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.

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

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.

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

Kinetic energy

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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

Zero-sum game

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state 1velocity = 0height = 0

state 2velocity = vheight = -h

h

state 2 state 1

Newton’s second law

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z

mg

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

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.

• 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

Force-length curve

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Force, F

Elongation, x

loading

Ideal spring

Force, F

Elongation, x

loading

unloading

Force-length curve

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dissipative spring

Force, F

Elongation, x

loading

unloading

energy dissipated by the spring

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

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

21Yang, Stabler, Journal of Electronic Materials. 38, 1245 (2009)

22What you need to know about energy, The National Academies.

23https://flowcharts.llnl.gov/ 23

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

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

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

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

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

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