CHAPTER 1

BASIC CONCEPTS OF THERMODYNAMICS

LEARNING OUTCOMES OF CHAPTER 1

Students should have thoseunderstandings of:

Applications of thermodynamics.

Basic consepts of system, energy,properties, state, process and cycles.

Units and dimensions in SI (SystemInternational).

2 important thermodynamic properties:temperature (T) and pressure (P) andhow to measure them.

THERMODYNAMICS & ENERGY

Definition of “thermodynamics” (Greekword):

– The ability to convert heat to power.– All aspects of energy and energy transfer

including power production / generation,refrigeration and property relation ofsubstances.

therme

(heat)

dynamics

(power)

BASIC LAWS OF THERMODYNAMICS

1st Law of Thermodynamics(=Conservation of Energy Principles)

2nd Law of Thermodynamics

Energy can change fromone form to anotherform with the amountof the energy keptconstant

Energy has its qualityand quantity: a realprocess occurs in thedecreasing quality ofenergy.

APPLICATIONS AREAS OF THERMODYNAMICS

Power plants

The human bodyAir-conditioning

systemsAirplanes

Car radiators Refrigeration systemsPower plants

The human bodyAir-conditioning

systemsAirplanes

Car radiators Refrigeration systems

Human body

Air conditioner / heater

Car radiator

Power PlantsRefrigeration system

Airplanes

DIMENSIONS & UNITS

DIMENSIONS

(= measure of physical quatity)

FUNDAMENTAL / PRIMARY

DIMENSIONS

DERIVED / SECONDARY DIMENSIONS

*

Mass (m), Length (L),Time (t), Temperature(T), Current (I) &Amount of matter (mol)

Velocity (v), Energy (E),Volume (V), Force (F),Power (P), etc.

* Derived dimensions = combination of a few primary dimensions. Eg: Velocity = Distance/Time = L/t

UNITS

(= magnitudes assigned to the dimensions)

UNIT ASAS / PRIMER

DERIVED / SECONDARY

UNITS*

-accompany primarydimensions

-accompany deriveddimensions

2 types of unit systems widely used:

i) English System / United States CustomarySystems (USCS)

ii) Metric System, SI (International System)

FUNDAMENTAL / PRIMARY

UNITS

Differences of Unit Systems

Fundamnetal / Derived Dimensions

SI Unit ES Unit

Mass (m) kg lbm, oz

Length (L) m ft, in

Time (t) s s

Temperature (T) K oC, oF, R

Ammount of matter (mol)

kmol lb mol

Velocity (v) ms-1 ft s-1

Energy (E) J (Joule) Btu, cal

Volume (V) m3 gal

Force (F) N (Newton) lbf

Power (P) W (Watt) hp

Pressure N/m2 (Pascal) psia, psig

Standard prefixes in SI units

Prefix Multipletera, T 1012

giga, G 109

mega, M 106

kilo, k 103

deci, d 10-1

centi, c 10-2

milli, m 10-3

macro, 10-6

nano, n 10-9

pico, p 10-12

Differences between SI and ES1) Force (F) = Mass x acceleration

F = ma (kgms-2)SI unit: newton (N). 1 N = force required to accelerate 1 kgmass at a rate of 1 m/s2.ES unit : pound-force (lbf). 1 lbf = force required to accelerate32.174 lbm (pound-mass) at a rate of 1 ft/s2.

1 N = 1 kgms-2 ; 1 lbf = 32.174 lbm. ft/s2

2) Weight (W) = a type of force W=mg (N)Weight (W) Mass (m)

(derived) (fundamental)Mass of a body is constant, but its weight can change dependingon gravitational acceleration (g) that varies with the placementof the body.

3) Work (W) = a form of energy = Force x Distance1 N.m = 1 J SI unit

ES unit: Btu (British Thermal Unit). 1 Btu = energy required toincrease the temperature of 1 lbm of water at 68oF by 1oF.Other unit : calorie (cal). 1 cal = energy required to increasethe temperature of 1 kg of water at 15oC by 1oC.

1 cal = 4.1868 J ; 1 Btu = 1.055 kJ

Dimensional HomogeneityIn engineering world, all equations must be dimensionallyhomogeneous every term in an equation must have thesame unit.

1) Addition, Subtraction & Equality OperationsEg : 4 s + 1.9 s - direct

1 kg + 2 lb - have to change to the same2 m + 1.5 ft unit10.6 N + 1.4 kgms-2 ?

2) Multiplication & Division OperationsEg : N X m2 = Nm2

kg x m2 m2

s

N x 1 ?m

SYSTEMSSystem = a quantity of matter or a region inspace chosen for study. It consists of:

– Surroundings = mass or region outside the system.

– Boundary = real or imaginary surface that separatesthe system from its surroundings – fixed or movable.

2 types of systems:– Closed systems / control mass

– Open systems/ control volumes

CLOSED SYSTEMSAlso known as control mass.Characteristics of closed systems:

– Contains a fixed amount of mass and no masscan cross its boundary.

– Energy in the form of heat or work can crossthe boundary.

– Volume of closed systems does not have tobe fixed.

In special case, when energy is notallowed to cross the boundary of closedsystems isolated system.2 common examples of closed systems:

– Closed/rigid tank– Piston-cylinder device

CLOSED SYSTEMS

Mass cannot cross theboundaries of a closedsystem, but energy can

An example of closed system witha moving boundary piston-cylinder device

OPEN SYSTEMSAlso known as control volumes.

Characteristics of control volumes:– Both mass and energy can cross its boundary called as

boundary surface.

– Its volume always fixed but its mass not necessarilyfixed.

Examples of control volumes:– Pumps, compressor, valves, heat exchangers, turbines.

Both mass and energycan cross theboundaries of controlvolume

ENERGY Exist in variable forms : heat, mechanical, kinetic,

potential, electric, magnetic, chemical and nuclear. Definition: Energy = Force x Distance (Unit = N.m = J) Total energy, E = amount of all forms of energies that

exist in a system.– Total energy based on a unit mass, e (kJ/kg):

Total energy can be divided into 2 groups:1) Macroscopic energies – related to motion and the influence

of some external effects such as gravity, magnetism,electricity, surface tension, kinetic and potential energies.

2) Microscopic energies – related to the molecular structure ofa system. Eg : chemical, nuclear, latent heat, sensible heat.The sum of microscopic energies internal energy, U.

mEe

Microscopic Energy The sum of microscopic energies internal

energy, U.

- Phase change of a system such asliquid phase changes to gas phase.

- Atom bonding in a molecule inchemical reactions.

- Strong bonds within the nucleus ofatoms.

Macroscopic Energy 2 main forms of macroscopic energies:

1) Kinetic energy – a system possesses as a result of itsmotion relative to some reference frame:

(kJ)

or, on a unit mass basis,(kJ kg-1)

with, V = velocity of the system relative to a fixedreference frame.

2) Potential energy – a system possesses as a result of itselevation in a gravitational field.

(kJ)

with, g = gravitational acceleration, z = elevation of thegravity centre of a system.

Other forms of macroscopic energies:– gravity, magnetism, electricity, surface tension.

2

2

KE mV

2

2

ke V

mgzPE

Total Energy By ignoring the effects of gravity, magnetism,

electricity and surface tension, hence the totalenergy is the sum of kinetic enery, potentialenergy and internal energy:

(kJ)

or, on a unit mass basis,(kJ kg-1)

Almost all closed systems remain stationary(KE=PE=0) during a process (unless stated) stationary systems. Hence, the change in totalenergy of a stationary system is equal to thechange of its internal energy:

mgzUUE mV 2

2

PEKE

gzuue V 2

2

peke

UE

Summary of Total Energy

ENERGY TOTALE=U+KE+PE

Microscopicenergy

Macroscopicenergy

Internal energyU

Kinetic energy, KEPotential energy, PE

Summary of Systems

SYSTEMS

CLOSED SYSTEMS

CONTROL VOLUMES

Isolated systemsE=0

Stationary systemsKE=PE=0

PROPERTIES OF A SYSTEM Any characteristic of a system property.

Eg of properties: pressure P, temperature T, volume V,mass m, viscosity, thermal conductivity, thermalexpansion coefficient, elevation etc.

PROPERTY

IntensiveProperty

ExtensiveProperty

-independent of themass of a system

Eg: Temperature TPressure PDensity

-depend on the sizeof a system

Eg: Mass mVolume VTotal Energy E

Definations of few properties Density, = mass per unit volume.

(kgm-3)

Reciprocal of density specific volume, v (=volume per unit mass)

Relative density, s or specific gravity (SG) = ratio of the density

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

specified temperature (usually water at 4oC, H2O = 1000 kg/m3).

All extensive properties per unit mass specific properties Eg:

Specific volume v=V/m

Specific total energy e=E/m

Specific internal energy u=U/m

Vmρ

O2Hρ

ρ

sρSG

m

Vv

1

STATE & EQUILIBRIUM

For a system not undergoing any change, at this pointall the properties can be measured or calculatedthroughout the entire system a set of propertiesthat completely describes the condition the state ofthe system.

At a given state, all the properties of a system havefixed values. If the value of even one property changes,the state will change to a different state.

m = 2 kgT1 = 20oC

V1 = 1.5 m3

m = 2 kgT2 = 20oC

V2 = 2.5 m3

State 1 State 2

State

Equilibrium The word equilibrium implies a state of balance. In an

equilibrium state there are no unbalanced potentials (ordriving forces) within the system experiences nochanges when it is isolated from its surroundings.

Types of equilibrium states:– thermal equilibrium if the temperature is the same

throughout the entire system.– Mechanical equilibrium if there is no change in pressure

at any point of the system with time.– Phase equilibrium when the mass of each phase reaches

an equilibrium level and stays there such as water and iceinequilibrium.

– chemical equilibrium if its chemical composition does notchange with time, that is, no chemical reactions occur.

A closed system achieves thermal equilibrium

Any change that a system undergoes from one equilibriumstate to another process, and the series of statesthrough which a system passes during a process theprocess path.

When a process proceeds in an equilibrium state at alltimes, it is called a quasi-equilibrium process can beviewed as a sufficiently slow process that allows thesystem to adjust itself internally so that properties inone part of the system do not change any faster thanthose at other parts.

PROCESS & CYCLES

Process

A compression process in a piston-cylinder device:

Processes in which one thermodynamic property is keptconstant:

Process Constant propertyIsobaric pressureIsothermal temperatureIsochoric/isometric volumeIsentropic entropy

Example of Process

Cycles A system is said to have undergone a cycle if

it returns to its initial state at the end ofthe process for a cycle the initial andfinal states are identical.

Process A

Process B1

2P

V

PRESSURE Pressure = normal force exerted by a

fluid per unit area.

Pressure only deals with gas or liquid.Pressure in solids normal stress.

Unit SI : Pascal (Pa) = Nm-2

English System : psi = lbf/in2 (pound-force per square inch), psia, psig. Otherunits: bar, standard atmosphere (atm).

AFP

Area

ForcePressure

P1

Pa

Pb

Pc P2

P3

P1=P2P3

Pa=Pb=Pc

Pressure at any point in a fluid is the same in alldirections.

Pressure varies in vertical directions due to gravityeffects but does not vary in the horizontal directions.

Absolute pressure, Gage pressure & Vacuum pressure The actual pressure at a given position absolute pressure -

measured relative to absolute vacuum (i.e., absolute zeropressure).

Most pressure-measuring devices are calibrated to read zero inthe atmosphere, and so they indicate the difference between theabsolute pressure and the local atmospheric pressure gagepressure.

Gage pressure = Absolute pressure – Atmospheric pressure

Pressures below atmospheric pressure vacuum pressures -measured by vacuum gages that indicate the difference betweenthe atmospheric pressure and the absolute pressure:

Vacuum pressure = Atmospheric pressure – Absolute pressure

Absolute, gage, and vacuum pressures are all positive quantities. Must use absolute pressures in thermodynamic problems. In ES unit, gage pressure and absolute pressure are

differentiated by their respective units:– psig (pounds force per square inch gage) and– psia (pounds force per square inch absolute),– but SI unit gives identical units.

Relation between absolute pressure, gage pressure & vacuum pressure

Pvac = Patm – Pabs

(for P<Patm)

Pgage = Pabs – Patm

(for P>Patm)

Pressure-measuring devices

Manometer

Barometer

Bourdon Tube

Manometer Consists of a glass or plastic U-tube containing one or

more fluids such as mercury, water, alcohol, or oil. Measures small and moderate pressure differences. The height of the fluid in the tube represents the

pressure difference between the system and thesurroundings of the manometer which is equal to the gagepressure:

PatmghPPP ρatm1

.m/s 9.8 onaccelerati nalgravitatiog

tube,- Uin the points obetween tw fluid ofheight the

tube,manometer in the fluid theofdensity ρ

tank,in the pressure gas

pressure, catmospheri

2

1

atm

h

P

P

ghPP

ghPPP

ρ

ρ

atmgas

atm21

Barometer Measures atmospheric pressure,

hence atmospheric pressure alsoknown as barometric pressure.

Consists of a mercury-filled tubeinverted into a mercury containerthat is open to the atmosphere.

Referring to Figure 1:

The height and cross-sectioanalarea of the tube does not affectthe height of the liquid in thebarometer tube (Figure 2).

tube.barometer in height mercury

,onaccelerati nalgravitatio

density,mercury ρ

tube,barometer of area sectional-crossA with,

ρ

ρ

todue Force A at ight Mercury we

atm

atm

atm

atm

h

g

ghP

APghA

APW

P

Merkuri

A1 A2 A3

Figure 2

Merkuri

A

C

B

h h

Pat

m

W=ghA

Figure 1

Bourdon Tubes Another type of commonly used

mechanical pressuremeasurement device.

Consists of a hollow metal tubebent like a hook whose end isclosed and connected to a dialindicator needle.

Calibrated to read zero, so itmeasures gage pressure.

Modern pressure sensors

pressure transducers - convertthe pressure effect to anelectrical effect such as achange in voltage, resistance,or capacitance.– smaller and faster, and they

can be more sensitive,reliable, and precise

Types of Bourdon Tubes

TEMPERATURE Temperature is one of the thermodynamic

properties - a measure of “hotness” or“coldness” or the energy content of a body.

When heat is transferred to a body, E

T. The temperature difference causes the heat

transfer from a hot body (with highertemperature) to another cold body (with alower temperature).

Two bodies are in thermal equilibrium whenboth of the bodies achieve similartemperature.

Similar to pressure, temperature applied inthermodynamic problems must be in absoluteunits. Absolute temperature scale in SI unit isKelvin and Rankine in unit ES.

Temperature scalesUnit

Property

SI ES

Temperature scale

oC oF

Absolute temperature scale

K R

Melting point 0oC 32oF

Boiling point 100oC 212oC

Relation between temperature scales:T(oF) = 1.8T(oC) + 32 (oC to oF)T(K) = T(oC) + 273.15 (oC to K)T(R) = T(oF) + 459.67 (oF to R)T(R) = 1.8T(K) (K to R)

Kelvin and Celcius Magnitude for each part of

1 K and 1C is similar, thesame case with 1 R and 1F(Figure 1).

And also,

Figure 1

T K T T

T T

T

= ( C + 273.15) - ( C + 273.15)

= C - C

= C

2 1

2 1

T R T F

SYSTEMATIC PROBLEM SOLVING Complicated thermodynamic problems can be solved by a systematic

approach. The followings are the systematic steps that can be taken to solve

thermodynamic problems:1. Read and understand the requirement of the problem.2. Draw a realistic sketch of the physical system involved, and list the

relevant information on the figure. Indicate any energy and massinteractions with the surroundings. List the given information on thesketch. Also, check for properties that remain constant during aprocess (such as temperature during an isothermal process), andindicate them on the sketch.

3. State any appropriate assumptions and approximations made tosimplify the problem to make it possible to obtain a solution. Assumereasonable values for missing quantities that are necessary. Forexample, in the absence of specific data for atmospheric pressure, itcan be taken to be 1 atm.

4. Determine the unknown properties at known states necessary to solvethe problem from property relations or tables, and hence the phaseof the substance can be determined.

5. Determine the process and sketch the process on property figuressuch as P-v or T-v.

6. Apply all the relevant basic physical laws and principles (such as theconservation of mass), and reduce them to their simplest form byutilizing the assumptions made.

7. Substitute the known quantities into the simplified relations andperform the calculations to determine the unknowns.

8. Reasoning, verification and discussion: Check to make sure that theresults obtained are reasonable and intuitive, and verify the validityof the questionable assumptions.