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Physics of Technology PHYS 1800 Lecture 25 Heat Engines and the 2nd Law of Thermodynamics Introduction Section 0 Lecture 1 Slide 1 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 1 PHYSICS OF TOF ECHNOLOGY - PHYS 1800 PHYSICS TECHNOLOGY ASSIGNMENT SHEET Spring 2009Spring Assignment Sheet 2009 Date Day Lecture Chapter Feb 16 M Presidents Day 17 Tu Angular Momentum (Virtual Monday) 18 W Review 19 H Test 2 20 F* Static Fluids, Pressure Feb 23 M Flotation 25 W Fluids in Motion 27 F* Temperature and Heat Mar 2 M First Law of Thermodynamics 4 W Heat flow and Greenhouse Effect 6 F* Climate Change Mar 9-13 M-F Spring Break Mar 16 M Heat Engines 18 W Power and Refrigeration 20 F* Electric Charge Mar 23 M Electric Fields and Electric Potential 25 W Review 26 H Test 3 27 F* Electric Circuits Mar 30 M Magnetic Force Review Apr 1 W Electromagnets 3 F Motors and Generators Apr 6 M Making Waves 8 W Sound Waves 10 F* E-M Waves, Light and Color Apr 13 M Mirrors and Reflections Introduction Section 0 Lecture 1 Slide 2 15 W Refraction and Lenses 17 F* Telescopes and Microscopes Apr 20 M Review 22 W Seeing Atoms 24 F The really BIG & the really small INTRODUCTION TO Modern Physics PHYX 2710 May 1 F Final Exam: 09:30-11:20am No Class 8 5-8 5-8 9 9 9 10 10 10 No Classes 11 11 12 12 13 9-12 13 14 9-12 14 15 15 16 17 17 17 1-17 18 (not on test) 21 (not on test) Homework Due - 6 7 8 - 9 10 11 No test week 12 Fall 2004 * = Homework Handout *Homework Handout Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 2 Physics of Technology PHYS 1800 Lecture 25 Heat Engines and the 2nd Law of Thermodynamics Introduction Section 0 Lecture 1 Slide 3 Review of Thermodynamics INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 3 Describing Motion and Interactions Position—where you are in space (L or meter) Velocity—how fast position is changing with time (LT-1 or m/s) Acceleration—how fast velocity is changing with time (LT-2 or m/s2) Force— what is required to change to motion of a body (MLT-2 or kg-m/s2 or N) Inertia (mass)— a measure of the force needed to change the motion of a body (M) Energy—the potential for an object to do work. (ML2T-2 or kg m2/s2 or N-m or J) Work is equal to the force applied times the distance moved. W = F d Kinetic Energy is the energy associated with an object’s motion. KE=½ mv2 Potential Energy is the energy associated with an objects position. Gravitational potential energy PEgravity=mgh Spring potential energy PEapring= -kx Momentum— the potential of an object to induce motion in another object (MLT-1 or kg-m/s) Introduction Section 0 Lecture 1 Slide 4 Angular Momentum and Rotational Energy— the equivalent constants of motion for rotation (MT-1 or kg/s) and (MLT-2 or kg m/s2 or N) Modern Physics PHYX 2710 Pressure— forceINTRODUCTION dividedTOby the area over which the force is applied (ML-1T-1 or kg/m-s or N/m2 or Pa) Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 4 Dennison’s Laws Thermal Poker (or How to Get a Hot Hand in Physics) • 0th Law: Full House beats Two Pairs • 1st Law: We’re playing the same game (but with a wild card) • 2nd Law: You can’t win in Vegas. • 3rd Law: In fact, you always loose. • 0th Law: Defines Temperature • 1st Law: Conservation of Energy (with heat) • 2nd Introduction Section 0 Lecture 1 Slide 5 Law: You can’t recover all heat losses (or defining entropy) INTRODUCTION TO Modern Physics PHYX 2710 • Fall 2004 3rd Law: You can never get to absolute 0. Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 5 Heat • What is heat? • What is the relationship between quantity of heat and temperature? • What happens to a body (solid, liquid, gas) when thermal energy is added or removed? Thermal Energy Solid: Atoms vibrating in all directions about their fixed equilibrium (lattice) positions. Atoms constantly colliding with each other. Liquid: Atoms still oscillating and colliding with each other but they are free to move so that the long range order (shape) of body is lost. Introduction Section 0 Lecture 1 Slide 6 Gas: No equilibrium position, no oscillations, atoms are free and move in perpetual high-speed “zig-zag” dance punctuated by collisions. solid liquid INTRODUCTION TO Modern Physics PHYX 2710 gas Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 6 Heat k BT 12 mv 2 kB is Boltzmann’s constant + + + + Lecture 1 + + Section 0 + Introduction + + =1.38 10-23 J/K Solid Slide 7 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 7 Temperature and Heat • When two objects at different temperatures are placed in contact, heat will flow from the object with the higher temperature to the object with the lower temperature. • Heat added increases temperature, and heat removed decreases temperature. • Heat and temperature are not the same. • Temperature is a quantity that tells us which direction the heat will flow. Introduction Section 0 Lecture Heat is a form of energy. (Here comes conservation of energy!!!) 1 Slide 8 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 8 Joule’s Experiment and the First Law of Thermodynamics • Joule’s experiments led to Kelvin’s statement of the first law of thermodynamics. – Both work and heat represent transfers of energy into or out of a system. – If energy is added to a system either as work or heat, the internal energy of the system increases accordingly. • The increase in the internal energy of a system is equal to the amount of heat added to a system minus the amount of work done by the system. U = Q - W Introduction Section 0 Lecture 1 Slide 9 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 9 Gas Behavior and The First Law Consider a gas in a cylinder with a movable piston. If the piston is pushed inward by an external force, work is done on the gas, adding energy to the system. • The force exerted on the piston by the gas equals the pressure of the gas times the area of the piston: F = PA • The work done equals the force exerted by the piston times theIntroduction distance the Section 0 piston Lecture 1 moves: Slide 10 W = Fd = (PA)d = PV INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 10 Physics of Technology PHYS 1800 Lecture 25 Heat Engines and the 2nd Law of Thermodynamics Introduction Section 0 Lecture 1 INTRODUCTION TO Modern Physics PHYX 2710 Slide 11 Heat Engines Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 11 Heat Engines What is a heat engine? It is a device that uses input heat to generate useful work. e From the 1st Law (Conservation of Energy) W QH W Qnet U In cyclic engines we return to the original state every cycle so U 0 Introduction Section 0 and W QH QC Lecture 1 Slide 12 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 12 Heat Engines What is a heat engine? All heat engines share these main features of operation: – Thermal energy (heat) is introduced into the engine. – Some of this energy is converted to mechanical work. – Some heat (waste heat) is released into the environment Introduction Sectionat 0 aLecture 1 Slide 13 temperature lower than the input temperature. QH W QC INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 13 Efficiency Efficiency is the ratio of the net work done by the engine to the amount of heat that must be supplied to accomplish this work. W e QH Or from the 1st Law QH Section Q0C Introduction e Lecture 1 Slide 14 QH INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 14 A heat engine takes in 1200 J of heat from the hightemperature heat source in each cycle, and does 400 J of work in each cycle. What is the efficiency of this engine? a) b) c) 33% 40% 66% QH = 1200 J W = 400 J e = W /Introduction QH Section 0 Lecture 1 = (400 J) / (1200 J) = 1/3 = 0.33 = 33% Slide 15 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 15 How much heat is released into the environment in each cycle? a) b) c) d) 33 J 400 J 800 J 1200 J QC = QHIntroduction -W Section 0 Lecture = 1200 J - 400 J = 800 J 1 Slide 16 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 16 Carnot Engine and Carnot Cycle • Carnot considered the ideal (most efficient possible) engine for a give TH and TC. • Carnot engine has negligible work lost to friction, turbulence, heat loss, etc. • Carnot also reasoned that the processes should occur without undue turbulence. – The engine is completely reversible: it can be turned around and run the other way at any point in the cycle, because it is always near equilibrium. – This is Carnot’s ideal engine. • The cycle devised by Carnot that an ideal engine would have to follow is called a Carnot cycle. Lecture 1 Slide 17 • AnIntroduction (ideal,Section not0 real) engine following this cycle is called a Carnot engine. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 17 Carnot Efficiency • The efficiency of Carnot’s ideal engine (one using an ideal gas with PV=NkBT) is called the Carnot efficiency and is given by: QH QC TH TC eC QH TH • This is the maximum efficiency possible for any engine taking in heat from a reservoir at absolute temperature TH and releasing heat to a reservoir at temperature TC. Introduction Section 0 Lecture 1 Slide 18 • This provides a useful limiting case. • Even Carnot’s ideal engine is less than 100% efficient. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 18 Carnot Cycle 1. 2. 3. 4. Heat flows into cylinder at temperature TH. The fluid expands isothermally and does work on the piston. The fluid continues to expand adiabatically (without heat loss). Work is done by the piston on the fluid, which undergoes an isothermal compression. The fluid returns to its initial condition by an adiabatic compression. Introduction Section 0 Lecture 1 Slide 19 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 19 A steam turbine takes in steam at a temperature of 400C and releases steam to the condenser at a temperature of 120C. What is the Carnot efficiency for this engine? a) b) c) d) 30% 41.6% 58.4% 70% TH = 400C = 673 K TC = 120C = 393 K eC = (TH -Introduction TC ) / THSection 0 Lecture 1 Slide = (673 K - 393 K) / (673 K) = 280 K / 673 K = 0.416 = 41.6% 20 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 20 If the turbine takes in 500 kJ of heat in each cycle, what is the maximum amount of work that could be generated by the turbine in each cycle? a) b) c) d) 0.83 J 16.64 kJ 28 kJ 208 kJ QH = 500 kJ e = W / QH , Introduction Section 0 Lecture so W = e QH = (0.416)(500 kJ) = 208 kJ 1 Slide 21 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 21 Physics of Technology PHYS 1800 Lecture 25 Heat Engines and the 2nd Law of Thermodynamics Introduction Section 0 Lecture 1 Slide 22 Second Law of Thermodynamics INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 22 Second Law of Thermodynamics Heat (random motion) is a special form of energy that cannot be fully (with complete efficiency) transformed to other forms of energy. This leads to various forms of the Second Law of Thermodynamics. • You can’t recover all heat losses . • You can’t win in Vegas. • No engine, working in a continuous cycle, can take heat from a reservoir at a single temperature and convert that heat completely to work. • Therefore, no engine can have a greater efficiency than a Carnot Introduction Section 0between Lecture 1 the Slide 23 engine operating same two temperatures. • Define entropy (something that measures randomness or disorder in an object) to take account of this. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 23 Second Law of Thermodynamics • An engine with an efficiency greater than the Carnot engine would produce a greater amount of work than the Carnot engine, for the same amount of heat input QH. • Some of this work could be used to run the Carnot engine in reverse, returning the heat released by the first engine to the highertemperature reservoir. Introduction Section 0 Lecture 1 Slide 24 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 24 Second Law of Thermodynamics • The remaining work Wexcess would be available for external use, and no heat would end up in the lower-temperature reservoir. • The two engines would take a small quantity of heat from the higher-temperature reservoir and convert it completely to work. • This would violate the second law of thermodynamics. Introduction Section 0 Lecture 1 Slide 25 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 25 Physics of Technology Next Lab/Demo: Fluid Dynamics Temperature Thursday 1:30-2:45 ESLC 46 Ch 9 and 10 Next Class: Wednesday 10:30-11:20 BUS Slide 26318 room Review Ch 10 Introduction Section 0 Lecture 1 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Heat Engines Lecture 25 Slide 26