BMCT 2123 thermodynamics 1

BMCT 2123thermodynamics 1CHAPTER 1Basic Concepts of Thermodynamics

Mr MOHD HAFIDZAL BIN MOHD HANAFIFaculty of Mechanical EngineeringUTeM[hafidzal@utem.edu.my][019-3921546][06-2346885]

What is Thermodynamics?Definition of THERMODYNAMICS (Greek)

Basic science that deals with energy. We all know that energy is neither created nor destroyed but it is transformed.Science of energy

Thermos = heatDynamos = power

ClassicalStatistical

ContTwo ways to look at systems:MacroscopicClassical Thermodynamics

Atomic or MolecularStatistical Thermodynamics

Types of form - energyMacroscopicMicroscopic a system posses as a whole with respect to outside reference frame Example: potential energy & kinetic energy

It related with the molecular structurethe degree of molecular activity

Dimensions & UnitsFundamental (Primary)Time (s)Mass (kg)Length (m)Temperature (deg. C)Current (A)Derived (Secondary)Velocity (m/s)Force (N)Pressure (Pa)Energy (J)Frequency (Hz) Dimensions are names that characterize physical quantities. Units are those arbitrary magnitudes and names assigned to dimensions, which are adopted as standards for measurement.

Amount substance (mol)luminous intensity (candela)5

Prefixes

produce a multiple or submultiple of the original unit. 6

Weight vs. MassMass is an amount of matterWeight is a measure of how much force is applied to the matterYour mass is the same on the earth and on the moonYour weight is different!We often are not careful to make a distinction between mass and weight

F = mam is massa is accelerationOn the surface of the earth we usually call the acceleration, gg = 9.8 m/sec2g = 32.174 ft/sec2

Weight on the surface of the earthF = m gIn metric the units becomeKg m/sec2 N(N x 0.2248) lbfG universal gravitational constant

G universal gravitational constant9

Exercise!!!A man at sea level has a mass of 63kg.Find:-1. The man weight on earth (g)2. The man weight on moon (g/6)

Solution:-From Newton Law; F = ma = m g=63 kg x 9.8 m/sec2 = 617.4 N

2. From Newton Law; F = ma = m g=63 kg x 9.8 m/sec2 /6= 102.9 N

SystemsSystemA quantity of matter or a region in space chosen for studySurrounding The mass or region outside the systemBoundaryThe real or imaginary surface that separate system and surroundingcontact surface shared by both system and surrounding

Quantity of matter or a region of spaceClosed SystemOpen SystemSurroundings everything thats not the systemBoundaryStationaryMoving

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Cont2 types of system:Closed system

Open system

ANALYZING SYSTEM:

The approach is different for closed and open systemsEnergy is a lot harder to handle than matter, because it exists in many forms

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Closed SystemControl mass (fixed amount of mass)No mass transfer across boundaryAllow energy transfer across boundary Volume Constant Example: Piston system

Open System Control Volume volume fixedAllow both mass and energy transfer across boundaryMass constantExample : Pump , Compressor , Valve , Turbine

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Energy = EMacroscopic formsRespect to some outside reference frameMicroscopic Related to the molecular structure

The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame.

The microscopic forms of energy, are those related to the molecular structure of a system and the degree of the molecular activity, and are independent of outside reference frames.Microscopic forms of energy are related to the molecular structure of a system. The sum of all microscopic forms of energy in a system is called the internal energy U15

Macroscopic EnergyKinetic Energy (KE)KE = mV2/2

Potential Energy (PE)PE = mgz

Macroscopic energy via mass flow

Stored energy is called potential energy and moving energy is called kinetic energy.

Others macro energy = magnetism, electricity, surface tension, etc.

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Short Quiz60 kg

P.E = mgh = 60 kg x 9.81m/s2 x 2m= 1177.2 Joules

Use g = 9.81 m/s

2 meter

Calculate the P.E

END SLIDE of WEEK 117

Other Kinds of Macroscopic EnergyMagneticElectricalSurface TensionThese are specialized, and we dont usually need to include them

Microscopic EnergyKinetic energy of individual moleculesPotential energy of individual moleculesBinding forcesChemical EnergyNuclear EnergyEtc

Both macroscopic and microscopicforms of energy are static they can be stored in a system

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ExerciseDetermine the power required to accelerated a 900 kg car from rest to a velocity of 80 km/h in 20 s on a level road

Ea =1/2 x m (V22 V12) = (900kg)(80,000m/3600)2 - 0 2)( 1kJ/1000m2/s2) =222kJ

Average powera= Ea / t = 222kJ / 20s = 11.1kW

Properties of a SystemIntensive Does not depend on the systems sizeTemperaturePressureExtensiveDepends on the systems sizeVolumeMassTotal Energy

We often define properties in terms of other propertiesDensity = m/V

Specific Volumev = V/m = 1/r

We can define most extensive properties per unit massCalled specific propertiesu = U/m is the specific internal energy

StateA set of properties that describes the condition or state of a system.At a given state, all the properties of a system have a fixed valueIf you change a property, you have changed the stateThermodynamics deals with equilibrium states

EXAMPLE : At atm pressure

Water in compressed liquid state at room temp is 30 CWater in solid state below 0 CVapour state above 100 C26

Equilibrium

ProcessA change from one equilibrium state to anotherA path is the series of states the system passes through during a processWe often deal with Quasi-equilibrium processesProcess diagrams

Process Diagram

PT

State 1State 2Process Path

P1T1

T2P2

The P-V diagram of a compression process.

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CycleWhen a system returns to its initial state.

P1P2State 1State 2

Process Path IProcess Path II

Pressure The force exerted by a fluid per unit areaOnly meaningful for a gas or a liquidIn solids we talk about stress

Units of pressure

One Pascal isnt very much!!1 atm = 101,325 Pa1 atm = 101.325 kPa = 1.01325 bar = 14.7 psi

Pressure dont change in the horizontal direction, but change in vertical direction.

Example : P1 = P2 ,P2 P3 P2 P3

Pressure at any point in a fluid is the same in all directionsExamples : Pa = Pb = Pc

Figure 1: Pressure characteristic

Test your understanding35Pressure in a fluid increase as the depth increase

depthpressureFig. 2: Pressure of a liquid at restWHY???

PressureAtmosphere pressureAbsolute pressureGauge pressureVacuum pressure

Atmosphere pressureSymbol :- PatmStandard atmospheric pressureStandard value at sea level is 1 atmCommonly assume in engineering application 1 atm = 101.325kPa

Absolute pressureSymbol : - PabsMeasure relative to absolute vacuum pressure (absolute zero pressure)0 Pabs Max value is infinity but min value is 0

Gauge pressureSymbol : PgaugeMeasure pressure exceeds local atmospheric pressureP > PatmUsually measure with a pressure gaugeAtmospheric, absolute and gauge pressure are all positive quantity and related to each others by:- Pabs = Pgauge + Patm

Vacuum pressureSymbol : PvacPressure less than local atmospheric pressureP < PatmUsually measure with a vacuum gaugeAtmospheric, absolute and vacuum pressure are all positive quantity and related to each others by:- Pvac = Patm - Pabs

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Exercise A pressure gage connected to a tankreads 500 kPa at a location where theatmospheric pressure is 94 kPa. Determine the absolute pressure in the tank. Pgage = Pabs Patm

Devices to measure pressureBourdon tubeBarometerManometer

Barometer P=rghFor a given barometer the density and the acceleration due to gravity are constants, soPressure is directly proportional to heightPressure is often measured in mmHg1 atm = 760 mmHg

Barometer

h

A

Note: The pressure measured by a barometer is independent of cross sectional area

Measure the atmospheric pressure Volume of mercury at B is V = A h Mass of mercury at B is m, = m/Vm = V = A h Weight of mercury at B is W, F = m aW = m g = (Ah) g Pressure produced at B = PatmPatm = F/A = (Ah) g / A = gh

Patm = ghWhere, : density of mercuryg : acceleration due to gravityh : height of mercury above free surface

ExerciseIf atmospheric pressure is 1.01325 bar, what will be the right height of the mercury? Hg = 13 600 kg/m3g = 9.81 m/s2 1 bar = 105 Pa

Patm = gh

0.133 m47

ManometerUsed to compare pressuresA pressure measuring device that uses liquid as medium (mercury, water, oil or alcohol)

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Cont

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.

ExerciseA manometer is used to measure the pressure in tank. The fluid used has a specific gravity of 0.85, and the manometer height is 55 cm. If the local atmospheric pressure is 96 kPa, determine the absolute pressure within the tank.

= s x water = 0.85 x 1000 kg/m3

P = Patm + gh = 100.6 kPa.50

TemperatureMeasures the energy content of matter (degree of hotness or coldness)Difference in temperature causes heat transfer from hot body to cold body until thermal equilibrium is reached.Absolute unit for T is Kelvin and Rankine.

Zeroth Law of ThermodynamicsIf two bodies are in equilibrium with a third body, they are also in equilibrium with each otherBasis for thermometers

132If T1 = T3 & T2 = T3 THEN T1 = T2

ExerciseAir at a temperature of 350 enters an airconditioning unit and is cooled at 200 at theoutlet from the unit. What is the temperaturedifference between the inlet and the outlet in(a) Celsius (b) Kelvin ?

T1= 350C, T2= 200C

T= ??

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THANK YOUQ & A Session ???