concepts need to be clearly defined before we can continue.
Thermodynamics is the science of energy, including energy storage and energy in transit. The three forms of energy storage of greatest interest to us is Potential Energy, Kinetic Energy, and Internal Energy. The First Law of Thermodynamics states that energy cannot be created or destroyed, but can only change its form. Thus we will consider energy in transit in the form of Work (W) and Heat (Q).
In this course we use the International System (SI) units exclusively, with occasional lapses. In the US this is an ongoing battle causing much confusion in the global economy. Even the NCEES is confused at this point - the Fundamentals of Engineering (FE) Reference Handbook and exam contain exclusively SI units and then, when you reach maturity and are ready to take the Professional Engineering (PE) exam, you find that the English system of units (USCS) is acceptable, and in some cases used exclusively. This confusion reached a new climax when in the Fall of 1999, NASA's $125 million Mars Climate Orbiter broke up in the Martian atmosphere, because scientists in one of NASA's subcontractors failed to convert critical data from the English sytem to the SI system of units.
We begin with Newton's Second Law, as follows:
In order to get a feel for the magnitudes of the SI units, recall the legend that Newton was inspired by an apple falling on his head. This is discussed in a delightful website by Mike Guidry of the University of Tennessee on Sir Isaac Newton in which we see a cartoon showing the apple falling on Newton's head. Well, the weight of a small apple is approximately one Newton. Furthermore, the mass of a plastic bottle containing one liter of water is approximately one kilogram.
We now consider the work done (W), the energy in transit requiring both the applied force and movement, as follows:
In order to gain an understanding of the different forms of energy we consider the (tongue-in-cheek) example of an attempt to cook a turkey by potential energy. The turkey is brought to the top of a 100 m building (about 30 stories) and then dropped from the ledge. The potential energy is thus converted into kinetic energy, and finally on impact the kinetic energy is converted into internal energy. The increase in internal energy is represented by an increase in temperature, and hopefully, if this experiment is repeated enough times the temperature increase will allow the turkey to cook. This remarkable experiment was first reported by R.C.Gimmi and Gloria J Browne - "Cooking with Potential Energy", published in the Journal of Irreproducible Results (Vol 33, 1987, pp 21-22).
What a disappointment! At 0.33°C per fall it will require repeating the experiment 600 times just to reach the cooking temperature of 200°C.
Over the years we have developed a basic Units Survival Kit in order to help convert between the USCS (English) system and the SI (International) system of units, as well as to develop a feel for the magnitudes of the various units.
We find that with the above Survival Kit we can determine many unit conversions between SI & English units, typically as demonstrated in the following block:
As we progress and learn new concepts we will add to this Survival Kit.
For purposes of analysis we consider two types of Thermodynamic Systems:
The Closed System shown above can be defined by its various Properties, such as its pressure (P), temperature (T), volume (V) and mass (m). We will introduce and define the various properties of thermodynamic interest as needed in context Furthermore the properties can be either Extensive or Intensive (or Specific). An extensive property is one whose value depends on the mass of the system, as opposed to an intensive property (such as pressure or temperature) which is independent of the system mass. A specific property is an intensive property which has been obtained by dividing the extensive property by the mass of the system. Two examples follow - notice that specific properties will always have kilograms (kg) in the units denominater.
The State of a system is defined by the values of the various intensive properties of the system. The State Postulate states that if two independent intensive property values are defined, then all the other intensive property values (and thus the state of the system) are also defined. This can significantly simplify the graphical representation of a system, since only two-dimensional plots are required. Note that pressure and temperature are not necessarily independent properties, thus a boiling liquid will change its state from liquid to vapor at a constant temperature and pressure.
We assume that throughout the system Equilibrium conditions prevail, thus there are no temperature or pressure gradients or transient effects. At any instant the entire system is under chemical and phase equilibrium.
A Process is a change of state of a system from an initial to a final state due to an energy interaction (work or heat) with its surroundings. For example in the following diagram the system has undergone a compression process in the piston/cylinder device.
The Process Path defines the type of process undergone. Typical process paths are:
We assume that all processes are Quasi-Static in that equilibrium is attained after each incremental step of the process.
A system undergoes a Cycle when it goes through a sequence of processes that leads the system back to its original state.
The basic unit of pressure is the Pascal [Pa], however practical units are kiloPascal [kPa], bar [100 kPa] or atm [101.32 kPa]. The Gage (or Vacuum) pressure is related to the Absolute pressure as shown in the diagram below:
The basic method of measuring pressure is by means of a Manometer, as shown below:
The Atmospheric Pressure is measured by means of a Mercury Barometer as follows:
Temperature is a measure of molecular activity, and a temperature difference between two bodies in contact (for example the immediate surroundings and the system) is the driving force leading to heat transfer between them.
Both the Fahrenheit and the Celsius scales are in common usage in the US, hence it is important to be able to convert between them. Furthermore we will find that in some cases we require the Absolute (Rankine and Kelvin) temperature scales (for example when using the Ideal Gas Equation of State), thus we find it convenient to plot all four scales as follows:
Notice from the plot that -40°C equals -40°F, leading to convenient formulas for converting between the two scales as follows:
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