1. Heat & Thermodynamics Physics(P)

2. State Standards 3. Energy cannot be created or destroyed, although in many processes energy is transferred to the environment as heat. As a basis for understanding this concept: a. Students know heat flow and work are two forms of energy transfer between systems. b. Students know that the work done by a heat engine that is working in a cycle is the difference between the heat flow into the engine at high temperature and the heat flow out at a lower temperature (first law of thermodynamics) and that this is an example of the law of conservation of energy. c. Students know the internal energy of an object includes the energy of random motion of the object's atoms and molecules, often referred to as thermal energy. The greater the temperature of the object, the greater the energy of motion of the atoms and molecules that make up the object.

3. Additional Standards d. Students know that most processes tend to decrease the order of a system over time and that energy levels are eventually distributed uniformly. e. Students know that entropy is a quantity that measures the order or disorder of a system and that this quantity is larger for a more disordered system. f.* Students know the statement "Entropy tends to increase" is a law of statistical probability that governs all closed systems (second law of thermodynamics). g.* Students know how to solve problems involving heat flow, work, and efficiency in a heat engine and know that all real engines lose some heat to their surroundings

4. What is the Difference Between Heat and Temperature? Both are related to energy but there’s a big difference

5. Temperature Measure of how hot or cold an object is Measured by thermometers Work by expansion of a liquid Other types use bimetallic strip

6. Digital Thermometers Use “thermistors” - temperature dependant semiconductor resistors

7. Temperature Scales Fahrenheit T( 0 F) = 9/5T( 0 C) + 32 Celsius (centigrade) T( 0 C ) = 5/9[T( 0 F) – 32] (degree is 9/5 that of Fahrenheit) Kelvin (Celsius + 273)

8. Examples Zero degrees Celsius is what Kelvin? Answer: 273 o What is the boiling point of water in degrees Kelvin? Answer: 373 o 200 degrees Celsius is what in Kelvin? Answer: 473 o

9. Absolute Zero 0 degrees Kelvin = -273 Celsius Lowest possible temperature Molecular motion ceases Courtesy Michigan State University

10. Kinetic Theory of Heat All matter is made of tiny atoms and molecules, constantly in motion Faster is hotter gas solid

11. Temperature and Kinetic Energy In ideal gas temperature is proportional to average kinetic energy per molecule Closely related in liquids and gases

12. Temperature does not depend on the amount

13. Heat does depend on the amount Analogy: Heat is like the total height of students in this room, temperature is like their average height. There is twice as much kinetic energy of moving molecules in two liters of water as in one liter.

14. Which has more heat? A swimming pool full of ice water? A cup full of boiling water? Answer: the swimming pool, because it has so much more water.

15. Heat – Energy Transferred Definition: Energy that transfers because of temperature difference Heat flows governed by average molecular kinetic energy difference

16. Thermal Equilibrium Objects at same temperature are at thermal equilibrium – no heat flows.

17. Internal Energy Total of all forms of energy inside something Includes –Translational kinetic –Rotational kinetic –Potential

18. Summary The greater the temperature of an object, the greater the energy of motion of the atoms and molecules that make up that object. Internal energy of an object includes the energy of random motions of atoms and molecules in the object

19. Measuring Heat One calorie is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius. Kilocalorie raises the temperature of one kg of water by 1 0 C (also called Calorie or food calorie) One calorie = 4.186 joules One kilocalorie = 4186 joules

20. Density of Water Density is mass per unit volume D = M/V One gram per cubic centimeter One kilogram per liter One thousand kg per cubic meter

21. Calorie Questions How many calories are needed to raise the temperature of 10 grams of water by 1 0 C? 10 How many calories are needed to raise the temperature of one gram of water by 10 0 C? 10

22. A certain amount of heat raises the temperature of 1 liter of water by 2 0 C. What would be the temperature increase if the same amount of heat were added to 2 liters of water? Answer 1 0 C

23. Example A person consumes and expends about 2000 Calories per day. What is their thermal power output? 2000Cal/d x 1 d/24h x 1h/3600s x 4184J/C = 96.8J/s = 96.8 Watts

24. Specific Heat Capacity Different materials change their temperature by different amounts when they absorb the same amount of heat. Some have more ways of storing energy than others Water has very high specific heat (capacity) Metals have much less

25. Q = mc  T Q = mc  T expresses how heat absorption works. C is specific heat Question: A certain rock has a specific heat of 0.25 (water is 1.0) How much heat will be required to heat 5.0 kg rock from 20 to 80 0 C? Q = 5.0kg x 1000g/kg x 0.25 Cal/g 0 C x 60 0 C Q = 75,000 C = 7.5 x 10 4 Calories

26. Thermal Expansion Most materials expand when heated Only exception is water between 0 0 C and 40 0 C Expansion joints in bridges, cracks in sidewalks allow for expansion

27. Bimetallic Strip How your thermostat works

28. Don’t Let Your Car’s Engine Overheat Aluminum expands more than iron Pistons made of aluminum Cylinder made of iron

29. Mechanical Equivalent of Heat Discovered by James Joule Falling weight makes paddle turn 4.186 x 10 3 J = 1 kcal Interpretation: HEAT IS ENERGY TRANSFER Courtesy W. Bauer http://lecture.lite.msu.edu/~mmp/kap 11/cd295.htm

30. Joule’s Apparatus Link to Joule’s original article

31. Example When digested a slice of bread yields 100 kcal. How high a hill would a 60 kg student need to climb to “work off” this slice of bread? 100 kcal x 4.186 x 10 3 J/kcal = 4.2 x 10 5 J W = mgh h = W/mg = 4.2 x 10 5 / (60 kg)(9.80 m/s 2 ) = 714m = 7.1 x 10 2 m If the body is only 20 percent efficient in transforming the bread, how high need they climb?

32. Bullet in Block When a 10 g bullet traveling 500 m/s is stopped inside a 1kg wood block nearly all its KE is transformed to heat. How many kcal are released? KE = ½ mv 2 = 0.5 x 0.010 kg x (500) 2 = 1250 J 1250 J x 1 kcal/4186 J = 0.30 kcal

33. Thermodynamics Study of heat and its transformation into mechanical energy Based on conservation of energy Explains how engines like car motors work

34. First Law of Thermodynamics Generally, when you add heat to a system it changes into an equal amount of some other form of energy Heat added = increase in internal energy + external work done by the system

35. Work Done On and By Compressing a gas by pushing down on a piston = work done on A gas expands by pushing a piston up = work done by

36. Questions 20 J of heat is added to a system that does no work. What is the change in internal energy? Answer +20 J 20 J of heat is added to a system that does 10 J of work. What is the change in internal energy? Answer +10 J

37. 20 J of heat is added to a system that does 30 J of work. What is the change of internal energy? Answer -10 J 20 J of heat is added to a system that has 10 J of work done on it. What is the change of internal energy? Answer +30 J

38. Bicycle Pump What do you think happens when you operate the pump. Where does the work you do go? It goes to heat, some through friction, some to adiabatic compression of the air inside the pump What does “adiabatic” mean? Answer: No heat enters or leaves Q=0

39. Adiabatic Processes Compression or expansion of a gas so that no heat enters or leaves Example: gas in cylinder of car or diesel engine Why adiabatic? Because it happens too fast for much heat to enter or leave. In adiabatic compression, temperature rises. –In diesel engine, enough to ignite gas without spark plug A process can also be adiabatic if it happens inside a well insulated conatiner.

40. Courtesy “How Stuff Works”

41. Courtesy Shell Canada

43. Adiabatic Expansion Produces cooling Example: blow on your hand first with wide open mouth, then with puckered lips How do you explain the results?

44. The Chinook What would you expect to happen if cold air moves down the slopes of mountains Hint: it will be compressed by atmosphere into smaller volume Chinook wind is warm Common in Rocky mountains

45. Second Law of Thermodynamics Heat flows from hot to cold. By itself it will never flow from cold to hot. Question: Would it violate the First Law of Thermodynamics (energy conservation) if heat flowed from a cold object to a warm object touching it? Answer: No

46. Second Law Applied to Engines It is impossible to build a heat engine that changes heat completely into work. Courtesy University of Oregon Such an engine would be 100% efficient! Allowed by 1 st law, forbidden by 2 nd law

47. Heat Engine Some heat is converted to useful work The rest is exhausted on at a lower temperature (cause of thermal pollution) Efficiency = useful work / heat input –About 20-25% for gasoline engine –About 35-40% for diesel engine The energy exhausted is waste, cannot be recovered

48. Ideal (Carnot) Engine Ideal (Maximum possible efficiency) = (T hot – T cold )/T hot (Kelvin temperatures) What is the efficiency of a steam turbine (assumed ideal) operating between 400K (127 0 C) and 300K (27 0 C)? (400 – 300)/400 = ¼ or 25% What would be the efficiency if the turbine could operate at 600K? What would the exhaust temperature need to be for an engine to be 100% efficient? 1/2 0 K

49. Steam Turbine

51. Limits to Technology What factors limit the efficiency of an engine? –Friction –Temperature at which parts melt –Carnot efficiency What would be the advantages of a ceramic engine? Disadvantages?

52. Courtesy University of Colorado Can operate at 3000 degrees without cooling, is light and doesn’t need much cooling, but…

53. Heat Engine Summary Work done is difference between heat flow in at high temperature and the heat flow out at a lower temperature (conservation of energy)

54. Order and Disorder Useful energy tends to degenerate and become less useful Alternate statement of 2 nd Law: Natural systems tend toward disorder Question: Could all the air molecules in this room spontaneously concentrate at the top of the room (more orderly system)?

55. Entropy A measure of how much change occurs when energy spreads out according to the second law. More generally (and less accurately) a measure of disorder When disorder increases, entropy increases

56. Mess to Neat? Will this mess become neat all by itself? No way!

57. Will this dish reassemble all by itself? No Way, the Second Law of Thermodynamics prohibits it

58. Entropy Summary Entropy is a quantity that measures the order or disorder of a system This quantity is larger for a more disordered system The Second Law of Thermodynamics says that entropy tends to increase All real engines lose heat to their surroundings Courtesy California Science Standards in Physics

59. Courtesy University of Oregon Global Warming