thermodynamics notes 1

8/20/2019 Thermodynamics Notes 1

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Introduct ion to Thermodyn amics, Lecture 1-2 Prof. G. Ciccarelli (2012)

Why do we need to study thermodynamics?

Knowledge of thermodynamics is required to design any

device involving the interchange between heat and work,

or the conversion of material to produce heat

(combustion).

Examples of practical thermodynamic devices:


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What is thermodynamics?

The study of the relationship between work,

heat, and energy.

Deals with the conversion of energy from oneform to another.

Deals with the interaction of a system and it

surroundings.

System - identifies the subject of the analysis by defining

a boundary

Surroundings - everything outside the system boundary.


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As an example, consider an Internal Combustion (IC)

Engine

EXHAUST GAS

WORK(drive shaft)

HEAT

(hot engine block)

FUEL

AIR

SYSTEM


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Closed System - fixed non-changing mass of fluidwithin the system, i.e., no mass transfer across thesystem boundary but can have energy exchange

with the surroundings. Example: piston-cylinderassembly

Isolated System - a system that does not interact at

all with the surroundings, e.g., no heat transferacross system boundary

CYLINDER

GAS

PISTON

ENERGY

INSULATED WALLS

ENERGY


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Open System (Control Volume) - fixed volume inspace, mass and energy exchange permitted acrossthe system boundary. Example: jet engine

As far as the surroundings are concerned the control

volume is:

C T Air

Fuel

CombustionProducts

Air

Fuel

CombustionProducts


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System Properties – macroscopic characteristics ofa system to which a numerical value can beassigned at a given time without knowledge of the

history of the system, e.g., mass, volume, pressure

There are two types of properties:

Extensive – the property value for the system isthe sum of the values of the parts into which thesystem is divided (depends on the system size) e.g.,mass, volume, energy

Intensive – the property is independent of systemsize(value may vary throughout the system), e.g., pressure,

temperature

VS = VA + VB extensive

Vs, Ts, Ms V A T A M A

VB TB MB

System

SystemSystem


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MS = MA + MB extensive

TS TA + TB intensive

Units – SI used exclusively

Fundamental units:

Mass kilograms kg

Length meter m

Time seconds s

Temperature Celcius/Kelvin oC/oK

Derived units:

Force (F) Newton N

Pressure (P) Pascal Pa

Energy (E) Joule J

Newton’s Law states: Force = mass x acceleration

F = m a

[N] = [kg] [m/s2] 1 N = 1 kgm/s2

P = F / A

[Pa] [kgm/s2] [m2] 1 Pa = 1 kg/ms2


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E = F x

[J] = [kgm/s2] [m] 1 J = 1 kgm2/s2


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Property Definitions

In order to speak of an intrinsic property “at a point” we

must treat matter as a continuum, i.e., matter is distributedcontinuously in space

In classical thermodynamics a point represents thesmallest volume V’ for which matter can be

considered a continuum.

The value of the property represents an average overthis volume V’.

At any instant the density, ρ, at a point is defined as

lim

V V

M

V units: kg/m3

Mass, M , of the system with volume, V , is

V dV dM M dV

dM

Note: if is uniform over the volume M= V

Specific volume, , defined as


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1 units: m3/kg

Consider a small area A passing through a point in a fluid

at rest. Fluid on one side of the area exerts a compressiveforce F normal to the area

The pressure, P , at a point is defined as

P F

A A A

lim

units: 1 Pa = 1 N/m2

1 standard atmosphere = 101,325 Pa

1 bar = 100,000 Pa = 100 kPa = 0.1 MPa

Absolute pressure , P abs, measured relative to a perfect

vacuum

F

A


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Gauge pressure , P g , measured relative to the local

atmospheric pressure, P atm.

Note: P* g = P*abs - P atm

Gauge pressure measurement via a manometer

P* g = P*abs – P atm = gL g= 9.81 m/s2

Gas atP*abs

Patm

L

Mercury/water

Local atmosphericpressure, P atm

0 kPa perfect vacuum

P= P*

P* g

P* abs


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The density of mercury is 13.5 times the density of water

Note: 1 standard atmosphere = 760 mm (29.9 in.) mercury= 10,260 mm water


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Temperature, T , in units of degrees celcius, oC, is a

measure of “hotness” relative to the freezing and boiling

point of water

A thermometer is based on the thermal expansion of

mercury

Microscopic point of view:

Temperature is a measure of the internal molecularmotion, e.g., average molecule kinetic energy

At a temperature of – 273.15oC molecular motion ceases

Temperature in units of degrees kelvin, oK, is measured

relative to this absolute zero temperature, so

0oK = -273oC

ICE BATH BOILINGWATER

0oC

100oC


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in general, T in oK = T in oC + 273.13

System state – condition of the thermodynamic system as

described by its properties (P1,T1,….)

A system is at steady-state if none of the system

properties change with time

A system is in equilibrium if when the system is isolated

from its surroundings there are no changes in its

properties

Example: thermal equilibrium

TH TC

TH TC TE TE

Non-Equilibrium EquilibriumTC


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A process is the transformation of a system from one

state to another state.

A cycle is a sequence of processes that begins and ends atthe same state.

Example showing a cycle consisting of two processes

P1,T1 P1,T1 P2,T2

STATE 1P1, T1

STATE 2P2, T2

Process 1 Process 2

1

2

CYCLE

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