Definition and Basic Concept

Topics: Temperature, Thermodynamics, Pressure Pages: 17 (2384 words) Published: May 11, 2014
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Chapter 1
Definition and Basic Concepts
Thermodynamics is defined as the science of energy. Originally derived from Greek words : therme (heat) and dynamics (power) – descriptive of early attempts at conversion of heat into power. Modern interpretation includes all aspects of energy and energy transformations, power generation, refrigeration, and the relationships among properties of matter. There are two approaches in thermodynamics studies: (1) classical thermodynamics – a macroscopic approach where the whole system is viewed as one entity. It does not require knowledge of behavior of individual particles. It is the direct and easy method of solving engineering problems, (2) statistical thermodynamics – is a microscopic approach based on average behavior of large groups of individual particles. It is rather involved. In this course, the classical approach is adopted.

Many engineering systems and applications apply the principles of thermodynamics – refrigerator, air-conditioning system, water heater, computer, TV, internal combustion engines, rockets, jet engines, power plants and many more.

1.1

Dimensions and Units

A physical quantity is characterized by dimensions, and dimensions have units. There are seven primary or fundamental dimensions:
Dimension
Length
Mass
Time
Temperature
Electric current
Amount of light
Amount of matter

SI Unit
meter (m)
kilogram (kg)
second (s)
kelvin (K)
ampere (A)
candela (cd)
mole (mol)

English Unit
pound-mass (lbm)
foot (ft)
second (s)
rankine (R)

SI unit is based on decimal relationship between units. The standard prefixes in SI units : Multiple
1012
109
106
103
102
101
10-1
10-2
10-3
10-6
10-9
10-12

Prefix
tera, T
giga, G
mega, M
kilo, k
hecto, h
deka, da
deci, d
centi, c
milli, m
micro, 
nano, n
pico, p

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Secondary or derived dimensions are expressed in terms of the fundamental dimensions, for example: velocity V = (length)/(time) and energy E.
When writing equations, ensure that each term in an equation has the same unit – i.e. dimensionally homogeneous.
1.2

Systems and Control Volumes

System = a quantity of matter or a region in space chosen for study Surroundings = the mass or region outside a system
Boundary = real or imaginary surface that separates the system from its surroundings. The boundary can be fixed or movable. It is a contact surface shared by both the system and its surroundings. Mathematically it has zero thickness, contains no mass and zero volume. Closed System = (or control mass) consists of a fixed amount of mass, and no mass can cross its boundary. No mass can enter or leave a closed system, but energy (heat and work) can cross the boundary.

Open System = (or control volume) is a properly selected region is space, for which both mass and energy can cross the boundary.
Control Surface = boundaries of a control volume.
surroundings
imaginary boundary

CLOSED
SYSTEM

Heat
Work

min

real boundary

CV

mout

m = constant
boundary
Mass cannot cross the boundaries
of a closed system, but energy can

1.3

energy
A control volume (CV) with real and
imaginary boundaries (this is a nozzle)

Properties of a System

Property = any characteristic of a system. For example: pressure P, temperature T, volume V, and mass m.
Intensive property = property that is independent of the mass of a system, such as P, T. Extensive property = property that depends on the size – or extent – of the system, such as mass, volume.
Generally, uppercase letters denote extensive properties (m is an exception). Lowercase letters are used for intensive properties (P and T are exceptions).

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Specific property = extensive properties per unit mass. For example: specific volume (v = V/m) and specific internal energy (u = U/m).
To differentiate between intensive and extensive properties, divide the system into 2 equal parts. Each part will have the same value of intensive properties...
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