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1.8 Principles of heat and fluid flow 17
crosses the boundary of the system. In open system (control volume) quantity of
mass is not constant and mass can cross the boundary. Open system exchanges both
matter and energy with its surroundings while the closed system only exchanges
energy with its surroundings. The isolated system exchanges neither energy nor mat-
ter with its surroundings. An example of a true isolated system is the universe with
of energy stored in it, is an isolated system.
Each system is characterized by its properties. Thermodynamic properties are a
macroscopic characteristic of a living entity to which a numerical value is assigned
at a given time without knowledge of its history. Properties are either intensive (exist
at a point in space, like temperature, pressure, and density) or extensive [depends
on the size (or extent) of the system, like mass and volume]. There are a number
of different intensive properties that are used to characterize material behavior. The
three most important independent properties that usually describe a system are tem-
perature, pressure, and specific volume [52]. Temperature is the measure of the rela-
tive warmth or coolness of a body. In another word, it is the intensity of heat in an
object and is expressed mainly by a comparative scale and shown by a thermometer.
Pressure is the amount of force that is exerted on a surface per unit area and specific
volume is the number of cubic meters occupied by 1 kg of a particular substance.
The four laws of thermodynamics that describe the temperature (zeroth law of
thermodynamics), energy (first law of thermodynamics), entropy (second law of
thermodynamics) and entropy of substances at the absolute zero temperature (third
law of thermodynamics) are discussed in detail in the following sections.
1.8.3.1 Zeroth law of thermodynamics
This law serves as a basis for the validity of temperature measurement. The zeroth law
of thermodynamics indicates that if two bodies are in thermal equilibrium with a third
body, they are also in thermal equilibrium with each other. Replacing the third body
with a thermometer helps measuring temperature of a system. Temperature is measured
by means of a thermometer or other instrument having a scale calibrated in units called
degrees. The size of a degree depends on the particular temperature scale is being used.
1.8.3.2 The first law of thermodynamics
Based on experimental observation, the energy can neither be created nor destroyed;
it can only change forms. The first law of thermodynamics (or the conservation of
energy principle) states that during an interaction between a system and its surround-
ings, the amount of energy gained by the system must be exactly equal to the amount
of energy lost by the surroundings.
For a closed system (control mass), the first law of thermodynamics is shown as:
Net amount of energy transfer as heat Net change in amount of enery
and work to/or from the system = (increase/or decrease)with in the system Net amount of energy transfer as heat and work to/or from
the system =Net change in amount of enery(increase/or
or
decrease)with in the system
Q −W = ∆E (1.34) Q−W=∆E
