thermodynamics 1 - basic concepts of thermodynamics

Chapter 1 Thermodynamics 1 1

Thermodynamics I

Introduction 1. Basic Concepts of Thermodynamics 2. Energy, Energy Transfer, and General Energy Analysis 3. Properties of Pure Substances 4. Energy Analysis of Closed Systems 5. Energy and Mass Analysis of Control Volumes 6. The Second Law of Thermodynamics 7. Entropy Applications Examples


Overview Basic Concepts of Thermodynamics

1-1 Thermodynamics and Energy 1-2 Dimensions and Units 1-3 Closed and Open Systems/Control Volumes 1-4 Properties of a System 1-5 Density and Specific Gravity 1-6 State and Equilibrium 1-7 Processes and Cycles 1-8 Forms of Energy 1-9 Energy and Environment 1-10 Temperature 1-11 Pressure 1-12 The Manometer 1-13 Barometer and the Atmospheric Pressure


Thermodynamics and Energy

Thermodynamics is the science that primarily deals with energy.

The name Thermodynamics comes from the Greek words therm (heat) and dynamis (power).

Classical thermodynamics: A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles. It provides a direct and easy way to the solution of engineering problems and it is used in this text.

Statistical thermodynamics: A microscopic approach, based on the average behavior of large groups of individual particles.


Conservation of Energy Energy can change from one form to the other.

Energy cannot be created or destroyed.


Laws of Thermodynamics Zeroth Law of Thermodynamics:

If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

First Law of Thermodynamics: Expression of the conservation of energy. Asserts that energy is a thermodynamic property. The energy in the whole universe is constant. W=Q

Second Law of Thermodynamics: States that energy has quality as well as quantity and processes occur in the direction of decreasing quality.

Third Law of Thermodynamics: At the absolute zero of temperature the entropy of a perfect

crystal of a substance is zero.


Laws of Thermodynamics Second Law of Thermodynamics: Whenever a temperature difference exists, motive power can

be produced (Carnot). It is impossible for a self-acting machine, by any external

agency, to convey heat from a body at a low temperature to one at a higher temperature (Clausius).

The entropy of the universe goes to infinity (Clausius). We cannot transfer heat into work merely by cooling a body

already below the temperature of the coolest surrounding objects (Kelvin).

It is impossible to construct a system which will operate in a cycle, extract heat from a reservoir, and do an equivalent amount of work on the surroundings (Planck).

It is impossible for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed temperature (Kelvin-Planck).


Dimensions and Units Dimensions are used to

characterise physical quantities.

Units are arbitrary magnitudes assigned to dimensions.

Fundamental or Primary Dimensions are basic dimensions such as mass m, time t, or temperature T.

Derived or Secondary Dimensions, such as energy E or volume V, are derived from primary dimensions.

Seven primary dimensions and their SI units are:

Standard prefixes in SI units


Dimensions and Units

Force:

Weight

Specific Weight

Work

[ ]F ma N=

2[ ] ( 9.81 / )W mg N g m s= =

[ ]Work Fs Nm=

3 2[ / ] ( 9.81 / )w g N m g m s= =

The SI unit prefixes are used in all branches of engineering.


All engineering equations must be dimensionally homogeneous.

In other words every term in an equation must have the same unit.

Dimensionally inhomogeneous equations are definitely wrong, but a dimensionally homogeneous equations is not necessarily right.

Example 1: Example 2: A tank (volume = 2 m3) is filled with oil (density

= 850 kg/m3). What is the mass m of the oil?

Dimensional Homogeneity

kgkJkJE /725 +=

3/850 mkg= 32mV =kgmmkgVm 1700)2)(/850( Thus, 33 ===


May be simple or complex May have one or many parts May contain one or more

components May have one or more phases May be closed or open May have limits or a surface, in

general, boundaries, which can be fixed or moveable.

Systems

That portion of the universe selected for study.


System Boundaries Rigid Boundary:

Prevents system from changing shape or size. Adiabatic Boundary:

Prevents system from changing unless boundary is changed (Greek a-dia-bainein = not-through-to go)

Isolating Boundary: Both adiabatic and rigid.

Diathermal Boundary: Permits flow-through of heat.

Permeable Boundary: Permits the passage of matter, otherwise impermeable.


Closed Systems Mass cannot cross the boundaries of a closed system but energy can.

0; 0CV CMdE dmdt dt

=


Control Volumes - Open Systems

0; 0CV CVdE dmdt dt


Control Volumes - Open Systems


Systems - Examples


Properties of a System Any characteristic of a system in equilibrium is

called a property. Properties of a system can be either intensive or

extensive. Intensive Properties:

System size independent cannot be added up Eg temperature, pressure

Extensive Properties: System size dependent can be added up e.g. volume, mass

Specific properties: Extensive properties per unit mass


DENSITY AND SPECIFIC GRAVITY Volume V is the space that a matter with the mass m fills. Density is mass per unit volume Specific volume is volume per unit mass.

Specific gravity: The ratio of the density of a

substance to the density of some standard substance at a specified temperature

(usually water at 4C).

Density Specific volume


State and Equilibrium State

Consider a system that is not undergoing any change. The properties can be measured or calculated throughout the entire system. This gives us a set of properties that completely describe the condition or state of the system. At a given state all of the properties are known; changing one property changes the state.

Equilibrium A system is said to be in thermodynamic equilibrium if it maintains thermal (uniform temperature), mechanical (uniform pressure), phase (the mass of two phases, e.g. ice and liquid water, in equilibrium) and chemical equilibrium.


State Postulate A system is described by its properties. Once a sufficient number of properties are known, the state is

specified and all other properties are known. The number of properties required to fix the state of a simple,

homogeneous system is given by the state postulate: The thermodynamic state of a simple compressible

system is completely specified by two independent intensive properties.


Processes and Cycles Any change from one state

to another is called a process.

During a quasi-equilibrium or quasi-static process the system remains practically in equilibrium at all times.


Processes and Cycles In some processes one thermodynamic property is held

constant. Process Property held constant

isobaric pressure isothermal temperature isochoric volume isentropic entropy

Constant Pressure Process

Water

F

System Boundary


Processes and Cycles A process (or a series of

connected processes) with identical end states is called a cycle.

Process B

Process A

1

2 P

V


Carnot Cycle


Diesel Cycle


Heat and Work

Heat is energy transfer that appears at the boundary when a system changes its state due to a difference in temperature between the system and its surroundings.

Work is energy transfer that appears at the boundary when a system changes its state due to the movement of a part or the whole of the boundary under the action of a force.


Temperature All temperature scales are based on some

easily reproducible states such as the freezing and boiling points of water: the ice point and the steam point.

Ice point: A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm pressure (0C or 32F).

Steam point: A mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure (100C or 212F).

The temperature scales used in the SI system today is the Celsius scale.

The absolute temperature scale in the SI is the Kelvin scale, which is related to the Celsius scale by

The magnitudes of each division of 1 K and 1 0C are identical.


Absolute, Gage and Vacuum Pressures

absatmvac

atmabsgage

PPP

PPP

=

=


Pressure Pressure is the force exerted by a fluid per

unit area. The counterpart of pressure in solids is

stress. Unit:

The pressure inside a fluid increases with depth.

2

5

1Pa 1N/m1bar 10 Pa=0.1MPa=100kPa1atm 101,325Pa 1.01325bars

=

== =


The Manometer

In stacked-up fluid layers, the pressure change across a fluid layer of density and height h is gh.

Measuring the pressure drop across

a flow section or a flow device by a differential

manometer.

The basic manometer.

It is commonly used to measure small and moderate pressure differences. A manometer contains one or more fluids such as mercury, water, alcohol, or oil.


Pressure - Example A manometer is used to measure the pressure inside a tank. The fluid has a specific gravity S of 0.85, and the manometer column height is 55 cm. If the local atmospheric pressure is 96 kPa, determine the absolute pressure within the tank.

We assume that the gravitational acceleration is 9.81 m/s2.

S 33 /850)/1000)(85.0())((2

mkgmkgOHs ===

= + = + = + =

= + =

=

3 2

2

196.00 (850.00 )(9.81 )(0.55 )1000

100.6

atm atm atmmg hAgP P P P PA A

kg m kPakPa m Nm sm

kPa


Pressure Basic Barometer

Thermodynamics IOverview Basic Concepts of ThermodynamicsThermodynamics and EnergyConservation of EnergyLaws of ThermodynamicsLaws of ThermodynamicsDimensions and UnitsDimensions and UnitsDimensional HomogeneitySystemsSystem BoundariesClosed SystemsControl Volumes - Open SystemsControl Volumes - Open SystemsSystems - ExamplesProperties of a SystemDENSITY AND SPECIFIC GRAVITYState and EquilibriumState PostulateProcesses and CyclesProcesses and CyclesProcesses and CyclesCarnot CycleDiesel CycleHeat and WorkTemperatureAbsolute, Gage and Vacuum PressuresPressureThe ManometerPressure - ExamplePressure Basic Barometer

thermodynamics 1 - basic concepts of thermodynamics

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