Thermodynamics

1. Thermodynamics

2. Kinetic Molecular Theory of Heat • Atoms held by electromagnetic forces • Atoms vibrate • Thermal energy is a measure of this motion. • More motion = more heat • Add heat = increase motion and thermal energy. • Think atoms: When heat or pressure is applied to a gas what happens to the molecules • More energy -> more collisions and more volume • More motion = more heat

3. Temperature • Physics Definitions • A measure of average particle kinetic energy • A measure of thermal energy. • Not dependent on mass. • Example Definitions: • The degree of hotness or coldness of a body or environment. • A measure of the warmth or coldness of an object or substance with reference to some standard value. • A measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale. • A measure of the ability of a substance, or more generally of any physical system, to transfer heat energy to another physical system. • Any of various standardized numerical measures of this ability, such as the Kelvin, Fahrenheit, and Celsius scale.

4. Thermal equilibrium • The energy transfers back and forth between two objects are equal (two sides) • Biology: Concentration inside/outside cell • Chemistry: reactants to products • Physics: systems • OR net zero energy transfer • Scenarios • Ice on a table • Where is the heat? • Air • What is making the ice melt? • A warm can in a refrigerator • Can loosing heat, warming fridge

5. How does a thermometer work? • Liquid (usually an alcohol) encased in narrow glass tube • Liquid thermometers are based on the principal of thermal expansion • When a substance gets hotter, it expands to a greater volume. • As the temperature of the liquid in a thermometer increases, its volume increases.

6. Temperature Scales • Celsius: freezing point = 0 boiling point = 100 • Kelvin: absolute zero = 0 (atoms are motionless) • Kelvin = SI unit of temperature.

7. Compare Scales K = ºC + 273 °C = (°F - 32°)/1.8

8. Which is the smaller temperature increment - a degree Celsius or a degree Fahrenheit? Explain. Answer: degree Fahrenheit The degree Fahrenheit is the smaller increment. After all, there are 180 of these Fahrenheit divisions between the normal freezing point and the normal boiling point of water; there are only 100 of the Celsius divisions between these two temperatures. If more Fahrenheit divisions can be fit between these two divisions than Celsius divisions, then the Fahrenheit divisions must be smaller.

10. Perform the appropriate temperature conversions in order to fill in the blanks in the table below.

11. Answers:

12. Why important? • Only a few countries use Fahrenheit • Planning a trip to ski in Canada or Swiss Alps? • To understand physics concepts and to communicate scientifically

13. Thermometers are Speedometers

14. Consider two samples of different gases. One sample consists of helium atoms and the other sample consists of diatomic oxygen molecules. If the samples are at the same temperature, will the particles within the sample have the same average speed? • Answer: No • Temperature is a measure of the average kinetic energy of the samples. Translational kinetic energy depends upon both the mass of the particles and the average speed at which the particles move. In comparing two samples of different gases at the same temperature, the gas with the more massive particles has the slowest particle speeds. So in comparing the speeds of helium atoms and diatomic oxygen molecules, one must be conscious of the relative masses of the two particles. Helium particles, being roughly one-eighth the mass of diatomic oxygen molecules, will move with a considerably faster speed.

15. The particles in a sample of table salt (sodium chloride) are not free to move about. They are locked in place in a structure known as a crystal lattice. Can the particles of sodium chloride possess kinetic energy? • Answer: Yes • Even though they do no possess any translational kinetic energy, they still possess some vibrational kinetic energy. The sodium and chloride ions can wiggle about their fixed lattice positions. The back and forth vibrational motion of the particles is what gives them vibrational kinetic energy. This explains why a thermometer will register a temperature when placed in the sample of matter.

16. Heat and Heat Transfer • Hea t: energy flows between objects due to temperature difference • Flows always from hot to cold • Heat Transfer: • Conduction • Convection • Radiation

18. Heat Transfer and Temperature Heat lost by one must equal heat gained by other Both end up at same final temperature hot object cold object Heat insulation

19. For each of the following designations of a system and a surroundings, identify the direction of heat flow as being from the system to the surroundings or from the surroundings to the system.

20. For each of the following designations of a system and a surroundings, identify the direction of heat flow as being from the system to the surroundings or from the surroundings to the system.

21. Conduction • Heat transfer when two objects (solids) are in contact • Most materials that conduct electricity: • conduct heat well • Will see this next unit too • Should you walk on asphalt or grass in the summer to get to the mail? • Asphalt conducts heat really well (allows transfer) • How do people walk across hot coals? • White hot

22. Convection • Heat transferred by fluid motion • Liquid or Gases • Warm air rises • Lava Lamp

23. Radiation • Energy transfer by electromagnetic waves. • Sunlight, toaster, campfire • Solar collector

24. Explain why high quality thermos bottles have a vacuum lining as a major component of their insulating ability. • Answer: Conduction and convection are heat transfer methods which depend upon the presence of materials to transfer heat. By lining a thermos bottle with a vacuum lining, energy cannot escape the contents of the bottle by two of the three forms of heat transfer.

25. http://www.physicsclassroom.com/Class/thermalP/U18l1e.cfm

26. Specific Heat • C: material property, the amount of energy required to raise 1 kg by 1 K. • Unit = J/kg∙K • Examples: Water C = 4180 J/kg∙K Iron C = 450 Copper C = 385

27. Heat Transferred • Amount of heat gained or lost by an object Q = mC∆T • Q = heat transferred (unit = joules) • m = mass • C = specific heat • ∆T = change in temp.

28. Predict the effect of the following variations upon the rate at which heat is transferred through a rectangular object by filling in the blanks. • a. If the area through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ (increased, decreased) by a factor of _________ (number). • b. If the thickness of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ by a factor of _________. • c. If the thickness of the material through which heat is transferred is decreased by a factor of 3, then the rate of heat transfer is ________________ by a factor of _________. • d. If the thermal conductivity of the material through which heat is transferred is increased by a factor of 5, then the rate of heat transfer is ________________ by a factor of _________. • e. If the thermal conductivity of the material through which heat is transferred is decreased by a factor of 10, then the rate of heat transfer is ________________ by a factor of _________. • f. If the temperature difference on opposite sides of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ by a factor of _________.

29. Answers: • a. If the area through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is increased by a factor of 2. • b. If the thickness of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is decreased by a factor of 2. • c. If the thickness of the material through which heat is transferred is decreased by a factor of 3, then the rate of heat transfer is increased by a factor of 3. • d. If the thermal conductivity of the material through which heat is transferred is increased by a factor of 5, then the rate of heat transfer is increased by a factor of 5. • e. If the thermal conductivity of the material through which heat is transferred is decreased by a factor of 10, then the rate of heat transfer is decreased by a factor of 10. • f. If the temperature difference on opposite sides of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is increased by a factor of 2.

30. Example 1 kg. of Iron is heated from 37 ºC to 57 ºC. The specific heat of iron is 450 J/kgK. What was the heat transferred to the iron? Q = mC∆T = (1 kg)(450 J/kgC)(57 ºC – 37 ºC) = 1 x 450 x 20 = 9,000 J

31. Qlost = Qgained • mC∆Tlost = mC∆Tgained • mC(Tf-Ti)hot object = mC(Tf-Ti)cold material • Watch your +/- signs

32. Example Drop 0.020 kg Iron at 180 ºC into 0.40 kg water at 10 ºC. What’s the final temp? mC(Tf-Ti)iron + mC(Tf-Ti)water = 0 (0.02)(450)(Tf – 180) + (0.4)(4180)(Tf – 10) = 0 9(Tf -180) + 1672(Tf -10) = 0 9Tf – 1620 + 1672Tf – 16720 = 0 1681Tf – 18340 = 0 1681Tf = 18340 Tf = 18340/1681 Tf = 10.9 ºC

33. Thermal Expansion • As temp increases, matter expands • Increase length • Increase volume • Water exception – expands as it freezes

34. The First Law of Thermodynamics: • Energy can’t be created or destroyed, but can change form. • Amount of energy in a system remains constant

35. The Second Law of Thermodynamics • Heat does NOT flow from cold to hot • ANY engine generates waste heat • Random particles of gas never arrange in an ordered pattern spontaneously.