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Physics of Technology PHYS 1800

Physics of Technology PHYS 1800. Lecture 25 Heat Engines and the 2 nd Law of Thermodynamics. PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet. *Homework Handout. Physics of Technology PHYS 1800. Lecture 25 Heat Engines and the 2 nd Law of Thermodynamics. Review of Thermodynamics.

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Physics of Technology PHYS 1800

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  1. Physics of TechnologyPHYS 1800 Lecture 25 Heat Engines and the 2nd Law of Thermodynamics

  2. PHYSICS OF TECHNOLOGYSpring 2009 Assignment Sheet *Homework Handout

  3. Physics of TechnologyPHYS 1800 Lecture 25 Heat Engines and the 2nd Law of Thermodynamics Review of Thermodynamics

  4. 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) 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) Pressure— force divided by the area over which the force is applied (ML-1T-1 or kg/m-s or N/m2 or Pa)

  5. 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 Law: You can’t recover all heat losses (or defining entropy) • 3rd Law: You can never get to absolute 0.

  6. 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 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. Gas: No equilibrium position, no oscillations, atoms are free and move in perpetual high-speed “zig-zag” dance punctuated by collisions. liquid gas

  7. Heat + + + + + + + + + kB is Boltzmann’s constant =1.38 10-23 J/K Solid

  8. 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. Heatis a form of energy. (Here comes conservation of energy!!!)

  9. 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

  10. 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 the distance the piston moves: • W = Fd = (PA)d = PV

  11. Physics of TechnologyPHYS 1800 Lecture 25 Heat Engines and the 2nd Law of Thermodynamics Heat Engines

  12. Heat Engines It is a device that uses input heat to generate useful work. From the 1st Law (Conservation of Energy) In cyclic engines we return to the original state every cycle so What is a heat engine?

  13. Heat Engines 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 at a temperature lower than the input temperature. What is a heat engine?

  14. 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. Or from the 1st Law

  15. A heat engine takes in 1200 J of heat from the high-temperature heat source in each cycle, and does 400 J of work in each cycle. What is the efficiency of this engine? • 33% • 40% • 66% QH= 1200 J W = 400 J e = W / QH = (400 J) / (1200 J) = 1/3 = 0.33 = 33%

  16. How much heat is released into the environment in each cycle? • 33 J • 400 J • 800 J • 1200 J QC= QH - W = 1200 J - 400 J = 800 J

  17. 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. • An (ideal, not real) engine following this cycle is called a Carnot engine.

  18. 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: • 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. • This provides a useful limiting case. • Even Carnot’s ideal engine is less than 100% efficient.

  19. Carnot Cycle • 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.

  20. A steam turbine takes in steam at a temperature of 400C and releases steam to the condenser at a temperature of 120C. What is the Carnot efficiency for this engine? • 30% • 41.6% • 58.4% • 70% TH= 400C = 673 K TC= 120C = 393 K eC = (TH - TC ) / TH = (673 K - 393 K) / (673 K) = 280 K / 673 K = 0.416 = 41.6%

  21. 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? • 0.83 J • 16.64 kJ • 28 kJ • 208 kJ QH= 500 kJ e = W / QH , so W = e QH = (0.416)(500 kJ) = 208 kJ

  22. Physics of TechnologyPHYS 1800 Lecture 25 Heat Engines and the 2nd Law of Thermodynamics Second Law of Thermodynamics

  23. 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 engine operating between the same two temperatures. • Define entropy (something that measures randomness or disorder in an object) to take account of this.

  24. 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 higher-temperature reservoir.

  25. Second Law of Thermodynamics • The remaining work Wexcesswould 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.

  26. 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 318 room Review Ch 10

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