ENERGY

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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 two 90-minute lectures.

Zhigang Suo, Harvard University
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Energy
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 dont 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
m
When a mass m is lifted by a distance z, The
energy increases by mgz. We call this energy
the potential energy.
State 2
z
m
state 1
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Kinetic energy
velocity v
stationary
m
m
state 1
state 2
From the stationary state to a state of velocity
v, the energy increases by We call this
energy the kinetic energy.
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Zero-sum game
state 1 velocity 0 height 0
h
state 2 velocity v height -h
state 2 state 1
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Newtons second law
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 place to another
  place.
• Conservation of energy. When kinetic energy and
  potential energy convert to each other, their sum
  is fixed. Really?

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Joules discovery
decreases
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Internal energy
Isolated system
(isolated system) fluid paddle weight
(internal energy) (kinetic energy) (potential
energy) constant
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Internal energy and molecular motion
Even when a tank of water is stationary at a
macroscopic scale, water molecules undergo rapid
and ceaseless motion.
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(No Transcript)
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A game-changing ideaThe principle of the
conservation of energyA new zero-sum game

• Energy is additive
• An isolated system has a fixed sum 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.feynmanlec
tures.caltech.edu/I_04.html The Feynmans
Lectures on Physics ought to be required reading
for all engineers.
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Elastic energy

• 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

Isolated system
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Force-length curve
Ideal spring
Force, F
Force, F
loading
loading
unloading
Elongation, x
Elongation, x
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Force-length curve
dissipative spring
Force, F
energy dissipated by the spring
loading
unloading
Elongation, x
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Force-length curve
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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Electrical energy
(isolated system) (battery) (bulb)
(insulated room) (chemical energy of the battery)
(internal energy of the bulb) (internal
energy of the room) constant
Energy per unit time (power) going out the
battery VI
Isolated system
bulb
conductor of negligible resistance
current I
voltage V
battery
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Convert chemical energy to electrical energy
lithium-ion battery
wire
electrolyte
electrode
electrode

• Electrodes host lithium atoms.
• (lithium atom) (lithium ion) (electron)
• Electrolyte conducts lithium ions.
• Wire conducts electrons.

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Electromagnetic energy

• n number of photons
• Plancks constant
• frequency of the electromagnetic wave

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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
• ss is the surface energy (per unit area).

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Energy is an over-rated concept and an over-used
word. The word tells you nearly nothing about
the process.

• Chemical energy
• Elastic energy
• Kinetic energy
• Potential energy
• Thermal energy

Does the inventor of this toy really get helped
by all these words? I dont think so. Do these
words help us understand how this toy work? Not
really.
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Convert energy from one form to another
kinetic potential light electrical chemical nuclear thermal
kinetic turbine falling object solar sail motor explosion atomic bomb steam engine
potential rising object seesaw electric pump atomic bomb balloon
light tribo- luminescence light bulb chemo- luminescence atomic bomb fire
electrical generator hydro-electric photo-electricity electrical circuit discharge battery nuclear power station thermo- electricity
chemical photo- synthesis charge battery chemical reaction atomic bomb chemical reaction
nuclear nuclear reaction
thermal friction falling object radiator radiator fire atomic bomb heat exchanger
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https//flowcharts.llnl.gov/
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https//flowcharts.llnl.gov/archive.html
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What you need to know about energy, The National
Academies.
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Wasted energy
Yang, Stabler, Journal of Electronic Materials.
38, 1245 (2009)
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System

• A system can be any part of the world.
• The rest of the world is called the surroundings
  of the system.

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Experimental setup

• A fixed number of H2O molecules
• Cylinder
• Frictionless, perfectly sealed piston
• Weights
• Fire

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Isolated system

• (isolated system) (a fixed number of H2O
  molecules in the cylinder) (weights) (fire).
• An isolated system does not interact with the
  rest of the world.
• Inside the isolated system, energy flows from one
  part of the system (weights or fire) to another
  (water).

Isolated system
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Closed system

• (closed system) (a fixed number of H2O
  molecules in the cylinder).
• The closed system does not exchange matter with
  its surroundings.
• The closed system exchange energy with its
  surroundings.
• Weights transfer energy to the closed system by
  work.
• Fire transfers energy to the closed system by
  heat.

closed system
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Thermal system (not a standard terminology)

• (thermal system) (fixed amount of water)
  (fixed set of weights)
• The thermal system does not exchange matter with
  its surroundings.
• The thermal system does not exchange energy by
  work with its surroundings.
• The thermal system exchanges energy by heat with
  its surroundings.
• The system is said to be in thermal contact with
  its surroundings.
• Enthalpy
• (Internal energy of the thermal system)
  (internal energy of water) (potential energy of
  weights)
• Draw a free-body diagram of the piston. Balance
  forces acting on the piston mg PA
• (Potential energy of weights) mgh PAh PV
• (Internal energy of the thermal system) U
  PV.
• U, P, V are all properties of water, so that (U
  PV) is also a property of water.
• Give the quantity (U PV) a symbol H, and call H
  U PV the enthalpy of water.

thermal system
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Adiabatic system(not a standard terminology)

• (adiabatic system) water fire
• The adiabatic system does not exchange matter
  with its surroundings.
• The adiabatic system does not exchange energy by
  heat with its surroundings.
• The adiabatic system exchanges energy by work
  with its surroundings.
• The system is said to be in adiabatic contact
  with the surroundings.

adiabatic system
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Systems interact with the rest of the world in
various ways
Exchange matter Exchange energy by 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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Change the state of a closed system in two ways

• A closed system does not exchange matter with the
  rest of the world.
• The closed system exchanges energy with the rest
  of the world in two ways.

thermal contact adiabatic
contact exchange energy by heat exchange
energy by work
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Adiabatic work
Variations of Joules experiment
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The first law of thermodynamics
To make energy conserved, we discover a form of
energy internal energy.

• Name the states of a closed system using a set of
  independent properties.
• Internal energy of the closed system, U, is a
  property (i.e., a function of state) of the
  closed system.
• Experimental determination of internal energy.
  Insulate the closed system to make an adiabatic
  system. When we do work Wadiabatic to the
  adiabatic system, the system changes from state A
  to state B, and the change in internal energy
  equals the adiabatic work, U(B) U(A)
  Wadiabatic.
• Experimental determination of heat. Now let the
  closed system interact with the rest of the world
  by both adiabatic contact and thermal contact.
  When the closed system changes from state A to
  state B, in general, U(B) U(A) does not equal
  the work W. The difference defines heat, U(B)
  U(A) W Q.
• Work and heat are not properties of the closed
  system.
• The art of measuring internal energy and heat is
  known as calorimetry.

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Force-displacement work
(Isolated system) (weights) (ideal
spring) (closed system) ( ideal spring)

• Displacement x.
• Force acting on the spring by the weights F.
• work done to the spring by the weights Fdx.

Isolated system
closed system
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Pressure-volume work

• External force acting on the piston F
• height of the cylinder occupied by the gas z
• Work done by the external force -Fdz
• Volume V. dV Adz
• Pressure due to external force p F/A.
• work done by the external to the closed system -
  Fdz - pdV.
• Work done in a process integrate pdV along the
  path

state
process
state
closed system
F
z
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Voltage-charge work

• Charge in the battery Q
• Voltage acting on the bulb by the battery V.
• work done to the bulb by the battery VdQ.

closed system
bulb
conductor of negligible resistance
current I
voltage V
battery
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Mechanisms of transferring energy by work

• Identify a macroscopic quantity
• Displacement x
• Volume V
• Charge Q
• The change in energy associated with such a
  macroscopic quantity is called a type of work
• Force-displacement, Fdx
• Pressure-volume, PdV.
• Voltage-charge, VdQ.

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Mechanisms of transferring energy by heat

• Conduction. Energy flows via waves of atomic
  vibration. Matter does not flow
• Convection. Matter flows, energy flows with it.
• Radiation. Light, electromagnetic wave. With or
  without matter.

conduction convection
radiation
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Pure substance

• Thermodynamic states of the system
• Add a little heat and add a little weight to the
  closed system
• Isolate the system.
• The system isolated for a long time approaches a
  thermodynamic state of equilibrium.
• The states of the system has two independent
  variations.
• Thermodynamic properties of the system
• A property is a function of state.
• Intensive properties temperature, pressure.
• Extensive properties, volume, energy, enthalpy.
• Equations of state
• Use two properties (e.g., pressure P and volume
  V) as independent properties to specify (i.e.,
  name) all states.
• Any other property is a function of the two
  properties. T(P,V) and U(P,V).

P
state
V
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Internal energy of a pure substance

• Internal energy U is an extensive property.
• Internal energy per unit mass, u U/m
• For a state outside the dome, u(P,T)
• For a state inside the dome, u xuf (1-x)ug.

P 0.1 MPa
T
u
uf
ug
u
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Enthalpy of a pure substance

• Enthalpy H is an extensive property.
• Enthalpy per unit mass, h H/m
• For a state outside the dome, h(P,T)
• For a state inside the dome, h xhf (1-x)hg.
• Latent heat hgf hg - hf

P 0.1 MPa
T
u
hf
hg
h
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Choices of two independent variables5 variables
(PTvuh), 10 choices
liquid
gas
T
T
u
liquid
gas
liquid
gas
gas
P 0.1 MPa
P 0.1 MPa
liquid
u
h
v
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Three phases
u
gas
liquid
solid
v
intensive-intensive
extensive-intensive
extensive-extensive
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Summary

• Energy has many forms.
• Convert energy from one form to another form.
• Energy is additive.
• Transfer energy from one place to another place.
• Discover new form of energy by making the energy
  of any isolated system conserved.
• The energy of a closed system changes as energy
  transfers by work and heat.
• The first law defines internal energy and heat in
  terms of experimental measurements.
• Calorimetry is the art to measure internal energy
  and heat.
• Represent states of a pure substance on planes of
  various properties.