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

Experimental setup
• A fixed number of H
2
• Cylinder
O molecules
• Frictionless, perfectly sealed piston
• Weights
• Fire
System
• A system can be any part of the world.
• Here the system is a fixed number of H
2
O molecules in the cylinder.
• The rest of the world is called the surroundings of the system.
The system interacts with its surroundings
• Weights transfer energy to the system by work .
• Fire transfers energy to the system by heat .
Closed system
• The system exchange energy with its surroundings.
• The system does not exchange matter with its surroundings.
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.
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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 ways to measure and calculate energy.
That is all that matters.
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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 .
z m state 1 m
State 2
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stationary m state 1
velocity v m state 2
From the stationary state to a state of velocity v, the energy increases by
1
2 mv
2
We call this energy the kinetic energy .
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1
2 mv
2 + mgz = constant h state 1 velocity = 0 height = 0
1
2 mv
2 mgh = 0+0 state 2 state 1 state 2 velocity = v height = -h
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a = dv dt v = dz dt ma = f f = mg m dv dt
+ mg = 0 z d m dt dv v + mgv = 0 dt
1
è 2 mv
2 + mgz
ö
0
ø
1
2 mv
2 + mgz = constant mg
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• 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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• 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 x
ò ( ) dx
0
• 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
Isolated system
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Force, F
Ideal spring
Force, F loading
ò x
0
( ) dx
Elongation, x loading unloading
Elongation, x
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dissipative spring
Force, F energy dissipated by the spring loading unloading
Elongation, x
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dissipative spring
(isolated system) = weights + spring + (insulated room)
(potential energy of the weights) + (elastic energy of the spring) + (internal energy of the room) = constant
Force, F energy dissipated by the spring loading unloading
Elongation, x
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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 ought to be required reading for all engineers.
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1
2 mv
2 + mgz decreases
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Isolated system
(isolated system) = fluid + paddle + weight
(internal energy) + (kinetic energy) + (potential energy) = constant
U +
1
2 mv
2 + mgh = constant
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Even when a tank of water is stationary at a macroscopic scale, water molecules undergo rapid and ceaseless motion.
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(isolated system) = battery + bulb + (insulated room)
(chemical energy of the battery) + (internal energy of the room) = constant
Energy per unit time (power) going out the battery = VI
Isolated system bulb current I conductor of negligible resistance voltage V battery
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lithium-ion battery wire
Electron
 electrode
Lithium-ion
 electrolyte electrode
• Electrodes host lithium atoms.
• (lithium atom) = (lithium ion) + (electron)
• Electrolyte conducts lithium ions.
• Wire conducts electrons.

• 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:
U = s s
A
• s s is the surface energy (per unit area).
dU = Fdx
2 s s bdx = Fdx
2 s s b = F
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kinetic potential light kinetic turbine falling object solar sail potential rising object seesaw light triboluminescence electrical generator chemical hydro-electric photoelectricity photosynthesis nuclear thermal friction falling object radiator electrical chemical nuclear motor electric pump light bulb electrical circuit charge battery radiator thermal explosion atomic bomb steam engine atomic bomb balloon chemoluminescence atomic bomb fire discharge battery chemical reaction nuclear power station atomic bomb thermoelectricity chemical reaction fire nuclear reaction atomic bomb heat exchanger
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Open system
Isolated system
Closed system
Thermal system
Adiabatic system
Exchange matter yes no no no no
Exchange energy by work yes no yes no yes
Exchange energy by heat yes no yes yes no
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(Isolated system) = (weights) + (ideal spring)
(closed system) =( ideal spring)
• 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 closed system
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work per unit time (power) going out the battery = VI closed system bulb current I conductor of negligible resistance voltage V battery
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Variations of Joule ’s experiment
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System = water 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.
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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,
D
U = W.
• For a closed system, in general D
U is not equal to W.
• The difference defines heat, D
U = W + Q.
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heat
• Conduction
• Convection
• Radiation
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Yang, Stabler, Journal of Electronic Materials. 38, 1245 (2009)
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What you need to know about energy , The National Academies.
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https://flowcharts.llnl.gov/

• 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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