thermodynamics for mechanical and industrial engineering

THERMODYNAMICS:

Lesson 1

GETTING STARTED IN THERMODYNAMICS:

INTRODUCTORY CONCEPTS AND

DEFINITIONS

1 Taught by Meng Chamnan

What is Thermodynamics?

• The word thermodynamics stems from the Greek words therme (heat) and dynamis (force).

• The capacity of hot bodies to produce work.

• Thermodynamics is study about energy and

properties of a system


System

Closed System

Control Volume


Definition

• A property is a macroscopic characteristic of a system such

as mass, volume, energy, pressure, and temperature to

which a numerical value can be assigned at a given time

without knowledge of the previous behavior (history) of the

system.

• The word state refers to the condition of a system as

described by its properties.

• A process is a transformation from one state to another.

• A system is said to be at steady state if none of its properties

changes with time.

• A thermodynamic cycle is a sequence of processes that

begins and ends at the same state.


• Extensive property if its value for an overall system is the sum

of its values for the parts into which the system is divided (ex:

mass, volume, energy, and several other properties..)

• Intensive properties are not additive in the sense and may be

functions of both position and time, whereas extensive

properties vary at most with time. (ex: specific volume,

pressure, and temperature)

• The term phase refers to a quantity of matter that is

homogeneous throughout in both chemical composition and

physical structure.

• A pure substance is one that is uniform and invariable in

chemical composition.

• Thermodynamics equilibrium is a balance condition among

force (mechanical equilibrium), temperature (thermal

equilibrium), phase equilibrium (no phase change), and

chemical equilibrium (no chemical reaction)

6

Example 1: As illustrated in this figure, water circulates

between a storage tank and a solar collector. Heated water

from the tank is used for domestic purposes. Considering the

solar collector as a system, identify locations on the system

boundary where the system interacts with its surroundings

and describe events that occur within the system. Repeat for

an enlarged system that includes the storage tank and the

interconnecting piping.

Taught by Meng Chamnan 7

Example 2: A dish of liquid water is placed on a

table in a room. After a while, all of the water

evaporates. Taking the water and the air in the room

to be a closed system, can the system be regarded

as a pure substance during the process? After the

process is completed? Discuss.


Units and Dimensions


SI is the abbreviation for Système International d’Unités (International System of Units)

1 0.453592lb kg 1 0.3048ft m

2 21 1 32.174 / 32.174 /lbf lb ft s lb ft s


Newton’s second law is

expressed as the equality

F ma

2 21 1 1 / 1 /N kg m s kg m s

Example 3:

An object has a mass of 20 kg. Determine its

weight, in N, at a location where the acceleration of

gravity is 9.78 m/s2.


Pressure Definition

14

Pressure and Pressure Units


The pressure, p, at the specified point is defined as the limit

'lim normal

A A

Fp

A

21 1 /pascal N m

3 2

5 2

6 2

1 10 /

1 10 /

1 10 /

kPa N m

bar N m

MPa N m

5 2

2

1.01325 10 /

14.696 / .

N m

lbf in

1 standard atmosphere (atm)

Specific Volume


1/v

mn

M

molar basis in terms of the kilomole (kmol) or the pound mole

(lbmol)

The number of kilomoles of a substance, n, is obtained by dividing

the mass, m, in kilograms by the molecular weight, M, in kg/kmol

specific volume v is defined as the reciprocal of the density

SI units for density

and specific volume

are kg/m3 and m3/kg

Example 4:

Atomic and molecular weights of some common

substances are listed in Appendix Tables T-1 and T-

1E. Using data from the appropriate table,

determine:

(a) the mass, in kg, of 20 kmol of each of the

following: air, H2O, CO2.

(b) the number of lbmol in 50 lb of each of the

following: N2, NH3, C4H10.


Example 5:

A spherical balloon has a diameter of 10 ft. The

average specific volume of the air inside is 15.1

ft3/lb. Determine the weight of the air, in lbf, at a

location where g = 31 ft/s2.


Temperature Scale


273.15oT C T K

1.8oT R T K

459.67o oT F T R

1.8 32o oT F T C

A process occurring at

constant temperature is

an isothermal process.

Example 6:

Convert the following temperatures from oC to oF:

(a) 21 oC,

(b) 17.78 oC,

(c) 100 oC,

(d) 273.15oC.

Convert each temperature to oR.


thermodynamics for mechanical and industrial engineering

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