NAME : F.3C ( ) Marks: /50 Form 3 Physics Assessment on Heat Time allowed: 45 minutes Section A (34 marks) 1. An indoor swimming pool containing 2.5 x 10 6 kg of water uses 40 identical heaters to maintain the temperature of water so that it won t drop lower than 20 0 C. (Given specific heat capacity of water is 4200 Jkg -10 C -1 ) (a) On a day with temperature of 15 0 C, it takes 2 hours to heat the water before its temperature reaching 20 0 C for use. Pt = mcdt (i) Estimate the power of each heater. _ 40P (2 x 60 x 60) = 2.5 x 10 6 (4200)(5) _ P = 1.82 x 10 5 W (ii) The actual power is different from the value found in (i), explain and predict the difference. Actual power should be larger _ Heat loss to surroundings. _ (b) On average, the heaters are switched on for 30 minutes every 4 hours on one day. (i) Estimate the average rate of heat power loss to surroundings that day. _ 1.82 x 10 5 x 40 x 30 x 60 = P (4 x 60 x 60) _ P = 9.11 x 10 5 W (ii) If the cost of electricity for the heaters that day is $120, calculate the cost of electricity per kwh. _ (1.82 x 10 5 /1000) x 40 x 0.5 x (24/4) x C = 120 _ C = $5.5 x 10-3 per kwh. (iii) Suggest one method to reduce the electricity cost by reducing amount of electricity consumed. (1 mark) Use insulator to make the pool (c) In order to warm the water effectively, where should they install the heaters? Illustrate your answer by drawing convection current the position of one heater on the figure and explain your answers. Put the heater at the bottom. Heated water rises and convection current facilitates the uniform heating.
2. The figure shows a cooling curve of 100-g of melting wax. soli liquid Freezing solid 2 M (a) What is the melting point of the wax? (1 mark) 58 0 C (b) If the specific heat capacity of liquid wax is 500 Jkg -10 C -1, find the average rate of heat loss to surroundings in the first 2 min (liquid is cooling, no change of state) 0.1 (500) (80-60) = P (2 x 60) P = 8.33 W (c) Why the term average is used in part (b)? Explain clearly. The rate of heat loss decreases as temperature difference decreases Thus, it is not a constant and the value found is an average. (d) Use your answer in (b) to estimate the specific latent heat of fusion of wax. 8.33 (4 x 60) = 0.1 L L = 1.99 x 10 4 Jkg -1 (actual L should be smaller because rate is smaller at lower temperature) (e) Sketch the corresponding cooling curve on the above figure if the room temperature is lower. Temp difference increases, rate increases / freezing point constant 3. Consider a boy of mass 50 kg eats 0.5 kg of ice-cream at 0 0 C. The specific latent heat of fusion of ice-cream is 3.35 x 10 5 Jkg -1. The normal body temperature is 37 0 C. The specific heat capacity of water is 4200 Jkg -10 C -1 and the specific latent heat of vaporization of water is 2.26 x 10 6 Jkg -1. (a) Estimate the temperature drop of the boy after eating the ice-cream, taking the specific heat capacity of human the same as water. _ Heat loss by boy = Heat gained by ice cream _ 50 (4200)(37-T) = 0.5 (3.35 x 10 5 + 4200 T) T = 35.8 0 C dt = 1.2 0 C
(b) Is actual specific heat capacity of human higher or lower than that of water? Hence, deduce if the answer in (a) is larger or smaller than the actual value. Lower Value in (b) is smaller than actual value. (c) Besides eating ice-cream, sweating can also cause Evaporation cooling effect. Explain. The sweat evaporates from the body Takes away latent heat from the body (d) Calculate the amount of sweat evaporated on the body which cause the same cooling effect as in part (a). You may assume sweat contains mainly of water. Heat absorbed by sweat to evaporate = Heat loss by boy m [ 2.26 x 10 6 ] = 50 x 4200 x 1.2 m = 0.11 kg 4. Describe an experiment to find the specific latent heat of vaporization of water using the following apparatus. State clearly the measurements that you have to take and the instruments you need to make the measurements. Also, describe clearly how you can find the result from the data obtained. Suggest TWO improvements in the experiment. (6 marks) A beaker of water A top-pan balance An 1000-W immersion heater A stopwatch Put a beaker of water on a top-pan balance and insert the immersion heater into it. ½ M Switch on the heater and wait till the water boils. Record the mass of the beaker and water m i, using the top-pan balance ½ M Time using stopwatch at the same time and switch off the heater after 2 minutes ½ M Record the mass of the beaker of water m f again ½ M The specific latent heat = 1000 (2 x 60) / (m i m f ) Replace the beaker by a foam cup Replace the top-pan balance by an electronic balance Section B (16 marks) Write your answers of Section B in the spaces provided below. 1. C 2. C 3. B 4. 5. C A 6. B 7. A 8. B
1. In an experiment to measure the specific heat capacity of metal, which of the following methods can improve the accuracy of the results? (1) Wrap the metal block with some insulating materials. (2) Read the final temperature as soon as the heater is switched off. (3) Put an insulating tile beneath the metal block. A. (1) only B. (2) only C. (1) and (3) only D. (1), (2) and (3) 2. When a hot object is in contact with a cold one, which of the following will happen? (1) The internal energy of the hot object drops. (2) The rate of heat transfer to the cold object decreases. (3) The temperature increase of the cold object equals to the temperature decrease of the hot object. (different specific heat capacities) A. (1) only B. (3) only C. (1) and (2) only D. (1), (2) and (3) 3. The following table shows the melting points and boiling points of three substances L, M and N. Substance Melting point / C Boiling point / C L 660 2350 M 189 126 N 7 58 Which of them is/are in liquid (between melting and boiling point) state at 20 C? A L only B N only C L and M D L, M and N 4. When 1 kg of steam is mixed with 1 kg of water at 0 0 C, what will be resulted? A. water between 0 0 C and 100 0 C B. water at 100 0 C C. water and steam at 100 0 C D. water and steam lower than 100 0 C
5. The temperature time graph of a material is recorded as follows. temperature / C t 1 t 2 time / min Which of the following descriptions about the material during t 1 and t 2 is/are correct? (1) Its temperature remains unchanged. Change of state (2) Its state of matter remains unchanged. (3) Its internal energy remains unchanged. pe increasing A (1) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) 6. In an experiment to find the specific latent heat of fusion of ice, which of the following statements is/are correct? (1) If the control set up is not used, the result will be larger. M measured larger (2) If the mass is not measured after steady state is attained, the result will be smaller. E measured larger (3) Heat gained from surroundings will affect the result. E measured smaller A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only 7. Which of the following quantities are the same? (1) specific heat capacity of 1 kg of water at 0 0 C. (2) specific heat capacity of 2 kg of water at 100 0 C. (3) specific heat capacity of 1 kg of ice at 0 0 C. (4) specific heat capacity of 1 kg of steam at 100 0 C. A. (1) and (2) B. (1) and (3) C. (2) and (4) D. None of them
8. Two solid substances P and Q of equal mass are heated by two identical heaters until they boil. The temperatures of each substance at different times are recorded, as shown in the graph below. Which of the following statements is not correct? A The specific heat capacity of solid P is larger than that of solid Q. B The specific heat capacity of liquid P is larger than that of liquid Q. C The specific latent heat of fusion of P is larger than that of Q. D The boiling point of P is higher than that of Q. End of Test