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Thermodynamics

Thermodynamics. Heat and Temperature, Thermal Properties of Matter, The First Law of Thermodynamics, The Second Law of Thermodynamics, Heat Engines, Internal-Combustion Engines, Refrigerators, Carnot Cycle, Entropy. Homework#1. What is a heat engine? What is an internal-combustion engine?

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Thermodynamics

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  1. Thermodynamics Heat and Temperature, Thermal Properties of Matter, The First Law of Thermodynamics, The Second Law of Thermodynamics, Heat Engines, Internal-Combustion Engines, Refrigerators, Carnot Cycle, Entropy

  2. Homework#1 • What is a heat engine? • What is an internal-combustion engine? • How does an internal-combustion engine work? Enumerate. • How is heat transferred in a refrigerator? Enumerate.

  3. Example • A surveyor uses a steel measuring tape that is exactly 50.000m long at a temperature of 20°C. What is its length on a hot summer day when the temperature is 35°C? α= 1.2 x10-3/C°

  4. Example • Feed a cold, starve a fever: During a bout with the flu, an 80-kg man ran a fever of 2.0°C above normal, that is a body temperature of 39.0°C. Assuming that the body is mostly water, how much heat is required to raise his temperature by that amount? cwater= 4190J/Kg

  5. Thermal Equilibrium, Temperature Scales, Thermal Expansion, Quantity of Heat, Calorimetry and Phase Changes, Mechanisms of Heat transfer Temperature and heat

  6. Temperature and Thermal Equilibrium • Temperature is a measure of hotness or coldness. • When an interaction causes no further change in the system, then it is in the state of thermal equilibrium. • Two systems are in thermal equilibrium if and only if they have the same temperature.

  7. Zeroth Law of thermodynamics • If one system is initially in thermal equilibrium with two other systems, then, these two other systems are also in thermal equilibrium with each other.

  8. Thermometric Scales

  9. Thermal Expansion Linear expansion Volume expansion Thermal Stress

  10. Quantity of Heat (Q) • Energy transfer that takes place solely because of temperature difference is called heat flow or heat transfer, and energy transferred in this way is called heat. • Calorie (cal) is defined as the amount of heat required to raise the temperature of one gram of water from 14.5°C- 15.5°C

  11. Conversion factors for the Quantity of Heat 1 cal = 4.186J 1 kcal =1000 cal = 4186 J 1 Btu= 778 ft.lb = 252 cal = 1055 J

  12. Specific Heat Capacity (C) The amount of heat needed to raise one kilogram of a substance to 1C°

  13. Phase Changes • The term phase is used to describe the specific state of matter. • A transition from one phase to another is called a phase change or phase transition.

  14. Phase Change • Heat of fusion (Lf)- the heat required per unit mass in changing solid to liquid • Heat of vaporization (Lv)- the heat required per unit mass in changing liquid to gas • Heat of combustion (Lc) - the heat required per unit mass in complete combustion of one gram of gasoline

  15. Mechanisms of Heat transfer • Conduction occurs between a body or between two bodies in contact. • Convection depends on motion of mass from one region of space to another. • Radiation is heat transfer by EM radiation

  16. Example • A surveyor uses a steel measuring tape that is exactly 50.000m long at a temperature of 20°C. What is its length on a hot summer day when the temperature is 35°C? α= 1.2 x10-3/C° • 50.009m

  17. Example • Feed a cold, starve a fever: During a bout with the flu, an 80-kg man ran a fever of 2.0°C above normal, that is a body temperature of 39.0°C. Assuming that the body is mostly water, how much heat is required to raise his temperature by that amount? cwater= 4190J/Kg.K • 6.7x105J= 133kcal

  18. Molecular Properties of Matter, KMT, Heat Capacities, Phases of Matter Thermal properties of matter

  19. KMT • Gases are considered to be composed of minute discrete particles. • The molecule in a container are believed to be in ceaseless motion during which they collide with each other and the walls of the container

  20. KMT • The molecular collision is perfectly elastic. • The molecules average KE is proportional to any given absolute temperature. • All forces of attraction are negligible due to rapid molecular separation.

  21. Gas Laws • Boyle’s Law:P1 x V1 = P2 x V2 • Charles’ Law: V1/T1 = V2/T2 • Gay-Lussac’s Law: P1/T1 = P2/T2

  22. Ideal Gas law • Emphasizes on the amount of substance and its effect on the volume of a gas, represented by Avogadro’s law • nα V if PVαRT, then PVαnRT

  23. Graham’s law of diffusion • The rate of flow of a gas molecule is inversely proportional to the square root of its density or its molecular weight, at constant temperature and pressure. • r1/r2 = √ d2/d1 • r1/r2 = √ MW2/MW1

  24. Example • How much faster does H2 travel than O2 molecule at the same temperature and pressure if molecular weights of H2 and O2 are 2g/mole and 32g/mole respectively? • H2 molecules travel 4times faster than O2 molecules

  25. Root-mean-square speed of a gas molecule Where k is the ratio of R to NA (Boltzman constant)= 1.381x10-23 J/molecule.K

  26. First law of thermodynamics

  27. First Law of Thermodynamics Internal energy is the sum of heat exchange between the system and the surroundings and W done on or by the system Internal energy is the change in initial and final energies of the system

  28. Sign Convention • W done on the system: + • W done by the system: - • Heat added to the system: + • Heat released by the system: -

  29. First Law of Thermodynamics Internal energy is the sum of heat exchange between the system and the surroundings and W done on or by the system Internal energy is the change in initial and final energies of the system

  30. Sign Convention • W done on the system: - • W done by the system: + • Heat added to the system: + • Heat released by the system: -

  31. Heat engines, Internal-Combustion Engines, Refrigerators, Carnot Cycle, Entropy Second law of thermodynamics

  32. Second Law of Thermodynamics • It is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines. • In so doing, it goes beyond the limitations imposed by the first law of thermodynamics.

  33. Second Law of Thermodynamics: heat engines • This is sometimes called the "first form" of the second law, and is referred to as the Kelvin-Planck statement of the second law. • It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W . Some amount of heat QC must be exhausted to a cold reservoir.

  34. Second Law of Thermodynamics: heat engines

  35. Second Law of Thermodynamics: heat engines • The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal processes and two adiabatic processes. • the Carnot efficiency: the processes involved in the heat engine cycle must be reversible and involve no change in entropy.

  36. The 4-stroke engine cycle

  37. The 4-stroke engine cycle • Intake stroke: the inlet valve is open and a fresh fuel-air mixture is pulled into the cylinder by the downward motion of the piston.

  38. The 4-stroke engine cycle • Compression stroke: the piston moves upward, compressing the mixture. The temperature and pressure increase. Prior to the piston reaching the top of its travel, the spark plug ignites the mixture and a flame begins to propagate across the combustion chamber.

  39. The 4-stroke engine cycle • Expansion/Power stroke: the flame continues its travel across the combustion chamber. The high pressure in the cylinder pushes the piston downward. Energy is extracted from the burned gases in the process.

  40. The 4-stroke engine cycle • Exhaust stroke: the hot products flow rapidly out of the cylinder because of the relatively high pressure within the cylinder compared to that in the exhaust port.

  41. Second Law of Thermodynamics: Refrigerators • It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. • Energy will not flow spontaneously from a low temperature object to a higher temperature object.  • “second form” or Clausius Statement

  42. Second Law of Thermodynamics: Refrigerators

  43. 2 vs 7

  44. Second Law of Thermodynamics: Entropy • In any cyclic process the entropy will either increase or remain the same. • Entropy is a measure of the amount of energy which is unavailable to do work. • Entropy is a measure of the multiplicity of a system. • Entropy is a measure of the disorder of a system. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html

  45. Thermodynamics Processes • Adiabatic: no heat transfer into or out of the system- Q=0 • ΔU= Q + W= W • Isochoric: constant volume; it does no work- W=0 • ΔU= Q + W= Q

  46. Thermodynamics Processes • Isobaric: constant pressure • W= p(V2-V1) • Isothermal: constant temperature- ΔU=0 • Q=W

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