chapter 1

Introduction to Thermodynamics

Ability to acquire and explain the basic

concepts in thermodynamics

The student should be able to explain:

� System, boundary and surroundings.

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

� Intensive and extensive properties, zeroth law of thermodynamics

� Thermodynamics state (equilibrium)

� Process (isobaric, isochoric, isothermal), cycles, steady flow process

1.1 System, boundary and surroundings

1.2 Non-flow and flow processes

1.3 Intensive and extensive properties1.3 Intensive and extensive properties

1.4 Thermodynamic states and equilibrium

What is Thermodynamics?

Greek Words

Therme

(heat)

Dynamis

(Power)

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The study of:

� Energy

� Transformation of useless energy (heat) to useful

one (work or power)

� Interaction between energy and matter (liquids and

gases)

� 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

SystemSystemSystemSystem

region chosen to study the

SurroundingsSurroundingsSurroundingsSurroundingsregion outside the system

Boundary

to study the changes of a

physical property

Real or imaginary surface that separates the system from its surroundings

Boundaryfixed

movable

1.2 Non-flow and flow processes

Types 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

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 whole with respect to some outside reference frames, e.g. KE, PE

- related to molecular structure of a system and the degree of molecular activity- independent of outside reference frames

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 gravitational

field

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)

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

molecular vibration

sensible energy

depend 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

PEmolecular vibration

electron spin

nuclear spin

depend on the temperature

PropertyPropertyPropertyProperty - any characteristic of a system that describes a system

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

� Density (mass per unit volume), (kg/m3) depends on T & Pm

=ρ� Density (mass per unit volume), (kg/m3) depends on T & P

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

� Specific volume, (m3/kg)

V=ρ

OH

s

2ρ

ρρ =

m

V=ν

Properties

Intensive

Extensive

independent of the

size/extent of the

system

dependent on the

size/extent of the

system

T, P,ρ

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)

� State State State State a set of properties that describe the condition of a system at certain time

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

� Equilibrium stateEquilibrium stateEquilibrium stateEquilibrium state steady state/ state of balance � Equilibrium stateEquilibrium stateEquilibrium stateEquilibrium state steady 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)

ProcessProcessProcessProcess Any change that a system undergoes from one equilibrium state to another

PathPathPathPath Series of states through which a system passes during a process

� Need to specify the initial & final states of the process, as well as the path it follows, and the interactions with the surroundings.

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

� Sufficiently slow process that allows the system to adjust to itself internally so that properties in one part of the system do not change any faster than those at other parts.

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Slow compression(quasi-equilibrium)

Very fast compression(non-quasi equilibrium)

� 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

Process B2

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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

Process B

Process A

1

2P

V

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

A

F

1 atm = 101,325 Pa = 1.01325 bars

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 vacuum

Gage pressure Difference between absolute pressure & local

atmospheric pressure

Vacuum pressure Difference between atmospheric pressure &

absolute pressure

� 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.

� Small to moderate pressure difference are measured by a manometer and a differential fluid column of height h corresponds to a pressure difference between the system and the surrounding of the manometer.

Manometer

22

∆P g h kPa= ρ ( )

Bourdon Tube

Modern pressure sensors:

1) Pressure transducers

2) Piezoelectric material

Other Pressure Measurement Devices

23

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

chamber.

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chamber.

Solution:

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

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

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Solution:Solution:Solution:Solution:

Pabs = Patm - Pgage= 98 kPa - 30 kPa = 68 kPa

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

= 100 kPa + 240 kPa

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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

Both a gage and a manometer are attached to a gas tank to measure its pressure. If the pressure gage reads 80 kPa, determine the distance between the two fluid levels of the manometer if the fluids is mercury whose density is 13,600 kg/m3.

P∆

Example 1.4Example 1.4Example 1.4Example 1.4

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hP

g=

∆

ρ

hkPa

kg

m

m

s

N m

kPaN

kg m s

m

=

=

80

13600 9 807

10

1

0 6

3 2

3 3

2.

/

/

.

� Measure of hotness and coldness

� Transfer of heat from higher to lower temp. until both bodies attain the same temp. At that point, heat transfer stops and the two bodies have reached thermal equilibrium

requirement: equality of temperature

� Zeroth Law of Thermodynamics:

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� Zeroth Law of Thermodynamics:

Two bodies are in thermal equilibrium when they have reached the same temperature. If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

Temperature scales: Celcius (°C)

Fahrenheit (°F)

Kelvin (K)

Rankine (R)

Conversion:

T (K) = T ( oC) + 273.15

T (R) = T (oF) + 459.67

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T (R) = 1.8 T(K)

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

Conversion:

T(K) = T(°C) + 273.15

T(R) = T(°F) + 459.67

∆T K = (T2°C +273.15) - (T1°C + 273.15)

= T2°C - T1°C

= ∆T°C

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= ∆T°C

∆T R = ∆T°F

Consider a system whose temperature is 18°C. Express this temperature 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



31

The temperature of a system drops by 27°F during a cooling process. 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

Technology