1. Chapter 21, Electric Charge, and electric Field

2. 19-5 Latent Heat Example 19-6: Determining a latent heat. The specific heat of liquid mercury is 140 J/kg·°C. When 1.0 kg of solid mercury at its melting point of -39°C is placed in a 0.50-kg aluminum calorimeter filled with 1.2 kg of water at 20.0°C, the mercury melts and the final temperature of the combination is found to be 16.5°C. What is the heat of fusion of mercury in J/kg?

3. Problem 20 20.(II) A 35-g ice cube at its melting point is dropped into an insulated container of liquid nitrogen. How much nitrogen evaporates if it is at its boiling point of 77 K and has a latent heat of vaporization of 200 kJ/kg? Assume for simplicity that the specific heat of ice is a constant and is equal to its value near its melting point.

4. Heat of fusion, L F : heat required to change 1.0 kg of material from solid to liquid Heat of vaporization, L V : heat required to change 1.0 kg of material from liquid to vapor 19-5 Latent Heat

5. The total heat required for a phase change depends on the total mass and the latent heat: 19-5 Latent Heat

6. Problem Solving: Calorimetry 1. Is the system isolated? Are all significant sources of energy transfer known or calculable? 2. Apply conservation of energy. 3. If no phase changes occur, the heat transferred will depend on the mass, specific heat, and temperature change. (continued) 19-5 Latent Heat

7. 4. If there are, or may be, phase changes, terms that depend on the mass and the latent heat may also be present. Determine or estimate what phase the final system will be in. 5. Make sure that each term is in the right place and that all the temperature changes are positive. 6. There is only one final temperature when the system reaches equilibrium. 7. Solve. 19-5 Latent Heat

8. The latent heat of vaporization is relevant for evaporation as well as boiling. The heat of vaporization of water rises slightly as the temperature decreases. On a molecular level, the heat added during a change of state does not increase the kinetic energy of individual molecules, but rather break the close bonds between them so the next phase can occur. 19-5 Latent Heat

9. 19-6 The First Law of Thermodynamics Example 19-7: Using the first law. 2500 J of heat is added to a system, and 1800 J of work is done on the system. What is the change in internal energy of the system?

10. Problem 34 34.(II) In an engine, an almost ideal gas is compressed ­adiabatically to half its volume. In doing so, 2850 J of work is done on the gas. (a) How much heat flows into or out of the gas? (b) What is the change in internal energy of the gas? (c) Does its temperature rise or fall?

11. The change in internal energy of a closed system will be equal to the energy added to the system minus the work done by the system on its surroundings. This is the law of conservation of energy, written in a form useful to systems involving heat transfer. 19-6 The First Law of Thermodynamics

12. The first law can be extended to include changes in mechanical energy—kinetic energy and potential energy: Example 19-8: Kinetic energy transformed to thermal energy. A 3.0-g bullet traveling at a speed of 400 m/s enters a tree and exits the other side with a speed of 200 m/s. Where did the bullet’s lost kinetic energy go, and what was the energy transferred?

13. 19-7 The First Law of Thermodynamics Applied; Calculating the Work The following is a simple summary of the various thermodynamic processes. and W=PdV

14. Charles Allison © 2000 Question Starting with a neutral rubber rod and piece of fur, you rub them together. If the rubber rod gets a charge of -7µC, what is the charge on the fur? A) There is not enough information to determine the answer B) -7 µC C) +7 µC D) -3.5 µC E) + 3.5 µC

15. Charles Allison © 2000 Question A metal sphere A has charge Q. Two other spheres, B and C, are identical to A except they have zero net charge. A touches B then the two spheres are separated. B touches C, then the two spheres are separated. What are the charges on each sphere. A) A=Q, B=0, C=0 B)A=Q/2, B=Q/2, C=Q/2 C) A=Q/3, B=Q/3, C=Q/3 D) A=Q/2, B=Q/4, C=Q/4 A B C

16. Charles Allison © 2000 21.1 Electric Charge, q or Q Charge comes in two types 1e = 1.6x10 -19 Coulombs (unit of charge) fundamental unit of charge Law of Conservation of Charge: The net charge on a closed system never changes. Charge can be transferred Charges are added algebraically +e-e

17. Charles Allison © 2000 21.1 Electric Charge Like charges Opposite charges Polarization +e -e repel attract 0e + + + - - - Charge separation in a neutral object

18. Charles Allison © 2000 Atom: Nucleus (small, massive, positive charge) Electron cloud (large, very low density, negative charge) 21 - 2 Electric Charge in the Atom

19. Charles Allison © 2000 Polar molecule: neutral overall, but charge not evenly distributed 21 - 2 Electric Charge in the Atom

20. Charles Allison © 2000 Ways to charge things 1) Charge Insulators by rubbing 2) Charge Conductors by contact 0e -e +e Charge transfers -e 0e -e

21. Charles Allison © 2000 21.3 Conductors and Insulators Conductors -Charges move easily -metals Insulators -Charges do NOT move easily -wood -glass -rubber

22. Charles Allison © 2000 Metal objects can be charged by conduction: 21-4 Induced Charge; the Electroscope

23. Charles Allison © 2000 They can also be charged by induction, either while connected to ground or not: 21-4 Induced Charge; the Electroscope

24. Charles Allison © 2000 Nonconductors won’t become charged by conduction or induction, but will experience charge separation: 21-4 Induced Charge; the Electroscope

25. Charles Allison © 2000 The electroscope can be used for detecting charge. 21-4 Induced Charge; the Electroscope

26. Charles Allison © 2000 The electroscope can be charged either by induction or by conduction. 21-4 Induced Charge; the Electroscope

27. Charles Allison © 2000 The charged electroscope can then be used to determine the sign of an unknown charge. 21-4 Induced Charge; the Electroscope

28. Charles Allison © 2000 Question What is the correct FBD for charge 3 where F 31 is the force on Q 3 due to Q 1 and F 32 is the force on Q 3 due to Q 2 ? a) b) c) d) +-+ Q 1 Q 2 Q 3 3 F 31 F 32 3 33 F 31 F 32 F 31 F 32 F 31 F 32 21-5 Coulomb’s Law