demos n.
Skip this Video
Loading SlideShow in 5 Seconds..
Demos PowerPoint Presentation


370 Vues Download Presentation
Télécharger la présentation


- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Demos • Shuttle Tile and Oven • Mohair fuzz • Candle • Fire Syringe • Deck of Cards • Pasco Cylinder and Temp Sensor • Bicycle Tire and Digital Thermometer • Steam Engine and generator + bulb • Stirling Engine • DI Water , Ethanol

  2. Thermodynamics, Heat Energy, and how we benefit from it.

  3. Outline • What is Thermodynamics • What is Heat and temperature • Definition • What makes Heat flow (heat transfer) • Specific Heat • How heat flows • Conduction • Convection • Radiation • Electromagnetic Spectrum • Laws of Thermodynamics and Entropy • Doing work on a Fluid • Fire Piston - before the invention of the Match Head • Some common thermodynamic cycles • Otto Cycle and other heat engines • Combustion Process • Stirling • Refrigeration and heat pumps • Why 100% efficiency is theoretically impossible (the Heat Tax) • Common engines • Steam Engine • Diesel Engine • Electric Engine

  4. What is Thermodynamics? Thermo means “heat" and Dynamics relates to "movement"; in essence thermodynamics studies the movement of heat energy and how that energy makes mechanical movement (i.e. does work).

  5. Thermodynamics is a science about the effects of changes in temperature, pressure, and volume and how these changes effect a physical system. (e.g. a car engine, an air conditioner)

  6. What is Heat? Heat Energy is a type of kinetic energy Heat Energy relates to Thermal Energy (or internal energy) Thermal Energy is the sum of the kinetic energy, ½ mv2 , of ALL the individual atoms in a system or object. Heat is the energy that flowsfrom one object to anotherdue to a temperature difference.

  7. When Energy flows from a hot object to a cold object, the energy is called Heat

  8. Before 1800, Heat was thought to be an invisible fluid that flowed between objects • Objects were thought to contain fixed quantities of heat. • Benjamin Thompson observed that canons bored with dull tools became very hot while those bored with sharp tools did not get as hot. The heat generated had nothing to do with the size of the canon. • Thompson suggested that heat came from friction (or mechanical energy).

  9. James Joule tests the predictions: James Joule’s experiment proved that heat was a form of energy. In this experiment the kinetic energy of the paddle is transferred to thermal energy in the water, as measured with a sensitive thermometer.

  10. Two objects in contact on a microscopic level: Fast moving Slow moving Fast moving atoms with a lot of random motion collide with slower moving atoms. As kinetic energy is transferred from the fast moving atoms to the slower moving atoms, we say that the warmer side gave up heat to the colder side and that heat was transferred.

  11. What is Temperature? Temperature is a measurement of the average thermal energy of the particles in a substance. Heat flows due to temperature differences. No heat is transferred between two objects that are at the same temperature (i.e. in thermal equilibrium). A cup of boiling water is at the same temperature as a gallon of boiling water, but the gallon of boiling water has more thermal energy than the cup.

  12. Which object has higher thermal energy?

  13. Heat Capacity • Heat Capacity of an object is the required energy needed to raise the object’s temperature by one degree. • A large quantity of matter has a larger heat capacity than something smaller • Our oceans and atmosphere have large heat capacities due to their large sizes.

  14. Specific Heat The measure of the heat energy required to increase the temperature of a unit quantity of a substance by one degree. Copper 0.385 Joule/gr oC Dry Air 1.0035 Joule/gr oC Humid Air 1.0102 Joule/gr oC Water 4.1813 Joule/gr oC Concrete 0.88 Joule/gr oC Sand 0.42 Joule/gr oC Second highest specific heat, next to Ammonia

  15. Specific Heat of Water • Very high • Earth’s ocean store vast amounts of thermal energy – these large heat reservoirs regulate the earth’s temperature. • Unfrozen lakes moderate surrounding climate • Water filled walls make good thermal mass

  16. Water filled walls as thermal mass

  17. Using plant material as solar mass McGill University, Montreal – Solar Decathlon 2007

  18. How does Heat flow? • Conduction – the transfer of heat energy by making direct contact with the atoms/molecules of the hotter object • Convection – the transfer of heat due to a bulk movement of matter from hotter to colder areas • Radiation – energy transferred by electromagnetic waves

  19. Conduction • When two objects are in direct contact, particles in the hotter object are moving faster and will collide with slower moving objects in the colder object. • When this happens, heat flows. • Energy is transferred from the hot object to the cold object.

  20. Touch the wood table and then touch the metal legs of the table… Both Objects are at the same temperature, but the metal feels colder, why? You are at a higher temperature than any non-living object in the room, therefore heat is transferred from your body to both the wood and the metal. The metal conducts heat better than the wood because there are a lot of free electrons in metals, therefore mobile electrons take heat from your hand faster than wood.

  21. The rate of heat transfer depends on: • The Temperature difference • And the Thermal Conductivity of the Materials Shuttle Tile white-hot at 2300 oF The tile is 10% pure silica fibers and 90% air. The high percentage of air makes the tiles very lightweight. Tile has very low thermal conductivity due to trapped air and the low conductivity of long glass fibers.

  22. Is air a good thermal insulator? • Thermal Insulation is the method of preventing heat from entering or escaping from a container. • Stagnant air is a good thermal insulator • Coats, feathers, fur, hair, fiberglass insulation, & straw bales all trap tiny pockets of air. • The ocean of air over your head helps keep the earth cool during the day and warm during the night. • Air has high specific heat.

  23. Heat energy is transmitted by collisions from neighboring atoms/molecules.

  24. More examples of Conduction

  25. Convection Buoyancy forces cause bulk movement of the water. www.physics.arizona.ed

  26. More examples of Convection Rising hot air and falling cool air sets up convection cells.

  27. Heat from the earth’s core comes from ancient energy left from earth’s formation and radioactive elements which decay and release heat. Heat from the earth’s core causes slow moving convection cells in the earth’s mantle. The earth’s crust spreads at mid-ocean ridges by 2-3 cm per year.

  28. Northern Atlantic cold water sinks. This pulls in warmer water from the Gulf Stream. This heat transfer phenomenon determines how warm or cool European climates will be. Fresh water is less dense than salt water. As Greenland’s ice fields recede more fresh water enters the North Atlantic possibly impacting normal oceanic convection patterns. Oceanic Convection

  29. Forced Convection • Forced Convection is not due to the natural forces of buoyancy induced by heating. • Instead, there is a external force that causes the fluid to convect, such as a fan or a pump.

  30. Convection Ovens A fan circulates the air so hot air is not trapped at the top of the oven. More cookies can be baked at one time and all will cook at the same rate.

  31. Ceiling Fans In both hot and cold weather, ceiling fans are useful for circulating air to force convection. Rooms with high ceilings are a problem during the winter as the hot air rises and moves away from the floor area.

  32. Heat Transfer from Radiation • All matter that has thermal energy will emit infrared electromagnetic radiation. • We can feel this when we put our hands close to a fire. • This type of heat transfer requires no medium. Electromagnetic radiation travels at the speed of light through a vacuum.

  33. Infrared Radiation All objects with thermal energy emit Infrared Radiation (even ice) Infrared radiation is invisible to our eyes but we can feel it as heat

  34. The Sun’s energy is transferred to earth by electromagnetic waves • Visible Light • Infrared radiation • Ultraviolet (UV)

  35. Electromagnetic Spectrum of Waves

  36. Ionizing radiation causes matter to ionize (can rip an electron off an atom) • Ionizing radiation carries more energy than those waves with larger wavelengths. • The sun’s UV waves are those responsible for burning and skin cancer. • Infrared is non-ionizing radiation. Non-ionizing radiation is everywhere and is considered to not be harmful.

  37. Laws of Thermodynamics Zeroth Law: If two objects are in thermal equilibrium with a third object, then they are also in thermal equilibrium with each other. Thermal equilibrium means an objects temperature, pressure, and volume are not changing.

  38. A cooling cup of coffee is NOT in thermal equilibrium with the room.

  39. / If two cups of coffee are at thermal equilibrium with the room, then the two cups are in thermal equilibrium with each other. The two cups of coffee have the same temperature. If the two cups are put in contact with each other no heat will flow.

  40. First Law of Thermodynamics: (The good news!) Energy is Conserved. Energy can not be destroyed. In an isolated system, the total energy stays the same. Energy can be converted from one form to another. Thermal Energy can be converted into another form of energy!

  41. What is Entropy? Entropy = total disorder of an object/system Disorder is the sum of the thermal energy plus the physical disorder. Entropy always increases with time!

  42. Examples of increasing entropy Playing “52 pick up”

  43. + Heat, light, ash, particulates, gases + Heat, light, ash, particulates, gases Direction Is possible Direction Is impossible

  44. Energy flows in one direction – towards a more disordered state

  45. The Second Law of Thermodynamics: (The bad news!) An isolated system gets more disordered with time. Entropy always increases with time.

  46. What does this mean to us? • It is impossible to construct an engine that converts all its thermal energy into useful work. The exhaust must be hotter than the incoming air. • 100% efficiency is impossible –there must be some unusable energy because entropy must increase. • We’re going to get old and die • The house is going to need cleaning again!

  47. Why is 100% efficiency theoretically impossible? • If machine operates in a cycle, some energy must be used to reset the machine. • Parts of machine will absorb some of the heat. • Exhaust must be hotter than incoming air, due to 2nd law. This hot exhaust represents wasted energy.

  48. Doing work on a Fluid When a fluid is compressed, work is done on the fluid. This work/energy is converted into thermal energy within the fluid. Each molecule has more kinetic energy so the temperature of the fluid increases.

  49. Fire Piston As air is rapidly compressed, it can reach 400-500 degrees, allowing tinder to ignite. The compressed air is the heat source as well as the oxygen needed to ignite the tinder. Fire piston are thought to be prehistoric fire starting devices, used in South East Asia and South Pacific. For more info see: