Basic Concepts of Thermodynamics

1. Basic Concepts of Thermodynamics
1

2. 1-1 Thermodynamics and Energy
• Thermodynamics is the science of energy (Probably we
may say the power of heat) .
• Energy is the ability to do work.
 Conservation of energy principle (first law of
thermodynamics): During an interaction (i. e. a process),
energy can change from one form to another but the total
amount of energy remains constant.
 Energy has quality as well as quantity; the second law
of thermodynamics.
2

3. Importance of thermodynamics
 All activities in nature involve interaction between energy and matter
(thermodynamics).
 Thermodynamics is encountered in many engineering systems and
other aspects of life.
 Energy conversion occurs in the cells of human body.
 Examples in an ordinary house: electric or gas range, the heating
and air-conditioning systems, the refrigerator, the pressure cooker,
the water heater, the shower, the iron, and even the computer and
the TV set.
 In automotive industry thermodynamics plays an essential role
regarding design and analysis.
 Power plants.
3

4. Micro vs. Macro
• Thermodynamic properties can be analyzed from a
microscopic or a macroscopic perspective:
- Classical thermodynamics treats matter as a ‘continuum’ –
macroscopic approach; a direct and easy way to the solution
of engineering problems.
- Statistical thermodynamics studies the statistical (i.e., random)
behavior of individual molecules – microscopic approach – and
then averages over all the molecules.
- We will focus entirely on the classical approach.
- Fortunately, the two approaches converge to the same answer.
4

5. 1-2 Dimensions and Units
•
Dimension: physical properties or characteristics of a system
-
Primary Dimensions: mass m, length L, time t, and temperature T
-
Secondary Dimensions (or derived dimensions): velocity V, energy E,
force F, and volume V
•
Units: magnitudes assigned to dimensions
•
Two Unit Systems: English and SI or System Internationale
English
SI
Conversion Factors
mass:
pound mass, lbm
kilogram, kg
1 lbm = 0.45359 kg
length:
foot, ft
meter, m
1 ft = 0.3048 m
force:
pound force, lbf
newton, N
1 lbf = 4.4482 N
work / energy:
British Thermal Unit, Btu
joule, J
1 Btu = 1055.06 J*
power:
Btu/h
watt, W
3.41214 Btu/h = 1 W
5

6. Important Note for Using British Units!
•
A conversion factor is needed when relating mass to either force, work,
or energy, i.e.,
Force = mass*acceleration/gc, where
gc = 1.0 (kg.m/s2) / N
= 32.17 (lbm.ft/s2) / lbf
•
A conversion factor is also used when dealing with ‘thermal energy’ such
that
One Btu (British thermal unit) = 778.169 ft-lbf
•
Horsepower ‘hp’ is also used as a unit such that
1 hp = 550 ft-lbf / s
6

7. Important terms:
Pound-force: the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate
of 1 ft/s2
Weight is a force (not mass) = mass x gravitational acceleration.
Specific Weight (w) is the weight per unit volume of a substance; w = density x
gravitational acceleration.
Joule is a unit of energy in the SI system; 1 J = 1 N . m
British thermal unit (BTU) is the unit of energy in the English system; 1 BTU =
the amount of energy required to raise the temperature of 1 lbm of water at 68 oF by 1
degree Fahrenheit.
1 Calorie (cal) is a unit of energy in the SI (metric) system defined as the amount
of energy required to raise the temperature of 1 g of water at 68 oC by 1 degree
Celsius.
Dimensional Homogeneity: Quantities added or subtracted from each other
must have the same units. The terms on both sides of an equation (separated
by either + or – sign) must have the same units for the equation to be correct
(see examples 1-1 and 1-2).
7

8. Standard Prefixes in SI units
8

9. Nomenclature
specific thermodynamic property, where x = u, h, s, …(J/kg)
x (lower case)
molar thermodynamic property, where X = U, H, S, …(J/mol)
X
total thermodynamic property, where X = U, H, S, …(J)
X
work, power (J, J/s)
W, W
heat, heat rate (J, J/s)
Q, Q
velocity (m/s)
V
volume, volumetric flow rate (m3, m3/s)
V,V
weight (N)
W
mole fraction: liquid or solid phase, vapor phase
x, y
molecular weight
M
mole, molar flow rate (mol, mol/sec)
n, n
mass, mass flow rate (kg, kg/s)
m, m
Dimension (example units)
Symbol
.
.
.
.
.
9

10. Closed and Open Systems
(a) Closed Systems:
controlled (fixed) mass.
Moving
boundary
Fixed
boundary
GAS
2 kg
3 m3
GAS
2 kg
1 m3
Closed
System
energy YES
(m= constant)
mass NO
If a closed system is not allowed to
exchange energy with its
surroundings, then it is called an
isolated system.
10

11. (b) Open system: controlled (fixed) volume
Water
Heater
(control
volume)
Control surface
Hot Water
Out
Cold Water
IN
Note:
Volume
is fixed
Examples of open systems: Pumps, compressors, turbines, boilers, Lungs of
humans, Nozzles, etc.
11

12. Properties of a System
Name of property Symbol definition Units (SI)
Density
Specific volume
Specific gravity, or
relative density
kg/m3
m3/ kg
s =

H2O
H2O=1000 kg/m3 @ 4oC
• Property: Any characteristic of a system.
Examples:
12

13. Classification of properties:
m
V
T
P

m/2 m/2
V/2 V/2
T T
P P
 
- Intensive properties: independent on system size (e.g., P, T, , e)
- Extensive properties: depend on system size (e.g., m, V, E)
Extensive
properties
Intensive
properties
13

14. Thermodynamic State and Equilibrium
• Thermodynamic state of a system – the condition of the system as
characterized by the values of its properties.
• There are many different types of equilibria that can be attained:
- Thermal: the temperature is the same throughout the system.
- Mechanical: the pressure is the same.
- Phase: no driving force for the total mass in each phase to change.
- Chemical: no driving force for chemical composition to change.
• We can show a state as a point on a phase diagram as long as the
continuum theory applies; thermodynamics deals only with
equilibrium states.
• Stable equilibrium state – a state in which the system is not capable of
any spontaneous change to another state without a finite change in
the surrounding. There are no driving forces to carry out a change.
14

15. Simple compressible system
A system is said to be a simple compressible system if it is not influenced
by electrical, magnetic, gravitational, motion, and surface tension effects.
- Typically, properties chosen are either :
≫ P & T
≫ P & specific volume (v = Volume / mass), or
≫ T & v
- The above 3 variables are the easiest to measure.
- The state of a two-phase system cannot be fully defined by P & T only.
State postulate: The state of a single-component, simple compressible
system, can be fully described by two intensive properties.
15