Thermodynamics I Chapter 3 Energy, Heat and Work
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Transcript of Thermodynamics I Chapter 3 Energy, Heat and Work
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Thermodynamics IChapter 3
Energy, Heat and Work
Mohsin Mohd SiesFakulti Kejuruteraan Mekanikal, Universiti Teknologi Malaysia
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Energy, Heat and Work(Motivation)
A system changes due to interaction with its surroundings.Energy interaction is a major factor.This chapter studies the nature of energy and its various forms and transfers so that we are able to follow its interaction with a system.
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ENERGY & THE 1st LAW OF THERMO.
1st Law : concerning quantity of energy
Energy is conserved(Amount of energy is constant, but can change forms)(e.g. potential, kinetic, electrical, chemical, etc.)
𝐸=𝑇𝑜𝑡𝑎𝑙 𝑠𝑦𝑠𝑡𝑒𝑚𝑒𝑛𝑒𝑟𝑔𝑦 [ 𝐽 ,𝑘𝐽 ]
𝑒= 𝐸𝑚 [ 𝑘𝐽𝑘𝑔 ] (𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐𝑒𝑛𝑒𝑟𝑔𝑦 )
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EnergyMacroscopic – system energy which value depends on a reference point (Kinetic Energy (KE), Potential Energy (PE)Microscopic – energy due to molecular interactions & activity (independent of any reference point) (Internal Energy, U)Total System Energy
[kJ/kg] [kJ]
upekeeUPEKEE
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Kinetic Energy (KE)
Potential Energy (PE)
2)VV(
mΔKEΔke
2V
mKEke
VV2mΔKE
2VmKE
21
22
2
21
22
2
)zg(zm
ΔPEΔpe gzmPEpe
)zmg(zΔPE mgzPE
12
12
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Internal Energy, UMolecular movement (vibration, collision, etc) sensible energy (molecular activity temperature)Bond Energy between molecules (phase change) latent energy (constant temperature)Bond Energy between atoms in a moleculechemical energyBond Energy between protons & neutrons in the nucleusnuclear energy
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Modes of Energy TransferEnergy Interaction
between System and SurroundingEnergy can cross the boundary (transferred) of a closed system by 2 methods;
HEAT & WORK
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12
2
1QδQ
kWskJ
tQQ
mQq
HEAT, Q [J, kJ]Heat - Energy that is being transferred due to a temperature difference
•Heat is a mode of energy transfer•Heat is not a property•Energy is related to states (property)•Heat is related to processes (not a property, depends on the path)
= rate of heat transfer
Amount of heat transferred during a process (depends on the path)
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Heat (ctd.)
systemQin= +ve Qout= -ve
Adiabatic Process (Q = 0)insulatedTsystem = Tsurrounding
Adiabatic ≠ Isothermal!(T can change by other methods;
energy can enter system by work)
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WORK, W [J, kJ]Work -–Energy that is crossing the boundary other than heat
(electrical, stirrer, shaft, moving piston, etc.)
-Not a property (related to process)-Mode of energy transfer
Win= -ve Wout= +vesystem
= rate of work = power
Depends on path
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Types of WorkWork
Electrical
MechanicalBoundary
Gravitational
Acceleration
Shaft
Spring
2
1VIdtWel
2
1dsFW
2
1dVpWb
)( 12 zzmgWg
)(21 2
12
2 VVmW
nW 2
)(21 2
122 xxkW
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Boundary Work for Polytropic ProcessesBoundary Work
General Polytropic Work
nVPVPW
1
1122 ( n 1 )
Const. Pressure Work (n=0))( 12 VVpW
Isothermal Work (n=1) ideal gas
Const. Volume Work (dv=0)𝑊 𝑏−𝑐𝑜𝑛𝑠𝑡𝑣𝑜𝑙=0
Area under P-v graph
𝑊=𝑚𝑅𝑇𝑙𝑛(𝑉 2
𝑉 1)=𝑚𝑅𝑇𝑙𝑛( 𝑃1
𝑃2)
𝑊 𝐵=𝑉 1
𝑉 2
𝑃𝑑𝑉
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∆𝐸=∑𝑄−∑𝑊⏟
1st Law Closed System
Qin
QoutWin
Wout
Net total energy transfer in the form of heat and work
Net change of system energy
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Q – W = U + KE + PE
Net transfer of energy Energy storage in different modes
Esystem= U + KE + PE
Esystem= U + KE + PE- Recall that System Energy consists of:
or
thus;
Q – W = Esystem= U + KE + PEOr can be called as the 1st Law/Energy Balance of a Closed System;
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Q – W = U + KE + PE [kJ]1st Law for Closed System in different forms;
q – w = u + ke + pe [kJ/kg]per unit mass;
Rate form;[kW]
Cyclic Process E =0
�̇�−�̇�= 𝑑𝑈𝑑𝑡 +
𝑑(𝐾𝐸)𝑑𝑡 +
𝑑 (𝑃𝐸 )𝑑𝑡
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summary;Q – W = U + KE + PE
mg(z2-z1)
2)( 2
12
2 VVm
m(u2-u1) water (Table)
mCv(T2-T1) ideal gas
Electrical
Mechanical
Boundary
etc…
2
1VIdtWel
2
1dsFW
2
1dVpWb