1. Introduction, Temperature and Pressure(1)(1)

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bull Sommerfield on Thermodynamics

bull ldquoThermodynamics is a funny subject The first time you go through the subject you dont understand it at all

bull The second time you go through it you think you understand it except for one or two small points

bull The third time you go through it you know you dont understand it but by that time you are so used to the subject that it doesnrsquot bother you any morerdquo

- Arnold Sommerfeld (see httpscienceworldwolframcombiographySommerfeldhtmlfor a biography)

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bull What is THERMODYNAMICS

bull The word Thermodynamics is made up of two words

bull Thermo - a Greek word for hot or heatbull Dynamics ndash the study of matter in motion

bull Therefore the word Thermodynamics means the study of heat related to matter in motion

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THERMODYNAMICS amp HEAT TRANSFER

bull American Oxford Dictionarybull Thermodynamics the branch of physical

science that deals with the relations between heat and other forms of energy (such as mechanical electrical or chemical energy) and by extension of the relationships and interconvertibility of all forms of energy

bull What does TVNZ saybull Video

httptvnzconzscience-made-simplethermodynamics-video-3697351

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THERMODYNAMICS amp HEAT TRANSFER

bull HEAT TRANSFER The study of the flow of heat within an object or from one medium to another due to their variation in temperature ndashRadiation - energy is emitted in the form of

electromagnetic waves or subatomic particles eg ndashheatwarmth felt from a flame or bonfire

sans touching it the heat from the microwave oven and the heat from the sun

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THERMODYNAMICS amp HEAT TRANSFER

bull TEMPERATURE AND THERMAL ENERGY bull Thermal energy ndash a form of kinetic energy

characterized by randomness of motion at the atomic and molecular level

bull Temperature ndash the degree or intensity of heat present in a substance or object the measure of the hotness or coldness of a body

bull Temperature Scale - a system of measuring the hotness or coldness of a substance

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THERMODYNAMICS amp HEAT TRANSFER

bull Heat ndash the energy transit from a body of higher temperature to one of lower temperature under the influence of the variation in temperature quantity of thermal energy absorbed or given off by a body

bull The basic unit of all energy including heat is the Joule (J) Units of heat can be KJ MJ

bull The symbol representing quantity of heat is ldquoQrdquo

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THERMODYNAMICS amp HEAT TRANSFERbull PRACTICAL TEMPERATURE MEASUREMENT bull A substance measuring temperature should

experience some recognisable change when its temperature changes The following changes can be used in measuring temperature

bull 1 Change in colourbull 2 Change of statebull 3 change in pressurebull 4 Change in electrical resistancebull 5 Change in sizebull 6 Generation of an electromotive forcebull 7 Change of degree of surface radiationbull etc etc

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THERMODYNAMICS amp HEAT TRANSFERbull PRACTICAL TEMPERATURE MEASUREMENT

bull 1 Change in colourThe appearance of a substance will change with temperature Eg a hot metal can be red hot (700⁰C) or white hot (1200⁰C) Not accurate at allbull 2 change in pressureThe temperature of fluid can be obtained by making a measurement of the fluid pressure Eg liquid gas or vapour-filled thermometerbull 3 Change in electrical resistanceThe Electrical resistance of most materials varies with temperature Eg RTD platinum resistance thermometer thermistors bull 4 Change in sizeExpansion of solids like metal rods Eg bimetallic thermometerExpansion of liquids Eg mercury-in-glass thermometer

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THERMODYNAMICS amp HEAT TRANSFERbull PRACTICAL TEMPERATURE MEASUREMENT bull 5 Generation of an electromotive forceAn emf is produce when two unlike metal wires(at different temperatures) are joined together Eg Thermocouplesbull 6 Change of stateSome materials change state permanently at a given temperature and this characteristic can be used when measuring temperature Eg Pyrometric cones called Seger Cones thermo coloursbull 7 Change of degree of surface radiationAs the temperature of a body rises so does the energy radiation from its surface Eg optical and radiation pyrometers infrared thermometerNOTEA thermometer is a temperature-measuring deviceA pyrometer is a thermometer used for measuring high temperatures usually above 700⁰C

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bull MEASURING UNITS (SI SYSTEM)bull Basic Unitsbull The basic units of the international system are Quantity Unit Mass Kg (Kilogramme) Length m (metre) Time s (second)bull Derived Unitsbull From the three units several other units are derived and used

in thermodynamics

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bull MEASURING UNITS (SI SYSTEM)bull Derived Units

bull Force (F) measured in Newtons (N) F = mass x acceleration (ma)bull Weight (w) is a force due to gravity w = mass x acceleration due to gravity

= m g bull Volume (V) measured in cubic metres (msup3)

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bull MEASURING UNITS (SI SYSTEM)bull Derived Units

bull Density (ρ) mass per unit volume and measured in Kgmsup3 Density = MassVolume

bull Density of water = 1000 Kgmsup3bull Relative Density (s) is the density of a substance

divided by the density of water Has no unitsbull Relative Density = density of a substance

density of water

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bull MEASURING UNITS (SI SYSTEM)bull Derived Units

bull Specific volume (v) is the volume per unit massbull Specific volume is the reciprocal of densitybull Specific volume (v) = Volume (V) mass (m) = 1 ρ

bull Specific Density is the density per unit massbull Specific Density = density (ρ ) mass (m)

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THERMODYNAMICS amp HEAT TRANSFERbull MEASURING TEMPERATURE bull Thermocouples (aka electronic thermometer) bull Used for the accurate temperature measurement over a wide

temperature range bull 2 wires made from dissimilar metals are joined at their ends to

form 2 junctions (hot and cold junctions)bull When there is a temperature difference between the two

junctions a small voltage difference (measured in millivolts) will arise causing an electric current to flow from the hot to the cold junction

bull If the cold junction is maintained at a known reference temperature the voltage difference is used to measure the temp of hot junction

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bull TEMPERATURE SCALESbull In the SI system two scales are used

1 Celcius (formerly known as Centigrade)2 Kelvin Scale

bull The imperial system uses the Fahrenheit Scale

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THERMODYNAMICS amp HEAT TRANSFER

bull TEMPERATURE SCALES 1 Fahrenheit Scale ndash (named after Ger

physicist Gabriel Daniel Fahrenheit ) based on the freezing and boiling point of water at normal atmospheric pressure ~ 32 ordmF - freezing pt and 212ordmF ndash boiling pt

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THERMODYNAMICS amp HEAT TRANSFER

bull TEMPERATURE SCALES bull 2 Celsius Scale ndash (named after Swed

astronomer Anders C Celsius ) based on the freezing and boiling point of water at normal atmospheric pressure ~ 0 ordmC - freezing pt and 100ordmC ndash boiling pt

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THERMODYNAMICS amp HEAT TRANSFER

bull TEMPERATURE SCALES 3 Kelvin Scale ndash (named after Brit physicist William

T Kelvin ) erased the arbitrary assigned reference points thus called not a relative scale but an Absolute temperature scale 0 = nothing ndash Absolute Zero ~ lowest temperature possible occurring when all random motion of molecules has ceased In this scale 273K ndash freezing pt of water 373K ndash boiling pt

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THERMODYNAMICS amp HEAT TRANSFER

bull CONVERSIONS ndash From degF to degC = (degF -32)59 ndash From degC to degF = (degC 95) + 32 ndash From degC to K = degC + 273 ndash From degF to K = (degF-32)59 + 273

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THERMODYNAMICS amp HEAT TRANSFER

bull Class Exercise

bull The difference in temperature between the input and output of a water heater is stated as 100 degF Convert this to degC

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bull PRESSUREbull Pressure is defined as Force per unit areabull The symbol of pressure is Pbull Mathematically P = F Abull where

F = the normal force in Newtons (N) A = area in square metre (msup2)

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THERMODYNAMICS amp HEAT TRANSFER

bull Units of PRESSUREF = the normal force in Newtons (N)

bull The unit of pressure is Nmsup2 bull This unit of pressure is sometimes called the

Pascal (Pa)bull The bar is commonly usedbull 1 bar = 10⁵ Nmsup2

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THERMODYNAMICS amp HEAT TRANSFER

bull PRESSUREbull Atmospheric pressure is the force per unit area

exerted against a surface by the weight of air above that surface in the Earths atmosphere

bull The standard atmosphere (symbol atm) is a unit of pressure and is defined as being equal to 101325 Pa or 101325 kPa

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THERMODYNAMICS amp HEAT TRANSFERbull PRESSURE MEASUREMENTbull Many techniques have been developed for the measurement

of pressure and vacuum Instruments used to measure pressure are called pressure gauges or vacuum gauges A vacuum gauge is used to measure the pressure in a vacuum

bull Common Pressure measuring instruments arebull 1 Barometer ndash used for measuring atmospheric pressurebull 2 Bourdon Gauge ndash used for measuring the difference

between the pressure of a fluid and atmospheric pressure Measures higher pressure differences

bull 3 Manometer ndash used to measure smaller temperature differences between the atmospheric pressure and pressure of a fluid

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THERMODYNAMICS amp HEAT TRANSFERbull PRESSURE MEASUREMENTbull The Bourdon Gauge is used to measure pressure

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THERMODYNAMICS amp HEAT TRANSFER

bull Bourdon Gauge bull It has a coiled tube whose one end is connected to the

system under consideration and other end is sealed bull With the application of the pressure in the tube it tends to

straighten up and this causes deflection of the sealed end The sealed end is connected to the indicating needle through a gear and linkage mechanism

bull The deflection of the sealed end results in movement of the needle which moves on a calibrated dial

bull Bourdon gauges can be used to measure a wide range of pressures

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THERMODYNAMICS amp HEAT TRANSFER

bull ABSOLUTE PRESSURE bull Although no pressure is an absolute quantity everyday pressure

measurements such as for tyre pressure are usually made relative to ambient air pressure In other cases measurements are made relative to a vacuum or to some other ad hoc reference When distinguishing between these zero references the following terms are used

bull Absolute pressure is zero referenced against a perfect vacuum so it is equal to gauge pressure plus atmospheric pressure

bull The absolute pressure - pa - is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum

bull Gauge pressure is zero referenced against ambient air pressure so it is equal to absolute pressure minus atmospheric pressure

bull Differential pressure is the difference in pressure between two pointsbull Absolute Pressure

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THERMODYNAMICS amp HEAT TRANSFER

bull ABSOLUTE PRESSURE bull Atmospheric Pressure

The atmospheric pressure is the pressure in the surrounding air It varies with temperature and altitude above sea level

bull Atmospheric pressure is also referred to as barometric pressurebull Standard Atmospheric Pressure

The Standard Atmospheric Pressure (atm) is used as reference for gas densities and volumes

bull The Standard Atmospheric Pressure is defined at sea-level at 273K (0oC) and is 101325 bar or 101325 Pa (absolute) The temperature of 293oK (20oC) is also used

bull In imperial units the Standard Atmospheric Pressure is 14696 psi1013 kPa = 1 atm = 14696 psi = 760 mmHg = 102 m water

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THERMODYNAMICS amp HEAT TRANSFER

bull ABSOLUTE PRESSURE

bull Absolute Pressure = Gauge pressure + Atmospheric Pressure

bull Pabs = Pgauge + Patm

Assume absolute pressure unless otherwise stated

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THERMODYNAMICS amp HEAT TRANSFER

bull ABSOLUTE PRESSURE

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bull PRESSURE VARIATION WITH DEPTH bull Variation of pressure with depth

bull True for all shapes of containers

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THERMODYNAMICS amp HEAT TRANSFER

bull PRESSURE VARIATION WITH DEPTH bull Variation of pressure with depth

P = ρ g hWhere

P = pressure in Newtonsρ = density of fluid (kgmsup3)h = height of fluid (m)g = acceleration due to gravity (mssup2)

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bull Tuto