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 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

2. 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.

3. 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

4. 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

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

6. Newton’s second law 6 z mg

7. 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? 7

8. Joule’s discovery 8 decreases

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

10. 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

11. 11

12. A game-changing idea The principle of the conservation of energy A new zero-sum game 12 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.feynmanlectures.caltech.edu/I_04.htmlhttp://www.feynmanlectures.caltech.edu/I_04.html The Feynman’s Lectures on Physics ought to be required reading for all engineers.

13. 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

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

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

16. 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. 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

18. 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. Electromagnetic energy 19 n: number of photons Planck’s constant frequency of the electromagnetic wave

20. 20 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).

21. Energy is an over-rated concept and an over-used word. The word tells you nearly nothing about the process. 21 Chemical energy Elastic energy Kinetic energy Potential energy Thermal energy Does the inventor of this toy really get helped by all these words? I don’t think so. Do these words help us understand how this toy work? Not really.

22. 22 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

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

24. 24 https://flowcharts.llnl.gov/archive.html

25. 25 What you need to know about energy, The National Academies.

26. 26 Yang, Stabler, Journal of Electronic Materials. 38, 1245 (2009) Wasted energy

27. System 27 A system can be any part of the world. The rest of the world is called the surroundings of the system.

28. 28 A fixed number of H 2 O molecules Cylinder Frictionless, perfectly sealed piston Weights Fire Experimental setup

29. Isolated system 29 (isolated system) = (a fixed number of H 2 O 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

30. Closed system 30 (closed system) = (a fixed number of H 2 O 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

31. Thermal system (not a standard terminology) 31 (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

32. Adiabatic system (not a standard terminology) 32 (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

33. Systems interact with the rest of the world in various ways 33 Exchange matterExchange energy by work Exchange energy by heat Open systemyes Isolated systemno Closed systemnoyes Thermal systemno yes Adiabatic systemnoyesno

34. Change the state of a closed system in two ways 34 thermal contact adiabatic contact exchange energy by heat exchange energy by work 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.

35. 35 Adiabatic work Variations of Joule’s experiment

36. 36 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 W adiabatic 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) = W adiabatic. 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. The first law of thermodynamics To make energy conserved, we discover a form of energy: internal energy.

37. Force-displacement work 37 Displacement: x. Force acting on the spring by the weights: F. work done to the spring by the weights: Fdx. (Isolated system) = (weights) + (ideal spring) (closed system) =( ideal spring) Isolated systemclosed system

38. Pressure-volume work 38 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 closed system state process F z

39. Voltage-charge work 39 battery bulb current I voltage V conductor of negligible resistance closed system Charge in the battery: Q Voltage acting on the bulb by the battery: V. work done to the bulb by the battery: VdQ.

40. Mechanisms of transferring energy by work 40 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.

41. 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. 41 conduction convection radiation

42. Pure substance 42 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, V,T,U P V state

43. 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 = xu f + (1-x)u g. 43 ufuf ugug u T u P = 0.1 MPa

44. 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 = xh f + (1-x)h g. Latent heat h gf = h g - h f 44 hfhf hghg h T u P = 0.1 MPa

45. a P a a T critical point Choices of two independent variables 5 variables (PTvuh), 10 choices 45 liquid gas T u P = 0.1 MPa T h u v gas liquid

46. Three phases 46 u v liquid solid gas intensive-intensiveextensive-intensiveextensive-extensive

47. 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. 47 Summary