An introduction to Thermodynamics

1. Chapter 4 Heat and Temperature An introduction to Thermodynamics

2. Usually attractive ; materials cling together Cohesion Attractive forces between “like” molecules Adhesion Attractive forces between “unlike” molecules Interactions can also be repulsive Water beading on wax ; Think about “Gortex”. Molecules Interact

3. Fig 4.3 Table 4.1 Comparison of Characteristics Three Phases of Matter Fluids:Abilityto Flow.

4. Temperature and Heatare different concepts Temperature (T) • A measure of the average kinetic energy of the molecules • Three Scales ( °F, °C, K )(See Fig 4.7) Heat (Q) • Measure of the internal energy (U) that has been absorbed or transferred from one body to another. Fig 4.10

5. Heat: “Energy in transit” Fig 4.9 Total PE + KE External energy versus Internal energy (U) • External energy can be transferred to Internal energy • Result: a temperature increase • See “Friction” in Fig 4.8.

6. Heating methods: gain of internal energy Heat (Q) (page 100) “Heating” = increasing internal energy “Cooling” = decreasing internal energy Think: “Process” ; “Direction of Flow” • Temperature difference between objects • Naturally from “higher” to “lower” • Energy-form conversion (thru work) • Mechanical, radiant, electrical.

7. Metric units calorie (cal) kilocalorie (kcal) Nutrition: Calorie, (Cal) English system British thermal unit (Btu) Measures of Heat(know the definitions) Mechanical equivalence (energy-form conversion) 4.184 J = 1 cal ; 4184 J = 1 kcal Example 4.4

8. Mechanical work and heat are different forms of the same thing

9. Table 4.2 Variables involved in finding Q • Mass (m) • Specific Heat (c) • Temperature change (ΔT) Fig 4.12

10. A Key Point! Calculating Amount of Heat

11. Determining Specific Heat

12. Heat flow(energy transfer processes) • Energy (heat) transfers take place because of a temperature difference by: • Conduction • Convection • Radiation

13. Conduction • Heat flowing through matter • Occurs easily in solids • Mechanism • Molecule-to-molecule contact • Warmer object to colder object • Poor conductors are good insulators (Air, styrofoam, wool, …) Fig 4.13

14. Conductivities: Table 4.3 on p 104

15. Insulation and R-Value

16. Convection: a “Vertical Current” Fig 4.16

17. Radiation(Radiant Energy)

18. Radiation(Radiant Energy) • Energy associated with EM waves • Emitted, absorbed, or reflected (scattered) • Can operate through space (or a vacuum) • Does not require matter for transfer

19. Fig 4.17 Phase Changes • Heat that is not associated with a Temp change • Energy Absorbed or Released • Latent Heat • “Hidden” energy • “Internal PE”

20. Heat, Temperature, and Phase Changes Fig 4.20 Must determine Q for: Changes in Temperature Changes in Phase

21. Refer to Table 4.4 Example 4.7: Energy and Making Ice

22. Example 4.7 Solution

23. Evaporation and Condensation • Individual molecules can change phase any time • Evaporation: liquid to gas (vapor) phase change • Higher energy molecules near the liquid surface can escape • Increases if? • Condensation: gas (vapor) to liquid phase change • Gas molecules near the surface lose KE to liquid molecules and return • Primarily occurs through?

24. The study of heat and its relationship to mechanical and other forms of energy Thermodynamic analysis includes: System and the Surroundings (everything else) Internal energy (U) Thermodynamics

25. The First Law of Thermodynamics • Energy supplied to a thermodynamic system in the form of heat, minus the work done by the system, is equal to the change in internal energy • Application of the Law of Conservation of Energy • Internal Energy in this case.

26. Equation 4.8

27. Heat naturally flows from a warm object to a cold object Another way to state it: It is impossible to convert heat completely into mechanical energy. The Second Law of Thermodynamics

28. Next: Chapter 5Waves and Vibrations