1

Basis concepts of thermodynamics such as:

System

Energy

Property

State,

Process

Cycle

Pressure, and temperature

1.1.- THERMODYNAMICS AND ENERGY:

Thermodynamics can be defined as the science of energy. Although everybody has a feeling of what energy is, it is difficult to give a precise definition for it. ENERGY CAN BE VIEWED AS THE ABILITY TO CAUSE CHANGES.

The name Thermodynamics stems from the Greek words THERME (Heat) and DYNAMIS (Power).

One of the most fundamental laws of nature is the CONSERVATION OF ENERGY principle. It simply states that during an interaction, energy can change from one form to another but the total amount of energy remains constant. That is, Energy cannot be created or destroyed.The FIRST LAW OF THERMODYNAMICS for example, is simply an expression of the conservation of energy principle, and it asserts that energy is a thermodynamic property.

The SECOND LAW OF THERMODYNAMICS asserts that energy has QUALITY as well as QUANTITY, and actual processes occur in the direction of decreasing quality of energy. For example, a cup of hot coffee left on a table eventually cool, but a cup of cool coffee on the same table never gets hot by itself.

Classical Thermodynamics: Macroscopic approach.Statistical Thermodynamics: Microscopic approach.

APPLICATION AREAS OF THERMODYNAMICSMany ordinary household utensils as heating and air-conditioning systems, the refrigerator,, the humidifier, the pressure cooker, the water heater, the shower, the computer, the TV and VCR set.

On a larger scale automotive engines, rockets, jet engines, and conventional or nuclear power plants. Should also mention the human body as an interesting application area of thermodynamics.

1.2.- A NOTE ON DIMENSIONS AND UNITS:

Any physical quantity can be characterized by DIMENSIONS. The arbitrary magnitudes assigned to the dimensions are called UNITS. Some basic dimensions such as mass m, length L, time t, and temperature T are selected as PRIMARY or FUNDAMENTAL DIMENSIONS, while others such as velocity (, energy E, and volume V are expressed in terms of the primary dimensions and are called SECONDARY DIMENSIONS, or DERIVED DIMENSIONS.

UNIT SYSTEMS:

English system, which is also known as the United States Customary System (USCS), and the metric SI (from Le Systme International dUnits), which is also know as the International System.

Table 1-1: The seven fundamental dimensions and their units in SIDimensionsUnits

Lengthmeter (m)

Masskilogram (kg)

Timesecond (s)

Temperaturekelvin (K)

Electric currentampere (A)

Amount of lightcandela (c)

Amount of mattermole (mol)

Table 1-2: Standard prefixes in SI units

MultiplePrefix

1012tera, T

109giga, G

106mega, M

103kilo, k

10-2centi, c

10-3mili, m

10-6micro, (

10-9nano, n

10-12pico, p

Some SI and English Units

In Si, unit of mass, length, and time are the kilogram (kg), meter (m), and second (s), respectively. The respective units in the English system are the pound-mass (lbm), foot (ft), and second (s).

The mass and length units in the two systems are related to each other by

1 lbm =0.45359 kg

1 ft = 0.3048 m

In the English system, force is usually considered to be one of the primary dimensions and is assigned a nonderived unit. This is a source of confusion and error that necessitates the use of a conversion factor (gc) in many formulas.

Force = (mass)(acceleration)

F= ma

In SI, the force unit is the newton (N), and it is defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s2. In the English system, the force unit is the pound-force (lbf) and is defined as the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s2.

That is,

1 N = 1 kg.m/s21 lbf = 32.174 lbm.ft/s2The term weight is often incorrectly used to express mass, particularly by the weight watchers, unlike mass, weight W is a force. It is the gravitational force applied to a body.

W = mg (N)

Work, which is a form of energy, can simply be defined as force time distance; therefore, it has the unit newton-meter (N.m), which is called a joule (J)1J = 1 N. m

1.3.- CLOSED AND OPEN SYSTEM

A THERMODYNAMIC SYSTEM, or simply a SYSTEM, is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the SURROUNDINGS.

The real or imaginary surface that separates the system from its surrounding is called the BOUNDARY. The boundary of a system can be fixed or movable.Systems may be considered to be closed or open, depending on whether a fixed mass pr a fixed volume in space is chosen for study.

A CLOSED SYSTEM (Also known as a CONTROL MASS) consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass can enter or leave a closed system. But energy, in the form of heat or work, can cross the boundary and the volume of a closed system not has to be fixed. If, as a special case, even energy is not allowed to cross the boundary, that system is called an ISOLATED SYSTEM.

CLOSED SYSTEM.- (CONTROL MASS)

An OPEN SYSTEM, or a CONTROL VOLUME, as it is often called, is a properly selected region in space. It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. Flow through these devices is best studied by selecting the region within the device as the control volume. Both mass and energy can cross the boundary of a control volume, which is called a CONTROL SURFACE.OPEN SYSTEM.- (CONTROL VOLUME)

1.4.- FORMS OF ENERGY.-

Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy E of a system.

MACROSCOPIC

KINETIC ENERGY

POTENTIAL ENERGY

MICROSCOPIC

INTERNAL ENERGY (Sum of all the microscopic forms of energy).

KINETIC ENERGY (KE).- Energy of a system result of its motion relative to some reference frame.

POTENTIAL ENERGY (PE).- Energy of a system result of its elevation in a gravitational field.

TOTAL ENERGY (E).- Consist of the kinetic, potential and internal energies.

1.5.- PROPERTIES OF A SYSTEM.-

Any characteristic of a system is called PROPERTY. Some familiar examples are pressure P, temperature T, volume V, and mass m.

The less familiar are viscosity, thermal conductivity and modulus of elasticity.

Not all properties are independent, however. Some are defined in terms of other ones.density

EMBED Equation.3 specific gravity or relative density

specific volume

INTENSIVE PROPERTIES.-

Independent of the size of a system.

pressure, temperature and density.

EXTENSIVE PROPERTIES.-

Dependent of the size or extent of a system.

mass, volume and total energy

Extensive properties per unit mass are called SPECIFIC PROPERTIES. Some examples of specific properties are specific volume , specific total energy and specific internal energy .

1.6.- STATE AND EQUILIBRIUM.-

STATE OF A SYSTEM.- Set of properties when the system not undergoing any change.

EQUILIBRIUM.- A system when is in equilibrium not undergoing any change.

Thermal equilibrium: temperature

Mechanical equilibrium: pressure

Phase equilibrium: two phases

Chemical equilibrium: chemical composition.

1.7.- PROCESS AND CYCLE.-

PROCESS.- Any change that a system undergoes from one equilibrium state to another.

PATH.- Series of states through which a system passes during a process.

QUASI-STATIC OR QUASI-EQUILIBRIUM, PROCESS.- When a process proceeds in such manner that the system remains infinitesimally close to an equilibrium state all times.

The prefix ISO- is often used to designate a process for which a particular property remains constant.

ISOTHERMAL PROCESS: temperature

ISOBARIC PROCESS: pressure

ISOCHORIC OR ISOMETRIC PROCESS: volume

CYCLE: A system undergoes a cycle if it returns to its initial state at the end of the process.

The P-V diagram of a compression process

A two-process cycle

A four-process cycle

1.8.- THE STATE POSTULATE.-

The state of a simple compressible system is completely specified by two independent, intensive properties1.9.- PRESSURE

Pressure is the FORCE EXERTED BY A FLUID PER UNIT AREA (with a gas or liquid).

The counterpart of pressure in solids is STRESS.

For a fluid at rest, the pressure at a given point is the same in all directions.

The pressure in a fluid increase with depth as a result of the weight of the fluid.

Since pressure is defined as force per unit area, it has the unit of newtons per square meter (N/m2), which is called pascal (Pa).

That is: 1Pa = 1N/m2Its multiples kilopascal (1kPa = 103Pa) and megaPascal (1MPa = 106Pa) are used commonly used.

Two other common pressure units are the bar and standard atmosphere1bar = 105 Pa = 0.1 MPa = 100 kPa

1atm = 101,325 Pa = 101.325 kPa = 1.01325 bars

In the English system, the pressure unit is pound-force per square inch (lbf/in2, or psi), and 1atm = 14.696 psi.The actual pressure at a given position is called ABSOLUTE PRESSURE, and it is measured relative to absolute vacuum, i.e., absolute zero pressure.

Most pressure measuring devices, however, are calibrated to read zero in the atmosphere, and so they indicate the difference between the absolute pressure and the local atmospheric pressure. This difference is called GAGE PRESSURE. Pressures below atmospheric pressure are called VACUUM PRESSURES and are measured by vacuum gages which indicate the difference between the atmospheric pressure and the absolute pressure.

Pgage = Pabs Patm (for pressures above Patm)

Pvac = Patm Pabs (for pressures below Patm)

MANOMETER.-

Small and moderate pressure differences are often measured by using a device know as a MANOMETER, which mainly consists of a glass or plastic U-tube containing a fluid such as mercury, water, alcohol, or oil.

P2 = P1AP1 = APatm + W

W = mg = Vg = Agh

P1 = Patm + gh (KPa)

La pressure difference can be expressed as:

P = P1 Patm = gh (kPa)

BAROMETER.-

The atmospheric pressure is measurement by a device called BAROMETER; thus the atmospheric pressure is often called the Barometric pressure.

APatm = W =ghAPatm = gh (kPa)

1.10.- TEMPERATURE AND THE ZERO LAW OF

THERMODYNAMICS.

Temperature is often defined as a measure of hotness or coldness.

Freezing cold, cold, warm, hot and red hot

Fortunately, several properties of materials change with temperature in a repeatable and predictable way, and this forms the basis for accurate temperature measurement. The commonly used mercury-in-glass thermometer, for example, is based on the expansion of mercury with temperature.

The ZERO LAW OF THERMODYNAMICS states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. If replacing the third body with a thermometer, the zeroth law can be related as: TWO BODIES ARE IN THERMAL EQUILIBRIUM IF BOTH HAVE THE SAME TEMPERATURE READING EVEN IF THEY ARE NOT IN CONTACT.TEMPERATURE SCALES.-

SI: Celsius scale (C)

English: Fahrenheit scale (F)

THE ABSOLUTE TEMPERATURE SCALES:

SI: Kelvin scale (K), which is related to the Celsius scale by:

T(K) = T(C) + 273.15

English: Rankine scale (R), which is related to the Fahrenheit scale by:

T(R) = T(F) + 459.67

T(K) = T(C)

T(R) = T(F)

T(R) = 1.8 T(K)

T(F) = 1.8 T(C) +32

h

Pabs

Patm

Pgage

Patm

Patm

Pvac

Pabs

Absolute

vacum

Absolute

vacum

Pabs = 0

z

P

4

2

1

V

P

1

3

V

P

1

2

Final state

Initial state

Process path

V

P

V2

V1

1

2

State 2

2 kg, 20 C,

3 m3

2 kg, 20 C,

1 m3

State 1

HOT WATER

OUT

WATER

HEATER

(CONTROL

VOLUME)

GAS

COLD WATER

IN

ENERGY YES

MASS YES

CONTROL

VOLUME

CONTROL

SURFACE

GAS

2 kg, 3 m3

FIXED

BOUNDARY

GAS

2 kg, 1 m3

MOVING

BOUNDARY

ENERGY YES

MASS NO

CLOSED

SYSTEM

m= constant

1

BOUNDARY

SURROUNDINGSG

BOUNDARY

SYSTEM

HEAT

PE = 7 units

KE = 3 units

PE = 10 units

KE = 0

Energy out

(4 units)

2

Energy storage

(1 unit)

Energy in

(5 units)

h

Patm

P1

A

W

h

h

Patm

A

W

PAGE 1Chapter 1

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