Thermo Lecture Notes CH1CH-1-2

Course Tutor: Jasim M. Mahdi / University of Baghdad

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year)

Introductory Concepts and Definitions Thermodynamics Thermodynamics is the study of energy and its transformations, and its relationship to the

properties of matter. Although, it is difficult to give a precise definition for energy, it can be

viewed as the ability to cause changes.

Thermodynamics studies the behavior of how objects and systems behave as energy is

transferred between them, what happen to objects as energy is added or subtracted.

Areas of Application of Thermodynamics: All natural processes are governed by the principles of thermodynamics. However, the

following engineering devices are typically designed based on the principles of

thermodynamics.

Automotive engines, Turbines, Compressors, Pumps, Fossil and Nuclear Power Plants,

Propulsion systems for the Aircrafts, Refrigeration, Air-conditioning and Heating Devices.

The principles of thermodynamics are summarized in the form of a set of axioms. These

axioms are known as four thermodynamic laws:

The zeroth law, the first law, the second law and the third law.


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The Zeroth Law deals with thermal equilibrium and provides a means for measuring

temperatures.

The First Law deals with the conservation of energy and introduces the concept of

internal energy.

The Second Law of thermodynamics provides with the guidelines on the conversion of

internal energy of matter into work. It also introduces the concept of entropy.

The Third Law of thermodynamics defines the absolute zero of entropy. The entropy of

a pure crystalline substance at absolute zero temperature is zero.

Thermodynamic system Thermodynamic system is defined as a quantity of matter or a region in space chosen for

study. The 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, real or imaginary. The boundary is the

contact surface shared by both the system and the surroundings, and through which energy and

mass may enter or leave the system. A system together with its surroundings is said to

constitute a universe.

A control volume with

Moving and fixed boundaries

A control volume with

real and imaginary boundaries

Imaginary boundary Real boundary

System a nozzle

System

Fixed boundary

Moving boundary

SYSTEM

BOUNDARY

SURROUNDINGS


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Types of Systems Systems may be considered to be closed or open:

Closed system (also known as a control mass) is one in which the system mass cannot cross

the boundary, but energy (in the form of heat or work), can.

As a special case of closed system, even energy is not allowed to cross the boundary, that

system is called an isolated system.

Open system or a control volume, as it is often called, is one in which mass can cross the

system boundary as well as energy.

Macroscopic and Microscopic Approaches It is well-known that a substance consists of a large number of particles called molecules. The

properties of the substance naturally depend on the behavior of these particles. For example,

the pressure of a gas in a container is the result of momentum transfer between the molecules

YES

mass

energy

Closed system

m = constant

NO

YES

energy

mass

Open System

(CV)

YES

mass

energy

Isolated system

m = constant

NO

NO


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and the walls of the container. However, one does not need to know the behavior of the gas

particles to determine the pressure in the container. It would be sufficient to attach a pressure

gage to the container. This macroscopic approach to the study of thermodynamics that based

on empirical laws to describe matter treated as a continuum, and does not require a knowledge

of the behavior of individual particles, is called classical thermodynamics. It provides a direct

and easy way to the solution of engineering problems. Another approach which concerns

directly with the structure of matter and characterize by statistical means the average behavior

of the molecules making up a system of interest, is called Microscopic approach , sometimes

called statistical thermodynamics.

Units: SI- units are used exclusively Fundamental units:

Mass kilograms kg

Length meter m

Time seconds s

Temperature Celsius / Kelvin oC / K

Derived units:

Force (F) Newton N

Pressure (P) Pascal Pa

Energy (E) Joule J

Newtons Law: Force = mass acceleration

F = m a

[N] = [kg] [m/s2] 1 N = 1 kgm/s2

P = F / A

[Pa] = [kgm/s2] [m2] 1 Pa = 1 kg/ms2

E = F x

[J] = [kgm/s2] [m] 1 J = 1 kgm2/s2


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Properties of a System Properties: are variables of a system to which a value can be assigned at given time without

knowledge of the history of the system.

Examples of properties: temperature, density, pressure, energy

Examples of non-properties: mass flow, heat, work

Not all properties are independent. Some are defined in terms of other ones. For example,

density is defined as mass per unit volume: Vm

= (kg/m3)

Sometimes the density of a substance is given relative to the density of a better known

substance. Then it is called specific gravity, or relative density (sg), and is defined as the ratio

of the density of a substance to the density of some standard substance at a specified

temperature (usually water at 4C, for which oH2 = 1000 kg/m3), That is:

OH2

sg

=

A more frequently used property in thermodynamics is the specific volume. It is the

reciprocal of density and is defined as the volume per unit mass:

==1

mV

Properties are considered to be either intensive or extensive.

Extensive properties are those whose values depend on the size or extent of the system, (i.e. the property is divided when the system is divided). Mass m, volume V, and total

energy E are some examples of extensive properties.

Intensive properties are those whose values are independent of the size or extent of the system, (i.e. the property doesnt change when the system is divided), such as

temperature, pressure, and density.

An easy way to determine whether a property is intensive or extensive is to divide the

system into two equal parts with a partition;


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Each part will have the same value of intensive properties as the original system, but half the

value of the extensive properties.

Extensive properties per unit mass are called specific properties. Some examples of specific properties are

o specific volume ( = V/m),

o specific total energy (e = E/m), and

o specific internal energy (u = U/m).

State and Equilibrium State: It is the condition of a system as described by the values

of its properties (P1,T1,.). The state of a simple compressible

system is completely specified by two independent, intensive

properties. A system is called a simple compressible system in

the absence of electrical, magnetic, gravitational, motion, and

surface tension effects. These effects are due to external force

fields and are negligible for most engineering problems.

Thermodynamics deals with equilibrium states. When the properties of the system show no

tendency to change, a state of thermodynamic equilibrium exists.

In an equilibrium state, there are no unbalanced potentials (or driving forces) within the

system.

A system that is in equilibrium experiences no changes when it is isolated from its

surroundings.

There are many types of equilibrium, and a system is not in thermodynamic equilibrium

unless the following condition satisfied: between the system and surroundings, there is no

difference in

1. temperature thermal equilibrium

2. pressure mechanical equilibrium

3. composition chemical equilibrium

m = 2 kg T2 = 20 C V2 = 2.5 m3

m = 2 kg T1 = 20 C V1 = 1.5 m3

(a) State 1 (b) State 2


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Processes and Cycles Process: It is any transformation of a system from one equilibrium state to another. The series

of states through which a system passes during a process is called the path of the process.

P- V diagram of a compression process of a gas

Cycle: It is a sequence of processes which return the system to its initial state.

Pressure Pressure (P): is defined as a normal force exerted by a fluid per unit area.

AFP = (Pa)

Units: 1 Pa = 1 N/m2

1 standard atmosphere = 101325 Pa

System

P

V

(1) (2)

1

2

V1 V2

Initial state

Final state

Process path

1

2

V

P

4

V

P

1

2

3

(a) A two-process cycle (b) A four-process cycle


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1 bar = 105 Pa = 100 kPa = 0.1 MPa

The actual pressure at a given position is called the 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 the

gage pressure. Pressures below atmospheric pressure are called vacuum pressures and are

measured by vacuum gages that indicate the difference between the atmospheric pressure and

the absolute pressure. Absolute, gage, and vacuum pressures are all positive quantities and are

related to each other by:

Pgage = Pabs - Patm > 0 (for pressures above Patm )

Pvac = Patm - Pabs < 0 (for pressures below Patm )

The pressure at a point in a fluid has the same magnitude in all

directions. The variation of pressure with elevation is given by

gdzdP

=

where the positive z direction is taken to be upward. When the

density of the fluid is constant, the pressure difference across a

fluid layer of thickness z is

P = P2 - P1 = gz

Patm

Patm

Pabs = 0

Patm

Pgage

Pabs Pvac

Pabs

2

z

P = Patm + gh

P = Patm 1


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Example The piston of the piston-cylinder devise containing a gas has a mass 60 kg and the

gravitational acceleration is 9.81(m.s-2). The cross-sectional area of the piston is 0.04 m2.

Determine the pressure inside the cylinder when the local atmospheric pressure is 0.98 bar.?

Solu.:

bar

AmgP

WPAPAF

patm

patmpp

117.11004.0

81.96098.0

P

0

5 =

+=

+=

+=

=

Temperature The concept of temperature is fundamental to thermodynamics. We know that a body at

high temperature will transfer energy to one at lower temperature. Consider two bodies with

different temperatures in contact with each other. Net energy transfer will be from the hotter

body to the colder body. At some point, the net energy transfer will be zero, and the bodies are

said to be in thermal equilibrium. Bodies in thermal equilibrium are defined to have the same

temperature. Temperature ( T),in units of degrees Celsius, oC, is a measure of hotness relative to

the freezing and boiling point of water. A thermometer is based on the thermal expansion of

mercury.

Gas

m = 60 kg

Ice Bath Boiling Water

0oC

100oC


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Microscopic point of view:

Temperature is a measure of the internal molecular motion, e.g., average molecule

kinetic energy, At a temperature of 273.15oC molecular motion ceases.

Temperature in units of degrees kelvin, K, is measured relative to this absolute zero

temperature, so

0 K = -273oC in general, T in K = T in oC + 273.13

Zeroth Law of Thermodynamics

If a system A is in thermal equilibrium with another system B and also with a third

system C, then all of the systems are in thermal equilibrium with each other. This is called the

zeroth law of thermodynamics. This is how a thermometer works. If a thermometer is placed in

a substance for temperature measurement, the thermometer's glass comes into thermal

equilibrium with the substance. The glass then comes into thermal equilibrium with the liquid

(mercury, alcohol, etc . . .) inside the thermometer. Because the substance is in thermal

equilibrium with the glass and the glass is in thermal equilibrium with the inner liquid, the

substance and liquid must be in thermal equilibrium by the zeroth law. And because they are

thermally equivalent, they must have the same temperature.

TA = TB

TA = TC

A B C

Zeroth law of thermodynamics: TA = TB = TC

Ice bath

0oC


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Energy Energy (E) is the capacity either latent or apparent to exert a force through a distance.

There are four primary forms of energy that are of interest in macroscopic thermodynamics:

a) Potential energy (P.E): is the energy due to elevation.

P.E = mgz (J)

b) Kinetic energy (K.E): is the energy of a moving mass.

K.E = 2mc2

1 (J)

c) Internal energy (U): is a measure of kinetic energy of the molecules and atoms that

make up the matter. It 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 internal energy of matter is measured by its temperature. Hot water has

more internal energy than the same amount of cold water.

d) Enthalpy (H): is the amount of energy in a system capable of doing mechanical work.

it is the sum of the internal energy and the flow work (PV)

H = U + PV (J)


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Tutorial # (1)

Problem 1

The level of the water in an enclosed water tank is (40 m) above ground level.

The pressure in the air space above the water is (120 kPa), and the density of the water

is (1000 kg/m3). What is the water pressure at a ground level?

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Problem 2

A gas is contained in a vertical frictionless piston-cylinder device. The piston

has a mass of (4 kg) and cross sectional area of (35 cm2). A compressed spring above

the piston exerts a force of (60 N). If the atmospheric pressure is (95 kPa), Determine

the pressure inside the cylinder.

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Problem 3

Two piston/cylinder arrangements, A and B, have their gas chambers connected

by pipe, see following figure. Cross-sectional areas are AA = 75 cm2 and AB = 25 cm2,

with the piston mass in A being mA = 25 kg. Assume outside pressure is 100 kPa and

standard gravitation. Find the mass mB so that none of the pistons have to rest on the

bottom.


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H.W # (1)

Q1: Four cubic meters of water at 25 C and 1 bar have a mass of 3990 kg. (a) List the values of two extensive and three intensive properties of the system. (b) If the local gravity g for the system is 9.7 m/s2, evaluate the specific weight.

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Q2: The acceleration of gravity is given as a function of elevation above sea level by the relation g = 9.81 - 3.3210-6 h m/s2, with h measured in meters. What is the weight of an airplane at 10 km elevation when its weight at sea level is 40 kN.

Ans.: 39.9 kN ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Q3: The lower half of a 10-m-high cylindrical container is filled with water ( = 1000 kg/m3) and the upper half with oil that has a specific gravity of 0.85. Determine the pressure difference between the top and bottom of the cylinder.

Ans.: 90.7 kPa ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Q4: A vertical, frictionless pistoncylinder device contains a gas at 250 kPa absolute pressure. The atmospheric pressure outside is 100 kPa, and the piston area is 30 cm2. Determine the mass of the piston. Ans.: 45.9 kg ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Q5: A valve in the cylinder pictured to the right has a cross-sectional area of 11 cm2 with a pressure of 735 kPa inside the cylinder and 99 kPa outside. How large a force is needed to open the valve?

Ans.: 699.6 N ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Q6: A vertical pistoncylinder device contains a gas at a pressure of 100 kPa. The piston has a mass of 5 kg and a diameter of 12 cm. Pressure of the gas is to be increased by placing some weights on the piston. Determine the local atmospheric pressure and the mass of the weights that will double the pressure of the gas inside the cylinder. Ans.: 95.7 kPa, 115.3 kg ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Q7: Two cylinders A and B are connected by a piston of two different diameters as shown in figure. Cylinder B contains a gas, while cylinder A contains oil that is pumped up to a pressure of 300 kPa by a hydraulic pump. The mass of the piston is 10 kg. Calculate the gas pressure in cylinder B. Ans.: 3.5 MPa

DB = 25 mm

Piston

Patm =100 kPa

DA = 100 mm

Pump

g

B

A

Thermo Lecture Notes CH1CH-1-2

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