Devil physics The baddest class on campus IB Physics

1. Devil physicsThe baddest class on campusIB Physics

2. Lsn 3-2, Thermal properties

3. Questions From Reading Activity?

4. Assessment Statements IB Topic 3.2., Thermal Properties of Matter Specific Heat Capacity, Phase Changes, and Latent Heat 3.2.1. Define specific heat capacity and thermal capacity. 3.2.2. Solve problems involving specific heat capacities and thermal capacities. 3.2.3. Explain the physical differences between the solid, liquid and gaseous phases in terms of molecular structure and particle motion.

5. Assessment Statements IB Topic 3.2., Thermal Properties of Matter Specific Heat Capacity, Phase Changes, and Latent Heat 3.2.4. Describe and explain the process of phase changes in terms of molecular behavior. 3.2.5. Explain in terms of molecular behavior why temperature does not change during a phase change.

6. Assessment Statements IB Topic 3.2., Thermal Properties of Matter Specific Heat Capacity, Phase Changes, and Latent Heat 3.2.6. Distinguish between evaporation and boiling. 3.2.7. Define specific latent heat. 3.2.8. Solve problems involving specific latent heats.

7. Assessment Statements IB Topic 3.2., Thermal Properties of Matter Kinetic Model of an Ideal Gas 3.2.9. Define pressure. 3.2.10. State the assumptions of the kinetic model of an ideal gas.

8. Assessment Statements IB Topic 3.2., Thermal Properties of Matter Kinetic Model of an Ideal Gas 3.2.11. State that temperature is a measure of the average random kinetic energy of the molecules of an ideal gas. 3.2.12. Explain the macroscopic behavior of an ideal gas in terms of a molecular model.

9. Objectives • State the basic definitions of calorimetry, such as specific heat capacity and specific latent heats of fusion and vaporization. • Understand why temperature stays constant during a phase change. • Outline methods for determining specific and latent heats experimentally.

10. Objectives • Solve calorimetry problems using Q = mcΔT and Q = mL. • State the factors that affect the rate of evaporation and distinguish evaporation from boiling. • Appreciate Boltzmann’s equation, the fundamental relationship between the absolute temperature and the average kinetic energy of the molecules.

11. Specific Heat Capacity, c • When you add heat (thermal energy) to an object it will, most of the time, heat up • Specific heat capacity is the amount of thermal energy needed to raise the temperature of a mass of one kilogram of a substance by one Kelvin.

12. Specific Heat Capacity, c • The units for specific heat capacity are J/kg•K • Does the temperature have to be in Kelvin? • Can it be Celcius/centigrade? • Can it be Fahrenheit?

13. Specific Heat Capacity, c • The units for specific heat capacity are J/kg•K • Does the temperature have to be in Kelvin? • Can it be Celcius/centigrade? YES • Can it be Fahrenheit? NO

14. Specific Heat Capacity, c • Specific heat capacity is a property of the material you are dealing with. • To be specific, specific heat capacity is specific to each specific material, specifically speaking.

15. Specific Heat Capacity, c What is the difference in thermal energy required to raise the temperature of 10kg of aluminum and lead by 10 K?

16. Specific Heat Capacity, c What is the difference in thermal energy required to raise the temperature of 10kg of aluminum and lead by 10 K? Aluminum = 91,000 J Lead = 13,000 J 78,000 J Al is 600% more than Pb

17. Specific Heat Capacity, c What is the difference in thermal energy required to raise the temperature of 1 mol of aluminum and lead by 10 K?

18. Specific Heat Capacity, c What is the difference in thermal energy required to raise the temperature of 1 mol of aluminum and lead by 10 K? molAl = 27 g = 246 J molPb = 207 g = 269 J = 23 J Pb is 9% more than Al How come?

19. Specific Heat Capacity, c • Thermal energy increases the kinetic energy of the molecules which causes temperature to increase. • 1 kg of each substance gives a large difference in the number of molecules • 1 mol of each substance gives the same number of molecules with a negligible difference in masses

20. Heat Capacity, C • Equal to mass times specific heat capacity • The amount of thermal energy required to change the temperature of a body by one Kelvin • Useful when dealing with a system of objects • If we know the heat capacity of the system,

21. Thermal Equilibrium • Thermal energy will always flow from a hot body to a colder body • As long as there is a means of conducting that energy, the energy will flow until both bodies are at the same temperature which is, thermal equilibrium

22. Change of State • One example is golfer Phil Mickelson moving from California to Texas to avoid paying state income taxes • What we are more concerned with is the change in the physical state of matter • There are four states of matter: • Solid • Liquid • Gas • Plasma

23. Solid • Liquid • Gas • Plasma Change of State • In general, these states of matter are a function of temperature with solids being the coldest and plasma being the hottest • When thermal energy is added to a body, it increases in temperature • Temperature increases until the body reaches a transition point determined by its physical and chemical properties • Further energy added will cause a change of state

24. Solid • Liquid • Gas • Plasma Change of State • Melting point • Point at which a substance transitions from a solid to a liquid or vice versa if thermal energy is being extracted • Once the melting point is reached, further energy is required to provide the work needed to overcome the intermolecular bonds which hold the solid together • Temperature will remain constant during this transition • Once melting is complete, additional energy added will increase the temperature of the liquid

25. Solid • Liquid • Gas • Plasma Change of State • Melting point • The thermal energy required to melt a given substance is called the specific latent heat of fusion, Lf • The units for specific latent heat of fusion are, J/kg

26. Solid • Liquid • Gas • Plasma Change of State • Boiling point • Point at which a substance transitions from a liquid to a gas or vice versa if thermal energy is being extracted • Once the boiling point is reached, further energy is required to provide the work needed to overcome the intermolecular forces which hold the solid together, i.e. to further separate them • Temperature will remain constant during this transition • Once boiling is complete, additional energy added will increase the temperature of the gas

27. Solid • Liquid • Gas • Plasma Change of State • Boiling point • The thermal energy required to turn a given liquid into a gas is called the specific latent heat of vaporization, Lv

28. Solid • Liquid • Gas • Plasma Change of State • Gas to Plasma • Not assessed by IB or discussed in Tsokos • At extremely high temperatures, electrons have so much energy that it overcomes the electromagnetic attraction of the nucleus • Electrons separate from the nucleus and we are left with a mixture of electrons and nuclei

29. Solid • Liquid • Gas Change of State • Sample Problem How much heat is required to completely vaporize 10 kg of lead at 25° C?

30. Solid • Liquid • Gas • Plasma Change of State • Specific Latent Heats • Amount of heat needed to change the state of a unit mass of a material at its specific melting / boiling point • Latent Heats (Not Specific) • The amount of heat needed to change the state of a material irrespective of mass

31. Measuring Specific Heats • In order to determine the specific heat of a substance, we need to measure • The amount of heat added • Mass of the substance • The change in temperature Calorimetry

32. Measuring Specific Heats • A second method is to drop a hot substance into a cold liquid whose specific heat is known • By obtaining an equilibrium temperature we can calculate the unknown specific heat Calorimetry

33. Stopped Here on 1/23/2013

34. Measuring Specific Latent Heats • A similar method can be used to determine the latent heat of fusion • Drop a cold solid substance at its melting temperature into a warm liquid mixture of the same substance • By measuring the temperature change of the liquid we can calculate the energy given up by the liquid that was used to melt the solid

35. Measuring Specific Latent Heats • Specific heat of vaporization can be measured using the device shown at right • Measure the amount of heat added to a liquid in a double container • As the liquid vaporizes, it leaves the inner container, condenses on the outer container, and then drips down and is collected in a beaker at the bottom

36. Evaporation • The molecules of a liquid are constantly moving at various speeds • At the surface of the liquid, the fastest of these molecules may contain enough energy to break free and enter the atmosphere as vapour (vapor) • When that molecule has left, the total kinetic energy of the liquid has decreased which means its temperature will drop

37. Evaporation • Rate of evaporation increases with surface area and temperature of the liquid • In an enclosed system, the escaping vapour creates vapour pressure above the surface of the liquid causing some of the molecules to re-enter the liquid • Evaporation continues until equilibrium is reached where as many molecules leave the liquid as those that return • The air is then considered saturated

38. Kinetic Theory of Gases • Explained through a simple mechanical model • Several basic assumptions must be made

39. Kinetic Theory of Gases • Basic Assumptions: • Gas consists of a large number of molecules • Molecules move with a range of speeds • Volume of individual molecules is negligible compared to volume of the container • Collisions between the molecules and molecules with the container are elastic • Molecules exert no forces on each other or on the container except when in contact

40. Kinetic Theory of Gases • Basic Assumptions: • Duration of collisions (impulse) is small compared to time between collisions • Molecules follow the laws of Newtonian mechanics

41. Kinetic Theory of Gases • Boltzmann Equation: • The v2 term is the average of the squares of the speeds of the molecules of the gas • This is called the root mean square (rms) speed • Not the average speed, but close enough that the terms are used interchangeably

42. Kinetic Theory of Gases • Boltzmann Equation: • k is the Bolzmann constant and is equal to 1.38 x 10-23 J/K • It is a ratio of the gas constant R to the Avogadro (avocado, acevedo) number • Note that temperature in this equation must be in Kelvin

43. Kinetic Theory of Gases • Boltzmann Equation: • The importance of this equation is that it shows how absolute temperature is directly proportional to the average kinetic energy of the molecules of a gas

44. Molecular Explanation of Pressure • Pressure in a gas is a result of collisions of the molecules with the walls of the container • Each collision results in a momentum change in the molecule • The wall must exert a force on the molecule to effect this change in momentum • Newton’s third law says that the molecule must then exert an equal and opposite force on the wall of the container

45. Molecular Explanation of Pressure • Pressure then is the total force created by all colliding molecules divided by the surface area of the container • Pressure results from collisions of molecules with the container, NOT from collisions with each other • Elastic collisions between molecules result in individual changes in velocity and energy, but momentum and kinetic energy are conserved

46. Molecular Explanation of Pressure • The two factors that affect pressure are speed of the molecules and frequency of the collisions • When the gas is heated, speed increases and collision frequency increases as a result • When gas is heated isothermally, speed stays the same but collision frequency increases due to less separation distance

47. Molecular Explanation of Pressure • The two factors that affect pressure are speed of the molecules and frequency of the collisions So what do you think will happen if a gas is compressed rapidly with a piston?

48. Σary Review • Can you state the basic definitions of calorimetry, such as specific heat capacity and specific latent heats of fusion and vaporization? • Do you understand why temperature stays constant during a phase change? • Can you outline methods for determining specific and latent heats experimentally?

49. Σary Review • Can you solve calorimetry problems using Q = mcΔT and Q = mL? • Can you state the factors that affect the rate of evaporation and distinguish evaporation from boiling? • Do you appreciate Boltzmann’s equation, the fundamental relationship between the absolute temperature and the average kinetic energy of the molecules?

50. Assessment Statements IB Topic 3.2., Thermal Properties of Matter Specific Heat Capacity, Phase Changes, and Latent Heat 3.2.1. Define specific heat capacity and thermal capacity. 3.2.2. Solve problems involving specific heat capacities and thermal capacities. 3.2.3. Explain the physical differences between the solid, liquid and gaseous phases in terms of molecular structure and particle motion.