Understanding Thermodynamics: Laws, Energy Conservation, and Entropy

1. Chapter 24 - thermodynamics

2. 24.1 – absolute zero • No upper limit of temperature • Since atomic motion determines temp, there is a lower limit – absolute zero • No atomic motion, no further energy could be extracted • -273 °C = 0 kelvin = 0 K • No negative numbers in kelvin scale

3. 24.2 – First law of thermodynamics • In past, heat was considered a fluid – a caloric • James Joule showed it was just energy • First law – heat added to system can 1. increase internal energy or 2. do work or 3. both • System = a collection of atoms, molecules or equipment • ΔU = Q + W • First Law = Conservation of Energy

4. Work - the system expanding or contracting - can change the internal energy as well • Work done to the system, ↑ internal energy • Work done by system, ↓ internal energy • Some work can generate heat and would have to be taken into account

5. 24.3 – adiabatic process • Gases are compressed or expanded so no heat is exchanged • Achieved in two ways: volume changes very quickly (a car engine) or system is insulated • Work goes entirely into/out of internal energy (↑or↓ temp), none gets leaked away (from system) as heat

6. Air temperature (internal energy) can be changed by: • Heat exchange – solar/terrestrial radiation, conduction with ground, or phase changes • Pressure changes • ΔT αΔP

7. In our atmosphere, as air rises, pressure is reduced, causing air to expand and cool • Adiabatic expansion, no heat is exchanged • The air (system) does work on surroundings (↑ volume), losing internal energy (↓ temperature) • About 10°C per 1 km

8. As air rises, water vapor condenses and it rains • The reverse happens as well – air warms as it descends and becomes dry • Called a chinook – from Rocky Mtns into Great Plains • Why warm and dry east of Sierras

9. 24.4 – second and third laws of thermodynamics • Second law – heat always flows from hot object to cooler one • First law says nothing about direction, as long as energy is conserved • Third Law – no system can reach absolute zero

10. 24.5 – heat engines & the 2nd law • Heat engine – a device that converts heat energy into work • Steam, internal combustion • Heat must flow from high temp. source, Thot(reservoir) to a low temp. sink, Tcold • In process, some energy can be extracted and used to do work

11. Heat energy is absorbed by engine, ↑ internal energy • Some of this is used to do work • The rest must be expelled • No heat engine can convert all heat energy into work – another statement of 2nd law • There is an upper limit to how efficient a heat engine can be • Called Carnot efficiency, only dependent upon high/low temps

12. In calcs. temps. must be in kelvins • The greater the temp difference, the higher efficiency • Why engines must run so hot

13. A steam turbine is a heat engine • Hot steam exerts more pressure on the “front” side of blades than the cooled steam on the “back” side does • Therefore, it does work • But, there must be ejected cooler steam • Otherwise, the turbine would just continue to warm until Thot= Tcold, no pressure diff.

14. 24.6 – order tends to disorder • All heat engines produce waste heat from organized fuel • The waste heat has less ability to do work – it degenerates • The energy is less organized, it has more random motion • All natural systems proceed to greater disorder • Nature proceed to what is most likely to happen • Work must be done to reorganize energy

15. 24.7 - entropy • A measure of disorder, > disorder = > entropy • Nature always proceeds to greater entropy • A system allowed to distribute its energy, ↑ entropy, leaving less energy available to do work • It’s possible to decrease entropy for one system, but at the expense of > ↑ in another