ENERGY I am teaching Engineering Thermodynamics using the textbook by Cengel and Boles. A few...
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![Page 1: ENERGY I am teaching Engineering Thermodynamics using the textbook by Cengel and Boles. A few figures in the slides are taken from that book, and most.](https://reader035.fdocuments.in/reader035/viewer/2022062315/5697bfa01a28abf838c9552f/html5/thumbnails/1.jpg)
ENERGY
I am teaching Engineering Thermodynamics using the textbook by Cengel and Boles. A few figures in the slides are taken from that book, and most others are found online.Similar figures can be found in many places.I went through these slides in one 90-minute lecture.
Zhigang Suo, Harvard University
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Energy
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The world has many parts: stars, planets, animals, molecules, electrons, protons...The parts move relative to one another, and interact with one another.The motion and interaction carry energy.
Energy is a fundamental concept. We don’t know how to define energy in more fundamental concepts.
But we do know how to measure and calculate energy. That is all that matters.
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Potential energy
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When a mass m is lifted by a distance z,The energy increases by
mgz.
We call this energy the potential energy.
mm
mm
z
state 1
State 2
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Kinetic energy
4
From the stationary state to a state of velocity v, the energy increases by
We call this energy the kinetic energy.
mm mm
state 1 state 2
velocity vstationary
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Zero-sum game
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state 1velocity = 0height = 0
state 2velocity = vheight = -h
h
state 2 state 1
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Newton’s second law
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z
mg
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Vocabulary
• Forms of energy (kinetic energy and potential energy)• Conversion of energy from one form to another form.• Transfer of energy from one part of the system to another part.• Conservation of energy. When kinetic energy and potential energy
convert to each other, their sum is fixed. Really?
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Joule’s discovery
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decreases
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Internal Energy
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(isolated system) = fluid + paddle + weight
(internal energy) + (kinetic energy) + (potential energy) = constant
Isolated system
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Even when a tank of water is stationary at a macroscopic scale,water molecules undergo rapid and ceaseless motion.
Internal energy and molecular motion
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A game-changing ideaThe principle of the conservation of energy
A new zero-sum game
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• An isolated system has a fixed amount of energy.• What if energy of all known forms is not conserved?• Discover another form of energy to make energy conserve.• But what qualifies as a new form of energy?• Anything that can convert to a known form of energy.• Sounds like a self-fulfilling prophesy. It is.
My view on the principle of the conservation of energy follows, I believe, Feynman. Read his tale of “Dennis the Menace”. http://www.feynmanlectures.caltech.edu/I_04.html
The Feynman’s Lectures on Physics ought to be required reading for all engineers.
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• Gradually add weights from different heights to pull the spring.• When the length of the spring is x, the amount of weights to maintain the length of the spring is F(x). • When the length increases by dx the potential energy of the weights reduces by F(x)dx.• The total reduction of the potential energy of the weights is
• The same amount of energy is added to the spring as elastic energy.• The spring is a lattice of atoms. The elastic energy is stored in the stretched atom bonds.• How do I know? Gradually remove the weights to place them back to the original heights.• (Isolated system) = weights + spring. • (energy of the system) = (potential energy of the weights) + (elastic energy of the spring) = constant
Elastic energy
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Isolated system
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Force-length curve
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Force, F
Elongation, x
loading
Ideal spring
Force, F
Elongation, x
loading
unloading
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Force-length curve
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dissipative spring
Force, F
Elongation, x
loading
unloading
energy dissipated by the spring
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Force-length curve
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(isolated system) = weights + spring + (insulated room)(potential energy of the weights) + (elastic energy of the spring) + (internal energy of the room) = constant
Force, F
Elongation, x
loading
unloading
energy dissipated by the spring
dissipative spring
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battery
bulb
current I
voltage V
conductor ofnegligible resistance
Energy per unit time (power) going out the battery = VI
Isolated system
Electrical energy(isolated system) = battery + bulb + (insulated room)(chemical energy of the battery) + (internal energy of the bulb) + (internal energy of the room) = constant
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Convert chemical energy to electrical energy
Electron
Lithium-ion
lithium-ion battery
electrolyte
electrodeelectrode
wire
• Electrodes host lithium atoms.• (lithium atom) = (lithium ion) + (electron)• Electrolyte conducts lithium ions.• Wire conducts electrons.
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Surface energy of liquid• Molecules on surface have different energy from those in the interior.• When the area of surface increases, more molecules come to the surface.• The extra energy of the surface is proportional to the area of the surface:
• s is the surface energy (per unit area).
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kinetic potential light electrical chemical nuclear thermal
kinetic turbine falling object solarsail
motor explosion atomic bomb steam engine
potential rising object seesaw electricpump
atomic bomb balloon
light tribo-luminescence
light bulb chemo-luminescence
atomic bomb fire
electrical generator hydro-electric photo-electricity
electricalcircuit
dischargebattery
nuclear power station
thermo-electricity
chemical photo-synthesis
chargebattery
chemical reaction
atomic bomb chemical reaction
nuclear nuclear reaction
thermal friction falling object radiator radiator fire atomic bomb heat exchanger
Convert energy from one form to another
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21Yang, Stabler, Journal of Electronic Materials. 38, 1245 (2009)
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22What you need to know about energy, The National Academies.
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23https://flowcharts.llnl.gov/ 23
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Systems interact with the rest of the world in various ways
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Exchange matter Exchange energyby work
Exchange energy by heat
Open system yes yes yes
Isolated system no no no
Closed system no yes yes
Thermal system no no yes
Adiabatic system no yes no
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System
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Experimental setup•A fixed number of H2O molecules•Cylinder•Frictionless, perfectly sealed piston•Weights•Fire
System•A system can be any part of the world.•The rest of the world is called the surroundings of the system.
Isolated system•An isolated system does not interact with the rest of the world.•No exchange of matter. Seal the cylinder.•No exchange of energy. Jam the piston. Insulate the cylinder.•Do whatever necessary to prevent the rest of the world from affecting the system.•Here, (isolated system) = (a fixed number of H2O molecules in the cylinder) + (weights) + (fire).•Within the isolated system, energy flows from one part of the system (weights or fire) to another (water).
Closed system•The system exchange energy with its surroundings.•The system does not exchange matter with its surroundings.•Here, (closed system) = (a fixed number of H2O molecules in the cylinder).•Weights transfer energy to the system by work.•Fire transfers energy to the system by heat.
Isolated system
closedsystem
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Transfer energy to a closed system in two ways—heat and work
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thermal contact adiabatic contact
• So far as water is concerned, the two ways of adding energy give the same result.• Internal energy is a property of the closed system.• Increase the internal energy of the closed system.• Work and heat are not properties of the closed system.• Thermal contact: transfer energy by heat.• Adiabatic contact: transfer energy by work.
System = water
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• For all adiabatic processes between two states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process.
• Determine the change in internal energy by adiabatic process, U = W.
• For a closed system, in general U is not equal to W.
• The difference defines heat, U = W + Q.
The first law of thermodynamics
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Adiabatic work changes internal energy
Variations of Joule’s experiment
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From isolated system to closed system
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• Force acting on the spring by the weights: F(x). • work done to the spring by the weights: F(x)dx.• Change in the elastic energy of the spring: dU = F(x)dx.
(Isolated system) = (weights) + (ideal spring)(closed system) =( ideal spring)
Isolated system closed system
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Electrical Work
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work per unit time (power) going out the battery = VI
battery
bulb
current I
voltage V
conductor ofnegligible resistance
closed system
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Mechanisms of transferring energy by heat
• Conduction
• Convection
• Radiation
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• Forms of energy.• Convert energy from one form to another.• Energy is additive.• Transfer energy from one place to another.• The energy of an isolated system is conserved.• The internal energy of a closed system changes due to heat and work.
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Summary