THERMODYNAMICS

Dr N S Senanayake

Basic concepts

Content Working substance Macroscopic and Microscopic analysis

Properties State Phases Processes System Control volume

Working substance This is the substance that facilitate the

work and heat transfer in a thermodynamic system

Working substances are in general fluids that are capable of changing its physical shape Pure substance-single substance or mixture

of substances which has the same molecular structure throughout its mass. Ex. Water or steam (air is not a pure substance)

Macroscopic and Microscopic Analysis

The analysis of a substance by its properties is called macroscopic analysis

The analysis done on the behavior of the individual atoms and molecules of a substance is called microscopic analysis

In Engineering Thermodynamics we mainly focus on Macroscopic Analysis

Property A property of a system is any quantity

that helps to defines a unique position (state) of a substance

Thermodynamic properties are classifies as: Intensive properties

Independent of the mass of the substance Ex: pressure, temperature, density

Extensive properties Dependent on the mass of the system Ex: Volume, Enthalpy, Internal Energy

Property cont.. Any property that determines the

transfer of Mass, Momentum or energy is referred to as Transport Property

Examples: thermal conductivity, viscosity

State State is the condition of existence of the

substance or a system Knowledge of thermodynamic properties

of a substance defines the “State” At a particular state a substance has

certain properties that are functions of that state, and certain relations exist between the properties.

Phase Substances can exit in 3 phases

Solid Liquid Vapor or gas

Process Process is an operation that changes the

state of a substance.

Process from state 1 to state 2

Constant

pressure

process

Constant

volume

process

Polytrophic

process

CycleThe thermodynamic cycle consists of a series of operations carried out on a substance (fluid) after which it returns to its original state.

system A thermodynamic

system is defined as a volume in space or a well defined set of materials.

The imaginary outer edge of the system is called its boundary.

The boundary can be fixed or moving.

Open and Closed systems A system in which

no matter enters or leaves (i.e. crosses the boundary) is called a closed system

Mass remains constant in the process

A system in which matter crosses the boundary is called an open system.

Mass changes in the process

Examples for closed and Open Systems

Closed

system Open

system

Closed

system

Hydrological cycle

Examples for closed and Open Systems

Steam power plant

Open syste

m

Closed system

Open syste

m

Control volume A volume of a system demarcated for

the purpose of study is called control volume.

Temperature Temperat

ure is one of the most intrinsic properties of a substance

A measure proportional to the kinetic energy associated with the disordered microscopic motion of atoms and molecules.

Internal Energy (U) The store of energy which results from the

motion of atoms and molecules of a substance is referred to as Internal Energy

The term “heat” is used to give energy transfer from the substance

Symbol U is used to give the internal Energy

Enthalpy (H) Enthalpy is a measure of the total energy of

a thermodynamic system It includes

the internal energy (the energy required to create a system), and

the amount of energy required to make space for it by displacing its surrounding and establishing its volume and pressure

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