Ch 5: Heat, Temperature and Thermodynamics

1. Ch 5: Heat, Temperature and Thermodynamics

2. Chapter 5 – Heat, Temperature and Thermodynamics The Three Basic Phases of Matter Microscopic behavior: Macroscopic result: Solid Atoms are tightly packed Material is non-compressible Atoms jostle about fixed positions Material does not flow

3. Chapter 5 – Heat, Temperature and Thermodynamics The Three Basic Phases of Matter Microscopic behavior: Macroscopic result: Liquid Atoms are tightly packed Material is non-compressible Atoms free to roam around Material does flow

4. Chapter 5 – Heat, Temperature and Thermodynamics The Three Basic Phases of Matter Microscopic behavior: Macroscopic result: Gas Atoms are widely separated Material is compressible Atoms free to roam around Material does flow

5. Chapter 5 – Heat, Temperature and Thermodynamics The Three Basic Phases of Matter Solid Liquid Gas Sequence of increasing molecule motion (and energy)

6. Chapter 5 – Heat, Temperature and Thermodynamics Internal Energy: The sum total of all the microscopic and random kinetic and potential energies of all the atoms in an object. The amount of internal energy depends on … … the amount of material has more internal energy than

7. Chapter 5 – Heat, Temperature and Thermodynamics Internal Energy: The sum total of all the microscopic and random kinetic and potential energies of all the atoms in an object. The amount of internal energy depends on … … the phase of the material has more internal energy than

8. Chapter 5 – Heat, Temperature and Thermodynamics Internal Energy: The sum total of all the microscopic and random kinetic and potential energies of all the atoms in an object. The amount of internal energy depends on … … the temperature has more internal energy than

9. Chapter 5 – Heat, Temperature and Thermodynamics Temperature: Temperature is a measure of average atom/molecule speeds. For Example: At 25 oF, air molecules have an average speed ~1080 mi/hr At 100 oF, air molecules have an average speed ~1160 mi/hr

10. Chapter 5 – Heat, Temperature and Thermodynamics Temperature: Temperature is a measure of average atom/molecule speeds. Temperature Measurement: Average molecule speeds are difficult to measure directly. Temperature is almost always measured indirectly (by one of it’s side effects). One such side effect is thermal expansion. Most materials expand when hotter and contract when cooler. Cool Iron bar

11. Chapter 5 – Heat, Temperature and Thermodynamics Temperature: Temperature is a measure of average atom/molecule speeds. Temperature Measurement: Average molecule speeds are difficult to measure directly. Temperature is almost always measured indirectly (by one of it’s side effects). One such side effect is thermal expansion. Most materials expand when hotter and contract when cooler. Hot Iron bar

12. Chapter 5 – Heat, Temperature and Thermodynamics Thermal expansion is a very small effect. For example: Steel expands 0.006% for every 10 oF From 20 oF to 100 oF: A 1.0 m steel bar expands 0.5 mm From 20 oF to 100 oF: A 500 ft steel bridge expands 3 in.

13. Chapter 5 – Heat, Temperature and Thermodynamics Thermal expansion is a very small effect. The effect is greatly exaggerated in a ‘liquid in glass’ thermometer.

14. Chapter 5 – Heat, Temperature and Thermodynamics Thermal expansion is a very small effect. Temperature Scales 212 oF 100 oC 32 oF 0 oC 9 5 F = C + 32 C = (F – 32) 5 9

15. Chapter 5 – Heat, Temperature and Thermodynamics Thermal expansion is a very small effect. Bi-metallic Strip or Spring Upon heating: metal 2 expands more than metal 1

16. Chapter 5 – Heat, Temperature and Thermodynamics Thermal expansion is a very small effect. Bi-metallic Strip or Spring Upon cooling: metal 2 contracts more than metal 1

17. Chapter 5 – Heat, Temperature and Thermodynamics How Cold Can It Get? Boiling pt for water 212 oF 100 oC Air molecules average 1260 mi/hr. 32 oF 0 oC Freezing pt for water -129 oF -89 oC Coldest naturally occurring on earth [Vostok Antarctica, July 1983] Air molecules average 880 mi/hr.

18. Chapter 5 – Heat, Temperature and Thermodynamics How Cold Can It Get? Boiling pt for water 212 oF 100 oC Freezing pt for water 32 oF 0 oC Coldest on earth -129 oF -89 oC Air starts to liquefy -321 oF -196 oC -346 oF -210 oC Air starts to freeze into a solid Molecule motion essentially stop -460 oF -273 oC

19. Chapter 5 – Heat, Temperature and Thermodynamics How Cold Can It Get? Boiling pt for water 212 oF 100 oC Freezing pt for water 273 K 32 oF 0 oC Coldest on earth -129 oF -89 oC Air starts to liquefy 77 K -321 oF -196 oC Air starts to freeze 63 K -346 oF -210 oC Molecule motion essentially stop 0 K -460 oF -273 oC Kelvin scale: K = C + 273 This is the coldest possible temperature. Referred to as ‘absolute zero’

20. Chapter 5 – Heat, Temperature and Thermodynamics Thermodynamics Heat plays a special role in the conversion of one form of energy into another. Other forms of energy (kinetic, chemical, potential, etc.) can be converted 100% into heat. There is no process whose sole effect is to convert heat 100% into other forms of energy. 2nd Law of Thermodynamics

21. Chapter 5 – Heat, Temperature and Thermodynamics 2nd Law of Thermodynamics Related to/caused by the difficulty in organizing a vast number of random motions. This is the fundamental reason that physical processes look ‘odd’ when viewed backward in time. [All other basic law’s of physics don’t ‘care’ which way time runs]

22. Chapter 5 – Heat, Temperature and Thermodynamics Thermodynamics Any machine or process that does convert heat into other forms of energy is referred to as a ‘heat engine’. Any heat engine has a maximum upper limit to it’s efficiency given by: ( Thot – Tcold ) Max. Eff. = x 100% Thot Thot & Tcold must be in K

23. Chapter 5 – Heat, Temperature and Thermodynamics Thermodynamics Any heat engine has a maximum upper limit to it’s efficiency given by: ( Thot – Tcold ) Max. Eff. = x 100% Thot Note that the efficiency is always less than 100%. Note that the efficiency = 0%, if Thot = Tcold. Real efficiencies are usually much lower.

24. Chapter 5 – Heat, Temperature and Thermodynamics Example: Find Max. Eff. of a Steam Generator Thot = 600 oC [ 870 K ] Tcold = 27 oC [ 300 K ] ( Thot – Tcold ) Max. Eff. = x 100% Thot ( 870 K – 300 K) Max. Eff. = x 100% 870 K 570 K Max. Eff. = x 100% = 0.65 x 100% = 65% 870 K