Chapter 18: THERMODYNAMICS

1. EQ: What does the first law of thermodynamics state? Chapter 18: THERMODYNAMICS

2. Absolute Zero There is no upper limit to temperature There is a lower limit: absolute zero. Absolute zero • Lowest limit of temperature: 0 K or -273.15 ºC • Molecules have lost all available kinetic energy.

3. Think About It What happens to a substance’s temperature as the motion of its atoms (kinetic energy) approaches zero?

4. Internal Energy • Internal (thermal) Energy is the grand total of all energies (kinetic and potential) inside a substance. • Kinetic energy: moving particles • Potential energy: stored energy in the bonds of the atoms

5. System • A system is a well-defined group of atoms, molecules, particles, or objects; • The system may be the steam in a steam engine, • the whole Earth’s atmosphere, • or even the body of a living creature. It is important to define what is contained within the system as well as what is outside of it.

6. The First Law of Thermodynamics ΔU = Q ± W change in = heat added ± work done on/by internal energy to the system the system Whenever heat (energy) is added to a system, it transforms to an equal amount of some other form of energy

7. Adiabatic Processes • Compressing or expanding a gaswhile no heat enters or leaves the system 0 (zero) ΔU = Q ± W change in = heat added ± work done on (+)/by(-) internal energy to the system the system so: ΔU = ± W

8. Adiabatic Processes Heat added to system= 0, so Increase in internal energy = work done on system Decrease in internal energy = work done by system Example: When we compress air using a bicycle pump, i.e., when we do work on the system, we heat the air up, i.e., increase its internal energy.

9. Meteorology and the First Law • Thermodynamics is useful to meteorologists when analyzing weather. • The first law of thermodynamics is expressed as: Air temperature rises as heat is added or as pressure is increased. • Heat may be added as • incoming solar radiation. • radiation back from Earth. • moisture condensation. • contact with ground.

10. Meteorology and the First Law • In the adiabatic form (i.e., when no heat is added), the first law of thermodynamics becomes: Air temperature rises (or falls) as pressure increases (or decreases). • Adiabatic processes in the atmosphere occur in large parts of the air, called parcels. • Parcels are large enough that outside air doesn’t appreciably mix with the air inside them. • They behave as if they are enclosed in giant, tissue-light garment bags.

11. Meteorology and the First Law • As parcels of air rise, they experience lower pressure and so they expand. • The expanding air cools down 10°C for every 1-km rise in altitude. • Air continues to rise and expand as long as it has a higher temperature than its surroundings. • When it gets cooler than the surroundings, it sinks

12. Meteorology and the First Law • As parcels of air rise, they experience lower pressure and cool. • As parcels of air drop, they experience higher pressure and heat up.

13. Temperature Inversion • When cooler air occurs at an altitude lower than warmer air, it’s called a temperature inversion. • If rising warm air is denser than upper layers of warm air, it will no longer rise. Example: Smog in LA is trapped by hot air from the desert coming over the mountains. Example: Smoke from a campfire sometimes may not rise.

14. EQ: What does the second law of thermodynamics state? Chapter 18: THERMODYNAMICS

15. Second Law of Thermodynamics Heat itself neverspontaneously flows from a cold object to a hot substance. Example: • In summer, heat flows from the hot air outside into the cooler interior. • In winter, heat flows from the warm inside to the cold exterior. Heat can flow from cold to hot only when work is done on the system or by adding energy from another source. • Example: heat pumps, air conditioners

16. Heat Engine • A heat engine is any device that converts internal energy into mechanical work. • In every heat engine, only some of the heat can be transformed into work.

17. Heat Engine • Every heat engine has • a reservoir of heat at a high temperature. • a sink at lower temperature. • Every heat engine • gathers heat from the reservoir at high temperature. • converts some of this heat into mechanical work. • expels the rest of the heat to the sink at lower temperature.

18. T - T = h o t c o l d I d e a l e f f i c i e n c y T h o t Second Law of Thermodynamics • The ideal (i.e., maximum possible) efficiency of a heat engine was determined by Carnot. • It depends upon the temperature of the hot reservoir (Thot) and the cold sink (Tcold). where, Thot and Tcold are in kelvin. • In real heat engines, the efficiency is actually less than ideal, due to friction.

19. Order Tends to Disorder Restatement of the second law of thermodynamics In natural processes, high-quality energy tends to transform into lower-quality energy—order tends to disorder. • Processes in nature moving from disorder to order do not occur without external assistance (work).

20. Order Tends to Disorder The first law of thermodynamics states that energy can be neither created nor destroyed. The second law adds that whenever energy transforms, some of it degenerates into waste heat, unavailable to do work. Another way to say this is that organized, usable energy degenerates into disorganized, nonusable energy. It is then unavailable for doing the same work again.

21. Order Tends to Disorder Push a heavy crate across a rough floor and all your work will go into heating the floor and crate. Work against friction turns into disorganized energy.

22. Order Tends to Disorder Organized energy in the form of electricity that goes into electric lights in homes and office buildings degenerates to heat energy. The electrical energy in the lamps, even the part that briefly exists in the form of light, turns into heat energy. This energy is degenerated and has no further use. The quality of energy is lowered with each transformation. Organized energy tends to disorganized forms.

23. Order Tends to Disorder Imagine that in a corner of a room sits a closed jar filled with argon gas atoms. When the lid is removed, the argon atoms move in haphazard directions, eventually mixing with the air molecules in the room. • The system moves from a more ordered state (argon atoms concentrated in the jar) to a more disordered state (argon atoms spread evenly throughout the room). • The argon atoms do not spontaneously move back into the jar to return to the more ordered containment. • With the number of ways the argon atoms can randomly move, the chance of returning to an ordered state is practically zero.

24. Order Tends to Disorder • Disordered energy can be changed to ordered energy only at the expense of work input. • Plants can assemble sugar molecules from less organized carbon dioxide and water molecules only by using energy from sunlight.

25. Entropy The measure of the amount of disorder in a system • If disorder increases, then entropy increases. • If work is put into the system, then entropy decreases

26. Entropy This run-down house demonstrates entropy. Without continual maintenance, the house will eventually fall apart.

27. Entropy Entropy normally increases in physical systems. However, when there is work input, as in living organisms, entropy decreases. All living things extract energy from their surroundings and use it to increase their own organization. This order is maintained by increasing entropy elsewhere.