1. Thermodynamics Introduction and Basic Conceptsby
Assit. Prof. Channarong Asavatesanupap
Mechanical Engineering Department
Faculty of Engineering
Thammasat University

2. INTRODUCTION
Thermodynamics is the study of energy conversion between heat and mechanical work, and subsequently the macroscopic variables such as temperature, volume and pressure.
One of the most fundamental laws of nature is the conservation of energy principle. It simply states that during an energy interaction, energy can change from one form to another but the total amount of energy remains constant. That is, energy cannot be created or destroyed.

3. Closed, Open, and Isolated Systems
A thermodynamic system, or simply 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 surroundings is called the boundary. The boundary of a system may be fixed or movable.
Surroundings are physical space outside the system boundary.
System
Boundary
Surroundings
Systems may be considered to be closed or open, depending on whether a fixed mass or a fixed volume in space is chosen for study.

4. A closed system consists of a fixed amount of mass and no mass may cross the system boundary. The closed system boundary may move.
Examples of closed systems are sealed tanks and piston cylinder devices (note the volume does not have to be fixed). However, energy in the form of heat and work may cross the boundaries of a closed system.

5. An open system, or control volume, has mass as well as energy crossing the boundary, called a control surface. Examples of open systems are pumps, compressors, turbines, valves, and heat exchangers.
An isolated system is a general system of fixed mass where no heat or work may cross the boundaries. An isolated system is a closed system with no energy crossing the boundaries and is normally a collection of a main system and its surroundings that are exchanging mass and energy among themselves and no other system.

6. Properties of a system volume V, mass m, density r, pressure P,
Any characteristic of a system is called a property. Some familiar properties are
volume V,
mass m,
density r,
pressure P,
temperature T
and etc.

7. Density is defined as mass per unit volume
20 C , 1 atm
r = 998 kg/m3
The reciprocal of density is the specific volume, which is defined as
Specific Gravity SG 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).

8. Temperature (T) is a measure of the average energy of motion, or kinetic energy, of particles in matter. (or a measure of hotness and coldness)
Temperature scales
common scale:
Celsius scale  °C SI unit
Fahrenheit scale  °F English unit
The Celsius scale is related to the Fahrenheit scale by
Thermodynamic scale(Absolute scale):
Kelvin  K SI unit
Rankine  R English unit

9. Temperature scales
The common scales are related to the absolute scale by
SI unit
English unit
Example: Water boils at 100C at one atmosphere pressure.
At what temperature does water boil in F, K and R.
T(°F) = 100x = 212 °F
T(K) = = k
T(R) = = F

10. Pressure(P) is the force per unit area applied in a direction perpendicular to the surface of an object N m2
P = F A
(Pa)
For English system,

11. Pressure scales Absolute scale:
Absolute pressure is the pressure that is measured relative to absolute
zero pressure (absolute vacuum).
Gage scale:
Gage pressure is the pressure that is indicated on a pressure-measuring
device (called a pressure gage). Generally, the device is calibrated to read zero in
the atmosphere.

12. Vacuum pressure
Pressures below atmospheric pressures are called vacuum pressures.
A device that is used to measure vacuum pressure is called a vacuum gage.
Pressure
Symbol
Absolute Pa
Gage Pg
Pressure
Eng. unit
Absolute
psia
Gage
psig

13. Example A pressure gage connected to a valve stem 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?
The pressure in psia is
What is the gage pressure of the air in the tire, in psig?

14. Intensive and Extensive properties
Intensive properties are those that are independent of the mass of a system.
Extensive properties are those whose values depend on the size—or extent—of the system.

15. U = CV T = m cV T [kJ/kg] cV = specific heat Capacity [kJ/kg-K] U
Internal energy (U)
is defined as the sum of all the microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and can be viewed as the sum of the kinetic and potential energies of the molecules.
Properties
Symbol
Unit
Extensive U
J [Joule]
Intensive
u=U/m
J/kg
U = CV T = m cV T [kJ/kg]
Gas:
where CV = heat capacity [kJ/K]
cV = specific heat Capacity [kJ/kg-K]

16. State, Equilibrium, Process, and Properties State State
A set of properties that completely describe the condition of the system is called “state of the system”
Equilibrium
A system is said to be in thermodynamic equilibrium if it maintains thermal (uniform temperature), mechanical (uniform pressure), phase (the mass of two phases, e.g., ice and liquid water, in equilibrium) and chemical equilibrium.
P=Patm and T=20C  Liquid

17. Process
Any change from one state to another is called a process. During a quasi-equilibrium or quasi-static process the system remains practically in equilibrium at all times. We study quasi-equilibrium processes because they are easy to analyze (equations of state apply) and work-producing devices deliver the most work when they operate on the quasi-equilibrium process.
Heating process
Liquid water
State 1
Steam
State 2
In most of the processes that we will study, one thermodynamic property is held constant. Some of these processes are
Constant Pressure Process
Water F
System
Boundary
Process
Property held constant
isobaric
pressure
isothermal
temperature
isochoric
volume
isentropic
entropy (see Chapter 7)

18. We can understand the concept of a constant pressure process by considering the above figure. The force exerted by the water on the face of the piston has to equal the force due to the combined weight of the piston and the bricks. If the combined weight of the piston and bricks is constant, then F is constant and the pressure is constant even when the water is heated.
We often show the process on a P-V diagram as shown below.

19. First Law of Thermodynamics
is an expression of the conservation of energy principle.
Ein – Eout = DEsys
where DEsys = Efinal – Einit

20. Closed System First Law
Energy can cross the boundaries of a closed system in the form of heat or work.
According to classical thermodynamics, we consider the energy added to be net heat transfer to the closed system and the energy leaving the closed system to be net work done by the closed system. So
where
Sign Convention
Heat transfer (Q) into a system is defined as positive
Heat transfer (Q) out of a system is defined as negative
Work (W) done by a system is defined as positive
Work (W) done by surrounding is defined as negative

21. Normally the stored energy, or total energy, of a system is expressed as the sum of three separate energies. The total energy of the system, Esystem, is given as
The change in stored energy for the system is
Now the conservation of energy principle, or the first law of thermodynamics for closed systems, is written as

22. + No motion (DKE=0) No elevation change (DPE=0)
If the system does not move with a velocity and has no change in elevation, the conservation of energy equation reduces to
No motion
(DKE=0) +
No elevation change
(DPE=0)

23. Q  Positive Tsys Tsurr Tsurr > Tsys Example Gas in a piston
Heat transfer into a system
Tsys
Tsurr
Q  Positive
Tsurr > Tsys

24. Tsys
Tsurr
Q  Negative
Tsurr < Tsys

26. Heat and work transfer into a system

27. Example A container of 3kg air is heated by a candle as shown in the figure. Determine the amount of heat if the temperature of air rises by 20C
Air in a container is considered as a closed system
First law for a closed system
Assumption: No motion of the system
2. No change of elevation
3. No work transfer
4. Air behaves like ideal gas

28. Qnet = CvDT = m cv DT Qnet = 3kg x 1.012 kJ/kg-K x 20 K
DU = CV DT (Ideal gas)
Qnet = CvDT = m cv DT
Qnet = 3kg x kJ/kg-K x 20 K
= kJ