AP Physics B Lecture Notes

1. Energy in Thermal Processes AP Physics B Lecture Notes

2. Energy in Thermal Processes Topics 11-01 Heat and Internal Energy 11-02 Specific Heat 11-03 Calorimetry 11-04 Latent Heat and Phase Change 11-05 Energy Transfer

3. Heat and Internal Energy Heat is random thermal Energy Unit of heat: calorie (cal) 1 cal is the amount of heat necessary to raise the temperature of 1 g of water by 1 Celsius degree. 1 kcal is the amount of heat necessary to raise the temperature of 1 kg of water by 1 Celsius degree.

4. Heat and Internal Energy The apparatus below is used to determine the mechanical equivalent of heat: Heat is a form of energy and can be equated to mechanical energy.

5. Heat and Internal Energy Definition of heat: Heat is thermal energy transferred from one object to another because of a difference in temperature. The sum total of all the energy of all the molecules in a substance is its internal (or thermal) energy. Temperature: measures molecules’ average kinetic energy Internal energy: total energy of all molecules Heat: transfer of energy due to difference in temperature

6. Energy in Thermal Processes 11-01 The amount of heat necessary to raise the temperature of 1 gram of water by 1°C is referred to as the (A) calorie. (B) kilocalorie. (C) British thermal unit. (D) joule.

7. Heat and Internal Energy Internal energy of an ideal (monatomic) gas: kinetic energy in terms of the temperature

8. Energy in Thermal Processes 11-02 The measure of the average kinetic energy of individual molecules is referred to as (A) internal energy. (B) thermal energy. (C) temperature. (D) heat.

9. Energy in Thermal Processes 11-03 The internal energy of an ideal gas depends on (A) its volume. (B) its pressure. (C) its temperature. (D) all of the above

10. Energy in Thermal Processes 11-04 An ideal gas with internal energy U at 200°C is heated to 400°C. Its internal energy then will be (A) still U. (B) 2 U. (C) 1.4 U. (D) 1.2 U.

11. Specific Heat (c) Specific heat of the material (Q) Thermal energy added (DT) Change in temperature (m) Mass of the object m

12. Specific Heat

13. Energy in Thermal Processes 11-05 Water has a higher specific heat capacity than iron. Now, consider equal masses of water and iron that are initially in thermal equilibrium. The same amount of heat, 30 calories, is added to each. Which statement is true? (A) They remain in thermal equilibrium. (B) They are no longer in thermal equilibrium; the iron is warmer. (C) They are no longer in thermal equilibrium; the water is warmer. (D) It is impossible to say without knowing the exact mass involved and the exact specific heat capacities.

14. Problem A 50 g piece of cadmium is at 20 oC. If 400 J of heat is added to the cadmium, what is its final temperature

15. Problem A 100 g lead bullet traveling at 300 m/s is stopped by a large tree. Half the kinetic energy of the bullet is transformed into heat energy and remains with the the bullet while the other half is transmitted to the tree. What is the increase in temperature of the bullet?

16. Problem A 3.0 kg block of iron is dropped from rest from the top of a cliff. When the block hits the ground it is observed that its temperature increases by 0.50 oC. Assume that all the potential energy is used to heat the block. How high is the cliff?

17. Problem A 1.5 kg copper block is given an initial speed of 30 m/s on a rough horizontal surface Because of friction, the block finally comes to rest. If the block absorbs 85% of its initial kinetic energy in the form of heat, Calculate its increase in temperature?

18. Calorimetry m1 m2 TH TL Conservation of thermal energy: Final Temperature:

19. Calorimetry (Problem) A 0.40 kg iron horseshoe that is initially at 500 oC is dropped into a bucket containing 20 kg of water at 22 oC. What is the final equilibrium temperature? Neglect any heat transfer to for from the surroundings.

20. Calorimetry (Problem) A 200 g block of copper at a temperature of 90 oC is dropped into 400 g of water at 27 oC. The water is contained in a 300 g glass container. What is the final temperature of the mixture

21. Latent Heat and Phase Change (m) Mass of the object (L) Latent heat of the fusion or vaporization (Q) Thermal energy added Energy is required for a material to change phase, even though its temperature is not changing.

22. Latent Heat and Phase Change Heat of fusion, Lf: heat required to change 1.0 kg of material from solid to liquid Heat of vaporization, Lv: heat required to change 1.0kg of material from liquid to vapor

23. Latent Heat and Phase Change Q2= mLf Q4= mLv 3.33 x 105 J 22.6 x 105 J 1 kg Ice -50 oC 150 100 50 0 -50 Q1= mcIDt Q3= mcWDt Q5= mcSDt 1.05 x 105 J 4.19 x 105 J 1.01 x 105 J

24. Latent Heat and Phase Change Heat required to convert 1 kg of ice at -50 oC to steam at 150 oC Q1 = 1.05 x 105 J Q2 = 3.33 x 105 J Q3 = 4.19 x 105 J Q4 = 22.6 x 105 J Q5 = 1.01 x 105 J 3.22 x 106 J

25. Latent Heat and Phase Change (Problem) A large block of ice at 0 oC has a hole chipped in it, and 400 g of aluminum pellets at a temperature of 30 oC are poured into the hole. How much of the ice melts?

26. Energy Transfer x Heat conduction can be visualized as occurring through molecular collisions. The heat flow per unit time is given by:

27. Energy in Thermal Processes 11-06 Equal masses of water at 20°C and 80°C are mixed. What is the final temperature of the mixture? (A) 40°C (B) 50°C (C) 60°C (D) 70°C

28. Energy Transfer The constant k is called the thermal conductivity. Materials with large k are called conductors; those with small k are called insulators.

29. Energy Transfer (Problem) Problem: A window has a glass surface of 1.6 x 103 cm2 and a thickness of 3.0 mm. Find the rate of heat transfer by conduction through this pane when the temperature of the inside surface of the glass is 20 oC and the outside temperature is 40 oC.

30. Energy Transfer (Problem) Problem: A glass window pane has an area of 3.0 m2 and a thickness of 0.60 cm. If the temperature difference between its faces is 25 oC, how much heat flows through the window per hour?

31. Energy Transfer Building materials are measured using R−values rather than thermal conductivity: Where, x is the thickness of the material.

32. Energy in Thermal Processes 11-07 The heat required to change a substance from the solid to the liquid state is referred to as the (A) heat of fusion. (B) heat of vaporization. (C) heat of melting. (D) heat of freezing.

33. Energy in Thermal Processes 11-08 When a liquid freezes (A) the temperature of the substance increases. (B) the temperature of the substance decreases. (C) heat energy leaves the substance. (D) heat energy enters the substance.

34. Energy in Thermal Processes 11-09 Phase changes occur (A) as the temperature decreases. (B) as the temperature increases. (C) as the temperature remains the same. (D) all of the above

35. Energy in Thermal Processes 11-10 By what primary heat transfer mechanism does one end of an iron bar become hot when the other end is placed in a flame? (A) natural convection (B) conduction (C) radiation (D) forced convection

36. Energy in Thermal Processes 11-11 If you double the thickness of a wall built from a homogeneous material, the rate of heat loss for a given temperature difference across the thickness will (A) become one-half its original value. (B) also double. (C) become one-fourth its original value. (D) none of the above

37. Energy Transfer Summary Internal energy U refers to the total energy of all molecules in an object. For an ideal monatomic gas, Heat is the transfer of energy from one object to another due to a temperature difference. Heat can be measured in joules or in calories. Specific heat of a substance is the energy required to change the temperature of a fixed amount of matter by 1° C.

38. Energy Transfer Summary In an isolated system, heat gained by one part of the system must be lost by another. Calorimetry measures heat exchange quantitatively. Energy in involved in phase changes even though the temperature does not change. Heat of fusion: amount of energy required to melt 1 kg of material. Heat of vaporization: amount of energy required to change 1 kg of material from liquid to vapor.

39. Energy Transfer Summary Heat transfer takes place by conduction, convection, and radiation. In conduction, energy is transferred through the collisions of molecules in the substance.

40. END