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					1.1 Thermodynamics and Energy
What is Thermodynamics?
Essentially, thermodynamics can be defined as the study of energy . Granted, this is a pretty broad definition, which suits it well, because thermodynamics is fundamentally a broad topic. The word thermodynamics always seems to crop up whenever matter and energy interact! In fact, thermodynamic considerations are key to making much of what you see around you come to life ...

The foundation of thermodynamics rests in a few basic concepts: ‚ First Law of Thermodynamics : A simple expression of the conservation of energy principle. Energy may transfer from form to another, but in such a way so that the total energy at any instant always remains constant (e.g. motion of a simple pendulum, shown below ...)

Max. pot. energy Max. kin. energy
‚ Second Law of Thermodynamics : The second law is a restriction of the first law, telling us whether or not a process is actually possible in the real world. For example, in the winter, your home loses heat to the cold surroundings. You'd make a pretty penny if could make a house get warm by itself in the winter! The scope of this course is limited to the study of classical thermodynamics , a macroscopic approach towards solving real world engineering problems. We don't need to know what every molecule is doing to analyze a system. A microscopic approach is used in statistical thermodynamics , where the average behavior of groups of molecules determine the thermodynamic characteristics of a system.

1.2 A Note on Dimensions and Units
All physical quantities are characterized by dimensions defined by the concept of units. Basic elementary dimensions are defined as primary dimensions (e.g.: mass, length, time, temperature ...) from which secondary dimensions are derived (e.g.: velocity, force, energy ...). There are two main system of units used today: the English and SI (Systeme International or metric) systems. Since the U.S. currently uses both systems, it is very important to know how to work with both system of units of measure.

The following table lists some important primary and secondary dimensions with English and SI units:

Dimension Primary:
mass length time temperature

English units
pound-mass, (lbm) foot, (ft) second, (s) degree Rankine, (oR)

SI units
kilogram, (kg) meter, (m) second, (s) kelvin, (K)

Secondary:
force energy pound-force, (lbf) British thermal unit, (Btu) newton, (N) joule, (J)

Very often, conversion of units between systems is necessitated to solve a problem. Conversion factors may readily be found on the inside cover of your text. A table of physical constants is available on the first page of the text, with values presented in both systems.

Some important things to know!
‚ Students often find it difficult to distinguish between pound-mass (lbm) and pound-force (lbf). They are two separate things altogether. A pound-force (lbf) is defined as the force required to accelerate a mass of 32.174 (lbm) at a rate of 1 (ft/s2 ). ‚ When solving equations, always include units! Since all terms in an equation must be dimensionally homogeneous, checking units serves as a value tool for spotting errors. ‚ The formula for the weight (W) of an object in a gravitational field, in many textbooks is incomplete. The complete formula considers a gravitational constant (gc ) given below ... where: gc = 32.174 (ft lbm/lbf s2 ) gc = 1 (kg m/N s2 ) Ignoring the gravitational constant in problems with English units will give you a result not consistent with units of force.

1.3 Closed and Open Systems
A thermodynamic system , or simply a system, is defined as a quantity of matter or a region in space chosen for study. The mass of the region outside the system is called the surroundings. The real or imaginary surface that separates the system from it surroundings is called the boundary. The boundary of a system may be fixed or movable. These terms are illustrated on the next page.

system system boundary
Two types of systems are considered in thermodynamics analyses:

surroundings

‚ Closed system (or control mass) : consists of a fixed amount of mass. No mass can enter or leave a closed system, but energy, on the other hand, may cross the boundary in the forms of heat or work. Note: The mass of a closed system is fixed (but not necessarily the volume, e.g. piston/cylinder device shown below).

2V V
moving boundary

fixed boundary

‚ Open system (or control volume) : consists of a properly selected region in space enclosing a device which involves mass flow such as a compressor, turbine, heat exchanger, etc ... Both energy and mass may cross the boundary of a control volume, which is called the control surface.

Hot Water out

WATER HEATER
(control volume)

Cold Water In

Important Point!
The thermodynamic relations that are applicable to closed and open systems are different! Therefore, it is extremely important that we recognize the type of system we have before we start analyzing it.

1.4 Forms of Energy
Energy exists in a number of forms, such as thermal, mechanical, kinetic, potential, chemical, etc ..., and their sum constitutes the total energy, E. Energy can generally be divided into two groups: ‚ Macroscopic: energy a system possesses as a whole with respect to some outside reference frame., such as kinetic energy (KE) and potential energy (PE). ‚ Microscopic: energy related to the molecular structure of a system and the degree of the molecular activity, and they are independent of an outside reference frame. The sum of all forms of microscopic energy is called the internal energy, U, of the system. The internal energy of a system is comprised of: v sensible energy: the portion of internal energy associated with the kinetic energy of molecules (i.e. translational, rotational, and vibrational kinetic energies). v latent energy: intermolecular forces between the molecules of a system. v chemical (or bond) energy : internal energy associated with atomic bonds in a molecule. During combustion processes, atomic bonds are broken and new ones are formed, altering the internal energy of the system. v Nuclear energy: energy harnessed from the bonds within the nucleus of an atom. Barring, special energy considerations (e.g. magnetic, chemical, surface-tension, etc ...), the total energy of a system can be expressed as ...

E = U + KE + PE

(1.4.1)

Forms of energy which can be contained in a system are called static forms of energy (e.g. internal, kinetic, potential energies). Dynamic forms of energy come from energy interactions, where energy crosses the system boundary during a process (e.g. heat transfer and work).

1.5 Properties of a System
Any characteristic of a system is called a property (pressure P, temperature T, volume V, mass m). Intensive properties are those which are independent of the size of a system, such as pressure, temperature, and density. Extensive properties are those whose values depend on the size of the system, such as mass, volume, and total energy. An easy way to determine if a property is intensive or extensive is to divide a system into two equal parts with a partition. Each part will have the same value of intensive properties as the original system, but half the value of the extensive properties.

m V T P ρ

0.5m 0.5V T P ρ
T

0.5m 0.5V T P
ρ

Extensive Properties Intensive Properties

Generally, uppercase letters denotes extensive properties (with mass, m, as an exception). Lowercase letters are usually reserved for intensive properties (with pressure, P and temperature, T, as a major

exception). Extensive properties per unit mass are called specific properties. Some examples are: specific volume, v (V/m), specific total energy, e (E/m), and specific internal energy, u (U/m).

1.6 State and Equilibrium
The condition of a system described by its' properties (temperature, pressure, etc ...) is defined as its' state. At a given state, all properties of a system have fixed values. The study of this course is limited to dealing with equilibrium states. In a equilibrium state there are no unbalanced potentials (or driving forces) within the system (e.g. the driving potential for heat flow is temperature difference). There are many types of equilibrium, but a system is said to be in thermodynamic equilibrium if the following relevant types of equilibrium are satisfied: ‚ thermal equilibrium : The temperature is the same throughout the entire system. ‚ mechanical equilibrium : The pressure at any point in the system does not change with time. It may however change with elevation due to gravitational effects (fluid statics), but is usually negligible in thermodynamics systems. ‚ phase equilibrium: The mass of each phase reaches an equilibrium level and stays there. ‚ chemical equilibrium : The chemical composition of the system does not change with time (i.e. chemical reactions do not occur).

Important Note!
The solution to all of the thermodynamic problems encountered in this course assume that all of the relevant equilibrium criteria are satisfied !!

1.7 Processes and Cycles
Any change that a system undergoes from one equilibrium state to another is called a process., and the series of states through which a system passes during a process is called the path of the process.

state 2 state 1 process path

When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasi-static, or quasi-equilibrium process . A quasi-equilibirium process can be viewed as a sufficiently slow process which allows the system to adjust itself internally so that properties in one part of the system don't change any faster than at other parts. Quasi-static processes are ideal processes and not true representations of actual system processes. They are very useful, easy to analyze and serve as an important comparison tool for evaluating efficiencies of actual systems.

Some important types of processes are listed below: ‚ isothermal process : constant temperature process ‚ isobaric process: constant pressure process ‚ isochoric (or isometric) process : constant specific volume process A system is said to have undergone a cycle if it return to its initial state at the end of a process.

P

3 4 2

1 V

1.8 The State Postulate
The number of properties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is completely specified by two, independent, intensive properties. A simple compressible system is a system in the absence of electrical, magnetic, gravitational, motion, and surface tension effects. The key word in the state postulate is independent. Two properties are independent if one can be varied while the other is held constant. For example, pressure and temperature during a phase-change process are dependent during the melting of ice because melting is an isothermal process!

Important Note!
Make sure you are able to distinguish between extensive and intensive properties!

1.9 Pressure
In a thermodynamic sense, pressure is defined as the force exerted by a fluid per unit area. For static fluids, the pressure at a point increases linearly with depth to counter the force produced by the weight of fluid at upper levels. Units of pressure include pascal (pa), kilopascal (kPa), bar, atmospheres (atm), and pound-force per square inch (psi). The actual pressure at a given point is called the absolute pressure., measured relative to absolute vacuum (zero absolute pressure). The difference between absolute and atmospheric pressures is called the gage pressure. The difference between atmospheric and absolute pressures (below atmospheric) is called the vacuum pressure. Absolute, gage, and vacuum pressures are all positive quantities and related by:

P gage = P abs − P atm P vac = P atm − P abs

(for pressures above Patm ) (for pressures below Patm )

(1.9.1) (1.9.2)

P gage P vac P atm P abs Absolute Vacuum P =0 abs Absolute Vacuum

P atm

P abs

Small and moderate pressure differences are measured with devices called manometers, which mainly consists of a glass or plastic U-tube containing a fluid such as mercury, water, alcohol, or oil. Consider the following pressure-measuring system and free-body diagram ...

P

atm

A

GAS
1 2

h W

P

1

Since fluid pressure does not vary in horizontal direction, P1 = P2 . Since the fluid column, h, is in static equilibrium, the sum of the forces in the vertical direction must equal zero. Therefore,

AP 1 = AP atm + W ,where: W = mg = Vg = Ahg
... or:

(1.9.3)

P = P 1 − P atm = gh

(1.9.4)

A barometer is an atmospheric pressure measuring device. It is similar to the barometer (usually mercury-filled), but is inverted so its' neck is immersed in a container open to the atmosphere. atmoshpere

C h B
mercury

A free-body analysis of the barometer gives ...

P atm = gh

(1.9.5)

1.10 Temperature and the Zeroth Law of Thermodynamics

The Zeroth Law of Thermodynamics states that if two bodies are in thermal equilibrium with a third body, then they are also in thermal equilibrium with each other. What does this imply? Well, if the third body is a thermometer, the Zeroth law can be restated as: Two bodies are in thermal equilibrium if they both have the same temperature reading even if they are not in contact. In other words, no temperature difference --> no heat transfer! This law is so trivial, it was actually overlooked when the first and second laws were presented--that's why it was labeled the Zeroth law ! Today, there exist several scales used to measure temperature. The Celsius (or Centigrade) and Fahrenheit scales, are SI and English temperature scales, based on two-points (freezing and boiling temperatures of water) and are not really used much in thermodynamics. The more appropriate SI and English scales are the Kelvin and Rankine scales, based on an absolute system. All motion (processes, life, etc ..) cease to exist at 0 (K or R -- there are no degree signs for absolute scales!). Get used to working with absolute temperatures!! Here are some useful temperatures conversions:

T(K) = T( o C) + 273.15 T(R) = 1.8T(K) T(K) = T( o C)

T(R) = T( o F) + 459.67 T( o F) = 1.8T( o C) + 32 T(R) = T( o F)


				
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posted:7/19/2009
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