Download Thermodynamics ERT 206 Energy Analysis and The First Law of

Thermodynamics ERT 206 Energy Analysis and The First Law of Thermodynamics HANNA ILYANI ZULHAIMI Outline: Ê  Forms of energy Ê  Physical Insight to internal Energy Ê  Energy Transfer by heat and work Ê  Enerngy transfer by heat Ê  Energy transfer by work Ê  First Law of Thermodynamics Ê  Energy change of a system Learning Outcome: Ê  At the end of this lecture, students should be able to: Ø  Introduce the concept of energy and define its various forms Ø  Discuss nature of internal energy Ø  Discuss the three mechanism of heat transfer: conducIon, convecIon and radiaIon Ø  Define concept of work and heat Ø  Introduce the first law of thermodynamics Ø  Understand the concept of control mass and control volume Forms of Energy Ê  Energy can exist in numerous forms such as thermal, mechanical, kineIc, potenIal, electric, magneIc, chemical and nuclear, and their sum consItutes the total energy, E of a system. Ê  Total energy, E on a unit mass basis denoted as: Ê  Thermodynamic provides no informaIon on the absolute value of total energy but it deals only with the change of the total energy. Energy can exist in light forms: numerous Thermal Mechanical KineIc PotenIal Electric MagneIc Chemical Nuclear Macroscopic forms of energy light Their sum consItutes the total energy, E of a system The total energy of a system on a unit mass: Elight (kJ/kg)
e=
m
Related to the moIon & the influence of some external effects such as : gravity, magneAon, electricity & surface tension Microscopic forms of energy Those related to the molecular light structure of a system and the degree of the molecular acIvity. Internal energy, U The sum of alight ll the microscopic forms of energy. Usually ignored. KineIc energy, KE The energy that a system possesses light as a result of its moIon relaIve to some reference frame. PotenIal energy, PE The energy that a system possesses light as a result of its elevaIon in a gravitaIonal field. Physical Insight to Internal Energy Ê  Internal energy is defined as the energy associated with the random, disordered moIon of molecules. Ê  It is sum of all the microscopic forms of energy. Ê  Internal energy involves energy on the microscopic scale. KineIc energy, KE PotenIal energy, PE Velocity 2
mV
KE = light 2
KineIc energy per unit mass: acceleraIon GravitaIonal PE = mgz
(kJ)
ElevaIon of the center of gravity of a light system relaIve to some arbitrarily selected reference level KineIc energy per unit mass: V2
ke =
2
pe = gz (kJ/kg)
Total energy of a system mV 2
E = U + KE + PE = U +
+ mgz
2
light Total energy of a system per unit mass: V2
e = u + ke + pe = u +
+ gz
2
(kJ)
(kJ/kg)
Flow of steam in a pipe Mass flow rate light  = ρA
c Vavg
 = ρV
m
Energy flow rate (kg/s)
light  e (kJ/s
E=m
@ kW)
Physical Insight to Internal Energy Ê  Many forms of energy: Ø  Nuclear Energy :The potenIal energy required to bind nucleons in the Ø 
Ø 
Ø 
Ø 
Ø 
nucleus Light Energy: The potenIal energy possessed by the oscillaIng electric and magneIc fields that make up electromagneIc radiaIon Chemical Energy: The potenIal energy stored in the electrostaIc bonding relaIonships among atoms in a molecule Electrical Energy : The potenIal energy involved in iniIaIng and maintaining electron flow Mechanical Energy :The energy generated (or stored) by machines which induces (or results from) concerted moIon processes in a system Heat Energy: The kineIc energy associated with random moIon of maWer including the vibratory and rotatory acIon of molecules Physical Insight to Internal Energy Physical Insight to Internal Energy Exersice A site evaluated for a wind farm is observed to have steady winds at a speed of 8 m/s. Determine the wind energy (a) per unit mass, (b) for a mass of 10 kg, and (c) for a flow rate of 1154 kg/s for air. c)
a)
V 2 8.5 m/s2 ⎛ 1 J/kg ⎞
e = ke =
=
= 36.1 J/kg
⎜
2 2 ⎟
2
2
1
m
/s
⎝
⎠
b)
E = me = (10 kg)(36.1 J/kg) = 361 J
1 kW ⎞
 e = (1154 kg/s)(36.1 J/kg)⎛⎜
E = m
⎟ = 41.7kW
⎝ 1000 J/s ⎠
Energy Transfer by Heat and Work Ê  total energy of the system Ø  staIc : energy stored in a system Ø  dynamic or energy interacIon: able to cross the system boundary. i. 
ii. 
heat transfer: the driving force is a temperature difference work: other than heat interacIon Heat and work are associated with a process, not a state! Energy Transfer by Heat and Work Heat transfer per unit mass light AdiabaAc process light Q
q=
(kJ/kg)
m
A process during which light there is no heat transfer Amount of heat transfer when heat transfer rate changes with Ame light  dt (kJ)
Q = ∫Q
t2
The system is well insulated so that only a negligible light amount of heat can pass through the boundary t1
Amount of heat transfer when heat transfer rate is clight onstant  Δt
Q=Q
(kJ)
During an adiabaIc process, a system exchanges no heat with its surroundings. Both the system and surroundings are at the same temperature and therefore light there is no driving force (temp diff) for heat transfer • 
Kinetic theory: Treats molecules as tiny balls
that are in motion and thus possess kinetic
energy.
• 
Heat: The energy associated with the random
motion of atoms and molecules.
Heat light Transfer Mechanisms Conduction: The transfer of energy from the more energetic particles of a light substance to the adjacent less energetic ones as a result of interaction between particles. Convection: The transfer of energy between a solid surface and the adjacent fluid that is light in motion, and it involves the combined effects of conduction and fluid motion. Radiation: The transfer of energy due to the light emission of electromagnetic waves (or photons). Work: The energy transfer associated with a force acting through a distance.
–  A rising piston, a rotating shaft, and an electric wire crossing the
system boundaries are all associated with work interactions
Work done per unit mass Power is the work done per unit Ime (kW) 17 Specifying the direcIons of heat and work. Ê  The first law of thermodynamics (the conservation of energy principle)
provides a sound basis for studying the relationships among the various forms
of energy and energy interactions.
Ê  The first law states that energy can be neither created nor destroyed during
a process; it can only change forms.
Energy cannot be created or destroyed; it can only change forms. 18 ΔEsystem
Internal, kineIc, and potenIal energy changes light 20 Ein and Eout
Ê  Heat transfer
Ê  Work
transfer
Ê  Mass flow
For constant rates, the total quanIIes during a Ime interval Δt are related to the quanIIes per unit Ime (kJ)
light The energy content of a control volume can be changed by mass flow as well as heat and work interacIons. A closed system (control mass) involves only heat transfer and work. Thank you J