Comparing the actual value and the experimental value on heat. By conservation of energy

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1 Topic: Heat 1. Temperature and thermometers a. Temperature: - measure degree of hotness. -measure the average kinetic energy of molecules in random motions. b. Fixed points: -Lower fixed point: temperature of pure melting ice at normal atmospheric pressure Upper fixed point: temperature of steam over pure boiling water at normal atmospheric pressure Factors affecting fixed points: -impurities -> boiling point, melting point -atmospheric pressure: larger atm -> boiling point, melting point 2. Heat and internal energy a. Heat: refers to the energy transfer (when thermal contact) between two objects - thermal equilibrium: no net flow of energy between two objects ie. Heat from A to B = heat from B to A b. Internal energy = total kinetic energy + total intermolecular potential energy c. Heat capacity and specific heat capacity i. Heat capacity ii. Specific heat capacity iii. d. * Mixing process Comparing the actual value and the experimental value on heat capacity/specific heat capacity: consider one of the parameters By conservation of energy m h c h ( T h T eq ) = m c c c ( T eq T c ) 3. Change of state a. latent heat: energy given out during the change of state--- no temperature change Reason: the latent heat released results in the decrease of PE between particles, average KE remains unchanged no temperature change. Cooling curve:

2 i. specific latent heat of fusion (l f ) /specific latent heat of vaporization (l v ) b. Evaporation i. Microscopic interpretation -takes place on the surface only -molecules at the surface absorb energy from the surroundings -remaining liquid becomes cooler ii. Factors affecting the rate of evaporation -air circulation -temperature -surface area -humidity 4. Transfer process a. Conduction: transfer of energy by collisions of molecules of (fast moving) electrons. Factors: i. temperature difference and distance between the two ends ii. presence of free electrons iii. cross-section area of conductor b. Convection: transfer energy by movement of fluid (liquid or gas) ability to set up a convection current. c. Radiation: energy transfer by EM wave (only process that does not need a medium of transfer). Black / dull objects: good radiation emitters and absorbers Shiny / silvery objects: poor radiation emitters and absorbers (by reflecting radiation)

3 Q1 Two solids A and B of the same mass are placed in hot bath and their temperatures plotted against time shown in above. a) (i) Which of the solids A and B has a higher melting point? (ii)the rate of heat supplied to A and B are the same. Which of the solids A and B has a higher (1) Specific heat capacity, (2) Specific latent heat of fusion? Explain your answer briefly. b) A student carried out an experiment to find the specific latent heat of fusion of ice by the method of mixture. He placed 0.02kg of ice at 0 into a plastic cup containing 0.05kg of water at 20. (i) Estimate the amount of heat required for the ice to change into water at 0?(Specific latent heat of fusion of ice = J kg -1 ) (ii) What is the amount of heat supplied by the water in the cup as it is cooled from 20 to 0? (specific heat capacity of water = 4200 J kg -1-1 ) (iii) What is the final temperature of the mixture? Explain briefly. (iv) The student found that he could not obtain the value for the specific latent heat of fusion of ice from this experiment. Explain the reason for this. (v) State how he can improve the experiment.

4 Q2 Explain the following cases: a) A test tube is filled with water and several cubes of ice are fixed at the bottom. The tube is then heated near at the top of the tube. When the water near the top boils, the ice cubes have not melted completely. b) When a pot of vegetables soup is heated above a stove, the tiny pieces of vegetables flow upwards and downwards in a circular path. c) Air- conditioners are usually installed at higher positions on the walls. Q3 The figure below shows a kind of pot. It is usually made of copper. In the central part of the pot, there is a chimney that helps the burning of the fuel below the pot. a) Briefly describe the advantage of using copper to make the hot pot in the heat transfer process. b) Briefly describe how the chimney can help in the burning process of the fuel. c) Will the chimney be useful when we use an electric hot plate or an induction heater? Briefly describe. Q4 The figure below shows a microwave oven. The inside surface of its door has a metal grill with many small holes. a) It is known that the oven emitted microwave of frequency 2.45 GHz. Find the wavelength of the microwave.

5 b) What is the function of the metal grill? c) Explain why metal containers should not be put into a microwave oven. Q5 When a person runs, he generates heat energy. Assume that the mass of the person is 65kg, his specific heat capacity is 3800 J kg -1-1 and the heat power he generates is 1300 W. a) Assuming there is no energy loss in the surrounding, find the temperature increase of the person after running for 40 minutes. b) In the reality, the person continuously releases heat to the surrounding. Discuss one mechanism (other than evaporation) for the person to release heat to the surroundings. Q6 State and explain whether each of the following statements is correct. a) When one end of a spoon is immersed in a glass of hot water, the other end will be heated up due to conduction only. b) When heat energy is transferred within a solid substance by means of conduction, the molecules do not move from one place to another. c) Metal is generally a good conductor of heat. d) Air is a good conductor of heat. e) Ice is a better conductor of heat than water. Q7 Tiffany likes having instant cup noodles as her lunch. She adds 300g of hot water at 100 into a cup containing 80 g of noodles at 25. Tiffany find that the temperature of the cooked noodles is about 82. a) Neglecting the heat capacity of the polystyrene cup, what is the specific heat capacity of the instant cup noodles? Given that the specific heat capacity of water is 4200 J kg b) Suggest why the cup is made of polystyrene. Q8 Ice cubes of mass 0.2kg at -5 are dropped into a glass which contains 0.3kg lemon tea at 80. What is the final temperature of the mixture? Neglect the heat absorbed by the lemon. Given Specific heat capacity of lemon tea and water =4200 J kg -1-1 Specific heat capacity of ice = 2100 J kg -1-1 Specific latent heat of fusion of ice = J kg -1

6 Q9 A student carries out an experiment to determine the specific latent heat of fusion of ice. The following shows his results: Mass of the beaker and water in experimental set = 0.065kg Mass of the beaker and water in the control = 0.019kg Final joulemeter reading = J Initial joulemeter reading = J a) What is the temperature of the water inside the beaker? b) Calculate the specific latent heat of fusion of ice according to the above data. c) Determine whether the value obtained in (b) is larger or smaller than the expected value. Explain the difference. d) What is the purpose of setting a control? e) Explain why would happen to the result if ice of -2 was used instead. f) What is the purpose of using crushed ice? Q10 Eric determined the specific latent heat of vaporization of water in the laboratory. The following is the result obtained: Initial joulemeter reading = J Final joulemeter reading = J Initial mass of boiling water = 8.50 kg Final mass of boiling water = 8.41 kg a) Calculate the specific latent heat of vaporization from the result obtained. b) The true value of specific latent heat of vaporization is 2.26 x 10 6 J kg -1. Suggest a reason that led to the difference when compared with (a). c) Terry suggests that the accuracy can be improved by covering the polystyrene cup with a lid. Do you agree? Explain.

7 Q11 (HKCEE 2007) Karen puts 0.12 kf of water at room temperature T 1 into the freezer of a refrigerator to make ice cubes. The cooling curve of the water is shown in Figure 6. Given: specific latent heat of fusion of ice = 3.34 x 10 5 J kg -1 a) State the physical meaning of temperature T 2. b) Find the latent heat released in the above process. c) If an ice cube from the freezer is placed at room temperature T 1, sketch a graph to show the expected change in temperature of the ice cube in Figure. 7. Q12 (HKCEE 2004) Mary wants to conduct an experiment to estimate the useful output of a microwave oven. She is provided with the apparatus and materials below: A beaker of 0.2kg water, thermometer, stop watch a) Describe how Mary should conduct the experiment. Specify all measurements Mary has to take and write down an equation for calculating the useful output power.

8 b) The value obtained by Mary is found to be smaller than the rated power of the oven. Suggest one possible reason to account for this difference. c) Mary uses the oven to defrost a piece of meat of mass 0.2 kg. The meat is taken from a freezer, the temperature of which is maintained at -20. Assume that 70% of the mass of the meat is made up of water. Given: Specific heat capacity of the frozen meat = 1700 J kg -1-1 Specific latent heat of fusion of ice = 3.34 x 10 5 J kg -1 (i) Find the energy required to raise the temperature of the meat from -20 to 0. (ii) Find the energy required to change the ice in the meat at 0 to water. (iii) Sketch a graph to show the variation of the temperature of the meat with time during the defrosting process. Q13 (HKCEE 2005) William makes a glass of hot tea. After a while, he adds some ice cubes into the tea. William uses a temperature sensor to measure the temperature of the tea. Figure 4 shows the temperature-time graph obtained. a) William stirs the tea throughout the experiment. Why does he need to do so? b) P, Q, R and S are four points on the graph. Stat the point which corresponds to each of the following: (i) The instant at which the ice cubes are added. (ii) The instant at which all the ice cubes melt. c) Explain why the temperature of the tea increases from R to S. d) Estimate the temperature of the surroundings.