The laws of thermodynamics

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The laws of thermodynamics

  1. 1. Connections between heat and work
  2. 2. Connections Between Heat and Work
  3. 3. In studying thermodynamics… Surroundings SYSTEM
  4. 4. Zeroth Law of Thermodynamics  If objects A, B and C are in contact with each other, and A is in thermal equilibrium with B and B is in thermal equilibrium with C, then A is in thermal equilibrium with C. A B C
  5. 5. Zeroth Law at Work
  6. 6. First Law of Thermodynamics Heat (Q) Internal Energy (U) Work (W)
  7. 7. First Law of Thermodynamics  The change in internal energy (ΔU) is equal to the difference of the heat (Q) added/removed to/from the system and the work (W) done on/by the system. ΔU = Q – W
  8. 8. Sign Conventions for the First Law For Heat (Q) + if the heat is added to the system if the heat is removed from the system For Work (W) + if work is done by the system if work is done on the system
  9. 9. Sample Word Problems on 1st Law  Suppose 2500 J of heat is added to a system and 1800 J of work is done on the system. What is the change in internal energy of the system?  You have a motor that absorbs 3000 J of heat while doing 2000 J of work. What is the change in the motor’s internal energy?  Say that a motor does 1000 J of work on its surroundings while releasing 3000 J of heat. By how much does its internal energy change?
  10. 10. The First Law at Work
  11. 11. Thermodynamic Processes  Adiabatic (constant heat)  Isothermal (constant temperature)  Isochoric (constant volume)  Isobaric (constant pressure)
  12. 12. Second Law of Thermodynamics  Review: How does heat flow?  Would it be possible for heat to flow from an area of lower to higher temperature?
  13. 13. Second Law of Thermodynamics  Clausius Statement Heat can flow spontaneously from a hot object; heat will not flow spontaneously from a cold object to a hot object.
  14. 14. Second Law and Heat Engines  It is a machine that turns energy into mechanical energy or motion, especially one that gets its energy from a source of heat, such as burning of a fuel.  Can be classified as external combustion and internal combustion engines.
  15. 15. Water-Tube Type  It is common with stationary engines and turbines.  Water is allowed to pass through tubes while the flames and hot gaseous products of combustion follow a path over around tubes.
  16. 16. Fire-Tube Type  Used in steam locomotives  Flames and heat are made to enter the tubes which are horizontally arranged in the boiler and are surrounded by water.
  17. 17. Other Classification of Steam Engines  Condensing type  Non-condensing type
  18. 18. Gasoline Engines  These are engines whose working substance is gasoline. It is internally burned unlike steam engines.
  19. 19. Parts of a Gasoline Engine
  20. 20. Intake Stroke
  21. 21. Compression Stroke
  22. 22. Power Stroke
  23. 23. Exhaust Stroke
  24. 24. Diesel Engines  These are engines whose working substance is diesel. It is internally burned unlike steam engines.
  25. 25. Diesel Engine Structure
  26. 26. How efficient are heat engines?
  27. 27. Thermal Efficiency of an Engine  It is defined as the ratio of the net work (W) done by the engine during one cycle to the energy absorbed at the higher temperature (QH) during the cycle.
  28. 28. Sample Problem on Thermal Efficiency  Find the efficiency of a heat engine that absorbs 2000 J of energy from a hot reservoir and exhausts 1500 J to the cold reservoir.  Your car is powered by a heat engine and does 3.0 x 107 J of work getting you up a small hill. If the heat engine is 80 percent efficient, how much heat did it use and how much did it exhaust?
  29. 29. Sadi Carnot  A French engineer who established the concept of an ideal engine known as the Carnot engine.  He developed the Carnot’s theorem which states that No real engine operating between two energy reservoirs can be more efficient than a Carnot engine operating between the same two reservoirs.
  30. 30. Basic Concept of the Carnot Engine  The ideal efficiency of an engine depends on the difference of the hot and cold reservoirs.  You can’t have it all.
  31. 31. Sample Word Problems on Carnot Efficiency  If an engine extracts heat from a 2730 K reservoir and expels heat at 1730 K reservoir, what is its efficiency?  How about if the engine extracts heat from a 10 730 K reservoir instead?  What if the engine reservoirs are working at the same temperatures?
  32. 32. Wait a minute…  It is possible to produce work from heat – that is heat transferred from a hot reservoir to a cold reservoir. Would it be possible to do the reverse?
  33. 33. Second Law of Thermodynamics  Kelvin-Planck Statement It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the absorption of energy from a reservoir and the performance of an equal amount of work.
  34. 34. The Impossible Heat Pump
  35. 35. Heat Pumps  These are heat engines running in reverse.  Heat is transferred from a cold reservoir to a hot reservoir by performing work.  This is done through the aid of phase change.  Examples are refrigerators and air conditioning units.
  36. 36. The Structure of a Heat Pump
  37. 37. Refrigerators
  38. 38. Questions to Ponder  What do you notice with heat engines and heat pumps?  What do they have in common?  What is its impact to the environment?
  39. 39. Entropy  Natural processes tend to undergo increased state of disorder.  A measure of the amount of energy in a physical system not available to do work.
  40. 40. Second Law of Thermodynamics  On entropy The total entropy of an isolated system that undergoes a change can never decrease.
  41. 41. Entropy increases…
  42. 42. Third Law of Thermodynamics  It is impossible to reach absolute zero.
  43. 43. Let’s wrap up…

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