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 2 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 3 When a mass m is lifted by a distance z, The energy increases by mgz. We call this energy the potential energy. m m m m z state 1 State 2

Kinetic energy 4 From the stationary state to a state of velocity v, the energy increases by We call this energy the kinetic energy. m m m m state 1state 2 velocity v stationary

Zero-sum game 5 state 1 velocity = 0 height = 0 state 2 velocity = v height = -h h state 2 state 1

Newton’s second law 6 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? 7

Joule’s discovery 8 decreases

Internal Energy 9 (isolated system) = fluid + paddle + weight (internal energy) + (kinetic energy) + (potential energy) = constant Isolated system

10 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 idea The principle of the conservation of energy A new zero-sum game 12 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”. 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 13 Isolated system

Force-length curve 14 Force, F Elongation, x loading Ideal spring Force, F Elongation, x loading unloading

Force-length curve 15 dissipative spring Force, F Elongation, x loading unloading energy dissipated by the spring

Force-length curve 16 (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

17 battery bulb current I voltage V conductor of negligible 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 Lithium-ion lithium-ion battery electrolyte electrode wire Electrodes host lithium atoms. (lithium atom) = (lithium ion) + (electron) Electrolyte conducts lithium ions. Wire conducts electrons.

19 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).

20 kineticpotentiallightelectricalchemicalnuclearthermal kinetic turbinefalling objectsolar sail motorexplosionatomic bombsteam engine potential rising objectseesawelectric pump atomic bombballoon light tribo- luminescence light bulbchemo- luminescence atomic bombfire electrical generatorhydro-electricphoto- electricity electrical circuit discharge battery nuclear power station thermo- electricity chemical photo- synthesis charge battery chemical reaction atomic bombchemical reaction nuclear nuclear reaction thermal frictionfalling objectradiator fireatomic bombheat exchanger Convert energy from one form to another

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

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

Systems interact with the rest of the world in various ways 24 Exchange matterExchange energy by work Exchange energy by heat Open systemyes Isolated systemno Closed systemnoyes Thermal systemno yes Adiabatic systemnoyesno

System 25 Experimental setup A fixed number of H 2 O 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 H 2 O 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 H 2 O molecules in the cylinder). Weights transfer energy to the system by work. Fire transfers energy to the system by heat. Isolated system closed system

Transfer energy to a closed system in two ways—heat and work 26 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

27 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

28 Adiabatic work changes internal energy Variations of Joule’s experiment

From isolated system to closed system 29 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 systemclosed system

Electrical Work 30 work per unit time (power) going out the battery = VI battery bulb current I voltage V conductor of negligible resistance closed system

Mechanisms of transferring energy by heat Conduction Convection Radiation 31

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. 32 Summary