1

11

Chapter Two: Basic Concepts of

Thermodynamics

22

• System is defined as a quantity of matter or aregion in space chosen for study.

• The mass or region outside the system is calledthe surroundings. They are separated byBoundary.

System

2

33

• Closed system (also known as a control mass)consists of a fixed amount of mass, and no masscan cross its boundary.

• Energy may cross the boundary of closed system

Closed System

44

In isolated system, even energy is not allowed tocross the boundary

Isolated System

NO

3

55

In Open system (also called control volume) Bothmass and energy can cross the boundary of acontrol volume.

Open System

66

System Properties

Any characteristic of a system is called a property. Somefamiliar properties are

– Pressure, P,

– Temperature, T,

– Volume, V,

– Mass, m.

– Density, specific gravity, or relative density, andSpecific volume

4

77

Types of PropertiesSpecific properties. Some examples of specific properties are

specific volume (v=V/m) and specific total energy (e=E/m).

Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density.

Extensive properties are those whose values depend on the mass of the system such as : volume V, and total energy E.

To determine if a property is intensive or extensive, divide the 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.

88

State and Equilibrium

• Set of properties that completely describes the condition, orthe state, of the system. Thermodynamics deals withequilibrium states.

• Equilibrium implies a state of balance.

• Thermal Equilibrium (temperature is the same through thesystem, Mechanical Equilibrium (no change in pressurethroughout the system with time).

• Phase equilibrium when the mass of each phase reaches anequilibrium level and stays there.

• Chemical equilibrium if its chemical composition does notchange with time.

5

99

Properties- is the state of a simple compressible system is

completely specified by two independent, intensiveproperties. A process is a system undergoes fromone equilibrium state to another. The path is theseries of states through which a system passes duringa process.

Volume

Pressure

initial

final

path

1010

Processes

- The prefix iso- is often used to designate a processfor which a particular property remains constant.

- An isothermal process, is a process during which thetemperature T remains constant;

- An isobaric process is a process during which thepressure P remains constant; and

- An isochoric (or isometric) process is a processduring which the specific volume v remains constant.

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1111

Steady and Unsteady States

• The term steady implies nochange with time.

• The opposite of steady isunsteady, or transient. The termuniform, however, implies nochange with location over aspecified region.

1212

Forms of Energy:

The total energy of a system is divided into two forms:

1. The macroscopic forms of energy are those whicha system possesses as a whole with respect tosome outside reference frame, such as kinetic andpotential energies .

2. The microscopic forms of energy are those which arerelated to the molecular structure of a system andthe degree of the molecular activity. The sum of allmicroscopic forms of energy is called the internalenergy of a system and is denoted by U.

7

1313

Forms of Energy:

• kinetic energy: The energy that a system possessesas a result of its motion relative to some referenceKE = ½ m v2 (J),

• potential energy: The energy that a systempossesses as a result of its elevation in agravitational field and is PE = mgz (J)

1414

Energy Balance

The total energy of a system consists of the kinetic,potential, and internal energies:

E = U + KE + PE .

Most closed systems remain stationary (no change intheir kinetic and potential energies). For stationarysystems E=U

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1515

Types of associated energies with U

1. Portion of the internal energy of a systemassociated with the kinetic energies of themolecules is called the sensible energy.

2. The internal energy associated with the phase of asystem is called the latent energy

3. The internal energy associated with the atomicbonds in a molecule is called chemical energy,

4. The energy associated with the strong bondswithin the nucleus of the atom itself is callednuclear energy.

1616

The largest source of air pollution is the motorvehicles, and the pollutants released by the vehiclesare usually grouped as hydrocarbons (HC), nitrogenoxides (NOx), and carbon monoxide (CO). There isalso (SOx) emitted to air from some ChemicalIndustries.

Acid Rain and Green house Effect.

Energy and Environment

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1717

Temperature and the Zeroth Law of Thermodynamics:

when an object is in contact with another object that isat a different temperature, heat is transferred fromthe object at higher temperature to the one at lowertemperature until both objects attain the sametemperature (Thermal Equilibrium). The zeroth lawcan be restated as two objects are in thermalequilibrium if both have the same temperaturereading even if they are not in contact.

1818

Absolute Temperature

-273.15

extrapolation

Experimental dataGas (A)

Gas (B)

Gas (C)

Gas (D)

P

T ( C )

10

1919

Temperature• We use four different measures of temperature:

Celsius (°C), Fahrenheit (°F), Kelvin (K),

Rankine (R).

• Kelvin and Rankine are absolute temperature

scales. Both are zero at absolute zero.

0 K=0 R

• Celsius and Fahrenheit are relative temperature

scales, and

0°C=273.15 K 0°C=32°F

0°F=460 R 100°C=212°F

2020

Temperature Conversion

Celcius

kelvin

1 DoC = 1 DK

Fahrenheit

Rankine

1 DoF = 1 DR

1 DoC = (1.8) DoF

1 DK = (1.8) DR

In ordinary usage the D symbol for the unit temperature

difference is omitted. Consequently, you have to infer from

the text whether oF, oC, oR, and K mean temperature or unit

temperature difference.

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2121

Temperature Conversion

2222

Example 1

Solution

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2323

Example 2

Convert 18 oC to oF

Convert 550 R to oC

4.64

32188.1

328.10

CTFT

2.328.1

32900

09046004.640

CT

FFT

2424

Pressure

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2525

Pressure

• Pressure is the force per unit area.

• Pressure has units of kPa, psi, atm, inches of Hg, mm of Hg, ft of H2O, and in of H2O, etc.

• Pressure is expressed as absolute pressure or relative pressure (i.e., the pressure above atmospheric pressure)

• Vacuum pressure is expressed in reference to atmospheric pressure.

• Differential pressure is the difference between two pressures.

2626

Pressure Conversion Factors

2

14.7 psi=1 atm

33.9 ft of H O=1 atm

29.92 of Hg=1 atm

101.3 kPa=1 atm

1.013 bar=1 atm

guage pressure + barometric pressure= absolute pressure

29.92 in. of Hg = 1 atm

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2727

• Pgage = Pabs - Patm (for pressures above Patm)

• Pvac = Patm - Pabs (for pressures below Patm)

P abs

P atm

P gage

P atm

P abs = 0

P atm

P abs

P vac

Absolute vacuum

Gage and Vacuum Pressure

2828

• Pressure in Depth : pressure in a fluid does not change in the horizontal direction. Pressure in a fluid increases with depth because more fluid rests on deeper layers, and the effect

of this “extra weight” on a deeper layer is balanced by an increase in pressure

• ∆P = P1 – P2 = ρgh = Pgage (h = pressure head)

• Manometers:

• P1 = P2 (horizontally) , But P2 = Patm + ρgh Figure (1)

• Example: A manometer is used to measure the pressure in a tank. The fluid used has a specific gravity of 0.85, and the manometer column height is 55 cm. If the local atmospheric pressure is 96 kPa, determine the absolute pressure within the tank.

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2929

Pressure Calculation Example

• Determine the pressure at the bottom of a 10 inch column of Hg (sp. g.=13.55).

30

Assignment One.

Problems:

2-59

2-67

2-69

2-75

2-103

Due date: 8/3/2011 (during lecture). Late submission will not be marked and student will get zero.

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31

Quiz One.

There will be quiz on next Tuesday, 8/3/2011. The quiz will be from Chapters 1 and 2.

3232

Pressure Example

• What is the absolute pressure in psi of a vacuum of 15 inch of Hg?

vacPatmPabsP

absP

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3333

Class Exercise

• Consider a diver swimming at 50 ft below the surface of the ocean. Determine the gauge and absolute pressure on diver in psi.

gage

atmPgPabsP

gP

3434

The Manometer

• The manometer is an instrument used to measure the pressure differences of liquids and/or gases.

• Small and moderate pressure differences can be measured by the manometer.

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3535

• SG = Density substance / water density

• Density of substance = 0.85 (1000kg/m3) =

850 kg/m3

• P = Patm + ρgh = 96 kPa + 850 kg/m3 * 9.81 m2/s *0.55m (1 kPa/1000N/m2) * ( 1 N / 1kg. m/s2) =100.6 kPa

Patm = 96kPa

h = 55 cm

SG = 0.85

P = ??

Example

Gas

3636

P1=Patm + ρ1gh1 + ρ2gh2 + ρ3gh3

P2= P1+ ρ1g(a + h) - ρ2gh – ρ1gaFluid

ρ2

a

h ρ1

21

h1

h2

h3

Fluid 1

Fluid 2

Fluid 3

P atm

1

A B

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3737

Example

P1 + ρwater gh1 + ρoil gh2 - ρmercury gh3 = Patm

ρwater=1000 kg/m3, ρmercury=13600 kg/m3 , ρoil=850 kg/m3

h1=0.1 m, h2=0.2 m , h3=0.35 m, Patm=85.6 kPa

Solving for P1 and substituting,

85.6 kPa + (9.81 m/s2)[(13,600 kg/m3) (0.35 m) -(1000 kg/m3) (0.1 m) – (850

kg/m3) (0.2 m)] [(1 N/1 kg · m/s2)( 1 kPa/1000 N/m2)] = 129.6 kPa

h3 h2

h1

1

2Water

Oil

Mercury

ρwater= 1000 kg/m3

ρmercury= 13600 kg/m3

ρoil= 850 kg/m3

3838

Example

P = Patm + W/A

= 0.97 bar + (60) (9.81)/(0.04) Pa[(1bar/105 Pa)] = 1.12 bar

Patm

W = mg

Patm = 0.97 bar

m = 60 kg

A = 0.04 m2

P=?

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3939

The atmospheric pressure (barometric pressure) is measured by a device called a barometer. As Torricelli measured the atmospheric pressure by inverting a mercury-filled tube into a mercury container that is open to the atmosphere. Note that the cross-sectional area of the tube have no effect on the height of the fluid column of a barometer

Patm = ρgh

Barometer and the Atmospheric Pressure

4040

• standard atmosphere, which is defined as the pressure produced by a column of mercury 760 mm in height at 0 °C

(ρ= 13,595 kg/m3) and (g = 9.807 m/s2). Called 760mmHg.

1 mmHg = 1 torr = 133.3 Pa

• 1 atm = 760 mm Hg = 760 mm torr

• If water instead of mercury were used to measure the standard atmospheric pressure, a water column will be about 10.3 m. How?

• As we go up atmospheric pressure drops. Why?

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End of Chapter Two