chapter 1 introduction to thermodynamics

8/6/2019 Chapter 1 Introduction to Thermodynamics

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Introduction to Thermodynamics


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Ability to acquire and explain the basicconcepts in thermodynamics


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The student should be able to explain:

System, boundary and surroundings.

Non-flow (control mass, closed) and flow (controlvolume, open) processes.

Intensive and extensive properties, zeroth law ofthermodynamics

Thermodynamics state (equilibrium)

Process (isobaric, isochoric, isothermal), cycles, steadyflow process


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1.1 System, boundary and surroundings1.2 Non-flow and flow processes

1.3 Intensive and extensive properties

1.4 Thermodynamic states and equilibrium


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What is Thermodynamics?

Greek Words

Therme

(heat)

Dynamis

(Power)

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The study of:

Energy

Transformation of useless energy (heat) to usefulone (work or power)

Interaction between energy and matter (liquids and

gases)


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HouseHouseHouseHouse----hold utensils appliances:hold utensils appliances:hold utensils appliances:hold utensils appliances:

Air-conditioner, heater, refrigerator

EnginesEnginesEnginesEngines::::

Automotive, aircraft, rocket

Plant/ FactoryPlant/ FactoryPlant/ FactoryPlant/ Factory

Refinery, power plants, nuclear power plant


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SystemSystemSystemSystem

region chosen

SurroundingsSurroundingsSurroundingsSurroundingsregion outside

the system

Boundary

to stu y t echanges of aphysicalproperty

Real orimaginarysurface thatseparates thesystem from itssurroundings


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Boundaryfixed

movable


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1.2 Non-flow and flow processesTypes of systems:

(a) isolated - no heat/ mass transfer across boundary

(b) closed(control mass) - only heat transfer across boundary

(c) open system(control volume) - heat & mass transfer across boundary

Non-flow processes Flow processes


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Forms of Energy

Forms of energy - thermal, mechanical, chemical, kinetic, potential,

electric, magnetic & nuclear

E = total energy i.e sum of all energy in a system

e = total energy = E (kJ/kg)

mass m

Forms of energy that make up the total energy of a system :

Energy form

macroscopic

microscopic

energy of a system as a wholewith respect to some outsidereference frames, e.g. KE, PE

- related to molecular structure of asystem and the degree of molecularactivity- independent of outside reference

frames


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Sum of all microscopic forms of energy = Internal Energy (U)

Macroscopic forms of energy

Kinetic energy (KE)

- result of motion relative to some

reference frame

KE = mv2/2 (kJ)

Potential energy (PE)

- due to elevation in a gravitationalfield

PE = mgh (kJ)

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Therefore, E = U + KE + PE (kJ)

where v = velocity of the system

relative to some fixed reference

frame (m/s)

m = mass of an object (kg)

where g = gravitational acceleration,9.81 m/s2

h = elevation of center of gravity of

a system relative to some

arbitrarily plane (m)


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Internal energy - sum of all microscopic forms of energy of a system

related to - 1) molecular structure

2) degree of molecular activity

I. EKE

molecular translation

molecular rotation

electron translation

sensible energy

de end on the

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Latent heat - Internal energy associated to with the phase of a system

- phase -change process can occur without a change in

the chemical composition of a system

PE molecular vibrationelectron spin

nuclear spin

temperature


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PropertyPropertyPropertyProperty - any characteristic of a system that describes a system

Some familiar properties are PPPP, TTTT, VVVV and mmmm. But can beextended to include less familiar ones such as viscosity,thermal conductivity, thermal expansion coefficient and etc

m

,

Specific gravity or relative density (ratio of the density of asubstance to the density of some standard substance at aspecified temperature) e.g. for water,

Specific volume, (m3/kg)

V

=

OH

s

2

=

mV=


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Properties

Intensive

Extensive

independent of the

size/extent of thesystem

dependent on the

size/extent of thesystem

T, P,

age,

colour

m

V

total E

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Specific properties - extensive properties per unit mass

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


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StateStateStateState a set of properties that describe the condition of asystem at certain time

At a given state, all the properties of a system have fixed values. Ifthe value of one property changes, the state will change to adifferent one.

E uilibrium stateE uilibrium stateE uilibrium stateE uilibrium state stead state state of balance

& no change with time

Thermal equilibriumThermal equilibriumThermal equilibriumThermal equilibrium T is the same throughout the system

Mechanical equilibriumMechanical equilibriumMechanical equilibriumMechanical equilibrium P is the same throughout

Phase equilibriumPhase equilibriumPhase equilibriumPhase equilibrium m of each phase unchanged

Chemical equilibriumChemical equilibriumChemical equilibriumChemical equilibrium chemical composition unchanged


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Thermal equilibrium(uniform temperature)


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ProcessProcessProcessProcess Any change that a system undergoes fromone equilibrium state to another

PathPathPathPath Series of states through which a systempasses during a process

Need to specify the initial & final states of the process, as wellas the path it follows, and the interactions with thesurroundings.


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When a process proceeds in such a manner that the system remainsinfinitesimally close to equilibrium state at all times.

Sufficiently slow process that allows the system to adjust to itselfinternally so that properties in one part of the system do not changeany faster than those at other parts.

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Slow compression(quasi-equilibrium)

Very fast compression(non-quasi equilibrium)


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The prefix iso- is often used to designate a process for which a particular

property remains constant.Isothermal Process a process when T remains constant

Isobaric P constant

Isochoric/ Isometric specific volume v remains constant

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A system is said to have undergone a cycle if it returns to its initial state at

the end of the process.

For a cycle, the initial & final states are identical

rocess

Process A

1

P

V


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Pressure

P = = Unit = N/m2 or Pa

Gas or liquid Pressure

Solids Stress

Common units

1 bar = 105 Pa

1 atm = 101,325 Pa = 1.01325 bars

Area

Force

AF

1 kgf/ cm2 = 0.9807 bar = 0.96788 atm English unit Ibf/in2 or psi

Absolute pressure Actual pressure at at given position &

measured relative to absolute vacuumGage pressure Difference between absolute pressure & local

atmospheric pressure

Vacuum pressure Difference between atmospheric pressure &

absolute pressure


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Absolute, gage & vacuum pressures are all +ve quantities & related to each

other by:

Pgage = Pabs - Patm (for pressure above Patm)

Pvac = Patm - Pabs (for pressure below Patm)

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In thermo, absolute pressure is always used unless stated.


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Small to moderate pressure difference are measured by amanometer and a differential fluid column of height h correspondsto a pressure difference between the system and the surroundingof the manometer.

Manometer

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P g h kPa= ( )


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Bourdon Tube

Modern pressure sensors:1) Pressure transducers

2) Piezoelectric material

Other Pressure Measurement Devices

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Example 1.1

A vacuum gage connected to a chamber reads 5.8 psi

at a location where the atmospheric pressure is

14.5 psi. Determine the absolute pressure in the

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chamber.

Solution:

Using Pvac = Patm - Pabs = 14.5 - 5.8 = 8.7 psi


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A vacuum gage connected to a tank reads 30 kPa at a locationwhere the atmospheric pressure is 98 kPa. What is the absolutepressure in the tank?

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Solution:Solution:Solution:Solution:Pabs = Patm - Pgage

= 98 kPa - 30 kPa = 68 kPa


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Example 1.3

A pressure gage connected to a valve stern of a truck tire reads 240 kPa at a

location where the atmospheric pressure is 100 kPa. What is the absolute

pressure in the tire, in kPa and in psia?

Solution:

Pabs = Patm - Pgage

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The pressure in psia is

Pabs = 340 kPa = = 49.3 psia

What is the gage pressure of the air in the tire, in psig?

Pgage = Pabs - Patm

= 49.3 psia - 14.7 psia

= 34.6 psig

= 100 kPa + 240 kPa

= 340 kPa

kPa

psia

3.101

7.14


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Both a gage and a manometer are attached to a gastank to measure its pressure. If the pressure gage reads

80 kPa, determine the distance between the two fluidlevels of the manometer if the fluids is mercury whosedensity is 13,600 kg/m3.

Example 1.4Example 1.4Example 1.4Example 1.4

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hg

=

h kPakg

m

m

s

N m

kPaN

k g m s

m

=

=

80

13600 9 807

10

1

0 6

3 2

3 3

2.

/

/

.


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Measure of hotness and coldness

Transfer of heat from higher to lower temp. until both bodies attainthe same temp. At that point, heat transfer stops and the two bodies

have reached thermal equilibriumrequirement: equality of temperature

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Two bodies are in thermal equilibrium when they have reached thesame temperature. If two bodies are in thermal equilibrium with athird body, they are also in thermal equilibrium with each other.

Temperature scales: Celcius (C)Fahrenheit (F)

Kelvin (K)

Rankine (R)


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Conversion:

T (K) = T (o

C) + 273.15T (R) = T (oF) + 459.67

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T (R) = 1.8 T(K)

T (oF) = 1.8 T(oC) + 32


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Conversion:

T(K) = T(C) + 273.15

T(R) = T(F) + 459.67

T K = (T2C +273.15) - (T1C + 273.15)

= T2C - T1C

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= TC

T R = TF


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Consider a system whose temperature is 18C. Express thistemperature in K, R and F.

AnsAnsAnsAns: 291 K, 523.8 R, 64.4: 291 K, 523.8 R, 64.4: 291 K, 523.8 R, 64.4: 291 K, 523.8 R, 64.4 ooooFFFF

Example 1.5Example 1.5Example 1.5Example 1.5

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The temperature of a system drops by 27F during a coolingprocess. Express this drop in temperature in C, K, R

AnsAnsAnsAns: 15: 15: 15: 15 ooooCCCC, 15 K, 27 R, 15 K, 27 R, 15 K, 27 R, 15 K, 27 R

....

chapter 1 introduction to thermodynamics

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