1. Change of Phase
Chapter 23

2. Evaporation
Evaporation – a change of phase from liquid to gas that takes place at the surface of a liquid
The molecules at the surface of a liquid may gain enough kinetic energy by being bumped by other molecules to break free of the liquid, comprising a vapor
The average kinetic energy of the molecules left behind is lowered, therefore evaporation is a cooling process

3. Evaporation

4. Condensation
Condensation – the changing of phase from a gas to a liquid
Vapor molecules can run into slower-moving particles of a cool surface and slow themselves down enough to reform as a liquid
The air always contains some water vapor
Saturated – at any given temperature, there is a limit to the amount of water vapor that can be in the air
Relative Humidity – indicates how much water vapor is in the air, compared with the limit for that temperature
At a relative humidity of 100%, the air is saturated
Fog is formed when water vapors stick together as they cool, moist air moves in close to the ground and water vapor condenses out to form a cloud

5. Condensation

6. Evaporation and Condensation Rates
Evaporation and Condensation are occurring continuously at equal rates
The molecules and energy leaving the liquid’s surface by evaporation are counteracted by as many molecules and as much energy returning by condensation
The liquid is in equilibrium – in a state of balance – since evaporation and condensation have canceling effects
If evaporation exceeds condensation, a liquid is cooled
If condensation exceeds evaporation, a liquid is warmed

7. Boiling
Boiling – change of phase which occurs beneath the surface from liquid to gas
The pressure of the vapor within the bubbles of a boiling liquid must be great enough to resist the pressure of the surrounding water
As atmospheric pressure is increased, the molecules in a vapor are required to move faster to exert increased pressure within the bubble, thereby increasing the boiling point of a liquid
Boiling, like evaporation, is a cooling process

8. Freezing Freezing – the change in phase from a liquid to a solid
When energy is extracted from a liquid, it freezes
If sugar or salt is dissolved in water, the freezing temperature is lowered; these molecules get in the way of the formation of ice crystals
The ocean, with its high salinity, rarely freezes, even on the top

9. Regelation
Regelation – the phenomenon of melting under pressure and freezing again when the pressure is reduced (specific to water)
This is do to the very open structure of ice crystals, the application of pressure lowers the melting point

10. Regelation

11. Energy and Changes of Phase
Energy must be put into a substance to change from a solid all the way to a gas, and energy must be extracted to change a substances phase from a gas to a solid
The phase change sequence is reversible! You can boil an ice cube, and then turn it back into an ice cube
While a substance changes phase, its temperature does not change
Much more energy is given off when water vapor condenses than when an equal mass of water freezes

12. Energy and Changes of Phase

13. Thermodynamics
Chapter 24

14. Absolute Zero
As the thermal motion approaches zero, the kinetic energy of the atoms approaches zero, and the temperature of the substance approaches a lower limit
Absolute Zero – no more energy can be extracted from a substance and no further lowering of its temperature is possible
Absolute zero corresponds with zero degrees on the Kelvin scale (0 K)
Unlike the Celsius scale, there are no negative numbers on the thermodynamic scale (Kelvin)

15. Absolute Zero

16. First Law of Thermodynamics
When the law of energy conservation is applied to thermal systems, we call it the first law of thermodynamics:
Whenever heat is added to a system, it transforms to an equal amount of some other form of energy
More specifically the first law states:
Heat added = (increase in internal energy) + (external work done by the system)
Adding heat is not the only way to increase the internal energy of a system
Changes in internal energy are equal to the work done on or by the system

17. Change in air temperature ~ pressure change
Adiabatic Processes
Adiabatic – the process of compression or expansion of a gas so that no heat enters or leaves a system
Adiabatic changes of volume can be achieved by performing the process rapidly so that heat has little time to enter or leave, or by thermally insulating a system from its surroundings
Adiabatic form of the first law:
Change in air temperature ~ pressure change
Adiabatic processes are happening all the time in the atmosphere and in your car

18. Adiabatic Cooling

19. Second Law of Thermodynamics
The second law of thermodynamics tells us the direction of heat flow in natural processes:
Heat will never of itself flow from a cold object to a hot object.
Heat only flows one way, from hot to cold

20. Heat Engines and the 2nd Law
Heat Engine – any device that changes internal energy into mechanical work
Mechanical work can be obtained only when heat flows from a high temperature to a low temperature
In every heat engine only some of the heat is transformed into work
Heat flows out of a high-temperature reservoir into a low-temperature reservoir
Every heat engine will (1) absorb heat from a reservoir of higher temperature, increasing its internal energy, (2) convert some of its energy into mechanical work, and (3) expel the remaining energy as heat into some lower-temperature reservoir (sink)

21. Heat Engines and the 2nd Law
When work is done by a heat engine running between two temperatures, T hot and T cold, only some of the input heat at T hot can be converted to work, and the rest is expelled as heat at T cold.
Carnot efficiency – the ideal efficiency of a heat engine:
Ideal efficiency = (T hot - T cold )/ T hot
The greater the temperature difference between the hot and cold reservoirs, the greater the efficiency
In practice , friction is always present in all heat engines, and efficiency is always less than ideal

22. Heat Engines

23. Order Tends to Disorder
Organized energy in the form of electricity that goes into electric lights degenerates to heat energy and has no further use.
The quality of energy is lowered with each transformation
Energy of an organized form tends to disorganized forms
Natural systems tend to proceed toward a state of greater disorder.
Disordered energy can only be transformed to ordered energy only at the expense of some organizational effort or work input

24. Entropy Entropy – the measure of the amount of disorder in a system
Whenever a physical system is allowed to distribute its energy freely, it always does so in a manner such that entropy increases while the available energy of the system for doing work decreases
Entropy will normally increase for physical systems
The first law of thermodynamics is a universal law of nature for which no exceptions have been observed, while the second law is a probability statement

25. Entropy

26. Assignment Read Chapters 23 & 24 (pg. 339-367)
Do Chapter 23 #26-47 (pg ); Appendix F #1-9 (pg )
Do Chapter 24 #30-43 (pg ); Appendix F #1-3 (pg. 682)