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Concepts and Definitions

Thermodynamics

The science of energy and entropy

the science that deals with heat and work and the

properties of substances that bear a relation toheat and work

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Concepts and Definitions

Stems from the greek word therme (heat) and

dynamis (power)

basis is experimental observation and formalized

into basic laws which are the First, Second, Third,and Zeroth laws of thermodynamics

the word thermodynamics was first used in a

publication by Lord Kelvin in 1849

The first textbook was written in 1859 by William

Rankine, at the University of Glasgow

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Applications of Thermodynamics

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The Thermodynamic System

A system is defined as a quantity of matter or

a region in space chosen for study. The mass

or region outside the system is called the

surroundings. The real or imaginary surfacethat separates the system from its

surroundings is called the boundary.

The extent of the system in space at any giventime is defined by the system boundary

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The envelope that represents the systemboundary which encloses the thermodynamicsystem is also known as the system control

surface The boundary can be fixed or movable

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Types of System

1) Closed System

2) Open System

3) Isolated System

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Closed System

Also known as control mass(amount of matter

inside control remains constant with time)

consists of a fixed amount of mass, and no

mass can cross its boundary. That is, no mass

can enter or leave a closed system, but energy,

in the form of heat or work, can cross the

boundary

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Closed System

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Open System

is a properly selected region in space. It

usually encloses a device that involves mass

flow such as a compressor, turbine, or nozzle.

Both mass and energy(in the form of work

and/or heat) can cross the boundary.

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Open System

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Open System

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Open System

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Isolated System

A system that is not influenced in any way by

the surroundings or environment no mass

and energy flow across the system boundary

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Macroscopic vs. Microscopic

Microscopic Point of View

System behavior is described by describing the

behavior of each molecule which comprise the

system Governing equations are written for each

molecule, e.g., equations for position, velocity,

etc.

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Macroscopic vs. Microscopic

Macroscopic Point of View The gross/average effects or time-averaged

influence of many molecules is used to describesystem behavior

Uses measurable parameters, e.g., pressure,temperature, etc. System volume should be very large compared

with molecular dimensions(System shouldcontain many molecules)

System is treated as continuous, disregardingthe action of individual molecules

Is completely independent of the assumptionregarding the nature of matter

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Properties and State of a Substance

Phase

A quantity of matter that is homogenous

throughout; solid, liquid, gas

When more than one phase is present, eachphase is separated by phase boundaries

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State of a Substance

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StateIndentified or described by certain

observable, macroscopic properties

called properties

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Property of a Substance

Any quantity that depends only on the state of

the system

Independent of the path by which the state is

arrived at.

*Given a state , each property has only one

definite value

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Some familiar properties are pressure P,temperature T, volume V, and mass m. The listcan be extended to include less familiar ones

such as viscosity, thermal conductivity, modulusof elasticity, thermal expansion coefficient,electric resistivity,and even velocity andelevation.

A property of a system has significance for theentire system only when the system is inequilibrium.

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2 General Class of Properties

Intensive Properties

independent of mass

Examples: Pressure, Temperature, Color, Odor,Ductility etc.

Extensive Properties

dependent of mass

Examples: Mass, Weight, Volume, Length

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Processes and Cycles

Occurs when a change in property occurs

Any change that a system undergoes from one

equilibrium state to another is called a

process, and the series of states through

which a system passes during a process is

called the path of the process

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Processes

Non-Quasi-Equilibrium Process

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Processes

Quasi-Equilibrium Process(ideal process)

When a process proceeds in such a manner that

the system remains infinitesimally close to an

equilibrium state at all times.

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Processes

The prefix iso- is often used to designate a

process which a particular property remains

constant.

Isothermal-Constant Temperature

Isobaric(Isopiestic)-Constant Pressure

Isochoric(Isometric)-Constant Volume

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Cycle

a series of processes, one after the other, such

that the initial and final states are the same

initial and final system compositions are

similar.

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Units for Mass, Length, Time, and Force

SI Units

Time second ( s )

Length meter ( m )

Masskilogram ( kg )

Forcenewton ( N )F = m a

1 (N) = 1 (kg) x 1 (m/s2)= 1 (kg-m/s2)

English Units

Time second ( s )

Length foot ( ft )

Mass pound mass ( lbm )

Force pound force ( lbf )F = m a

1 (lbf) = 1 (lbm) x 32.174 (ft/s2)= 32.174 (lbm-ft/s2)

For locations where theacceleration is different from

32.174 ft/s2 , the force or weight is F = m a / gc

Where gc = 32.174 lbm-ft/lbf-s2

Weight - is correctly used only asa force

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SI and English Units

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Mass v.s. Weight

The mass of a body remains the

same regardless of its location in

the universe. Its weight, however,

changes with a change in

gravitational acceleration

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Density and Specific Volume

Density is mass per unit volume

Specific Volume is the reciprocal of density

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Specific Gravity and Specific Weight

specific gravity, or relative density, and is defined as the ratio

of the density of a substance to the density of some standard

substance at a specified temperature.

Substances with SG of less than 1 are lighter than water, thus

they would float on water

The weight of a unit volume is called specific weight

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Pressure

Of a liquid or gas is defined as the normal component offorce per unit area

where

A = a differential area of a systemA= smallest area over which the fluid can be consideredas a continuumFn = component of force normal to A

Typical units,SI: 1 Pascal (Pa) = 1 Newton / m2 (N/m2)

English: pound-force / ft2 (lbf/ft2), pound-force / in2(lbf/in2)Others: 1 bar = 105 Pa = 0.1 Mpa

atm = 101,325 Pa = 14.696 lbf/in2

'

limn

A A

FP

A

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Fluid pressure in relation to a movable boundary

Under equilibrium conditions,pressure P exerted by the gas onall its boundaries is the same

With no heat transfer, the pressureis fixed by the external force Fextacting on the piston ; also, Fext =

Pressure x Piston Area (from FBDof piston)

Heating/cooling of the gas tends toincrease/decrease pressure andmove piston to the right/left suchthat Pressure x Piston Area = Fext

is satisfied.

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Pressure

Pressure is typically measured or indicated relative toeither of two references which are

Atmospheric Pressure typically sea level pressure atstandard conditions; measured by a barometerGauge pressure - indicates how much actual pressureis above atmospheric pressure; measured by apressure gauge

Vacuum pressure - indicates how much actualpressure is below atmospheric pressure; measured bya vacuum gauge

Absolute Zero Pressure zero pressure or perfectvacuum; measured by an absolute pressure gauge orcalculated from gauge/vacuum pressure

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Pressure

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Pressure Measurement

Using Dial Gauges

Consider the following

a. If Pi > Po ,

Pi = Po + Pg If Po = Patm , Pi, abs = Patm + Pg

b. If Pi < Po ,

Pi = Po - Pvac If Po = Patm , Pi, abs = Patm - Pvac

Tube

side

Dial side

Pd

= pressure

reading

= Pg or Pvac

Po = pressure outside Compartment= ambient pressure

Pi

= pressure

inside

compartment

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Pressure Measurement Example

A manometer is used to measure the pressure in atank. The fluid used has a specific gravity of 0.85, and

the manometer column height is 55 cm, as shown in

the figure. If the local atmospheric pressure is 96 kPa,

determine the absolute pressure within the tank.

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Pressure Measurement Example

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Equality of Temperature

Two bodies have equality of temperature if, when

they are in thermal equilibrium, no change in any

observable property occurs.

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The Zeroth Law of Thermodynamics

When two bodies have equality of temperature with

a third body, they in turn have equality of

temperature with each other.

"IfA is in thermal equilibrium with B and ifB is in

thermal equilibrium with C, thenA is in thermal

equilibrium with C."

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ME 63-Lecture 1-AY 20112012