NAME : F.5 ( ) Marks: /70 FORM FOUR PHYSICS REVISION TEST on HEAT Allowed: 70 minutes This paper consists of two sections. Section A (50 marks) consists of the structure-type questions, and Section B (20 marks) consists of multiple choice questions. Section A (50 marks) 1. A gas kept in a container with fixed shape and size is heated. (a) State the change, if any, of (i) the volume of gas (1 mark) (b) Unchanged (1 M) (ii) the total kinetic energy of the gas molecules (1 mark) Increased (1 M) (iii) the total potential energy of the gas molecules (1 mark) Unchanged (1 M) The pressure of the gas is used as thermometric property to measure temperature. The method of two fixed points is used to calibrate the thermometer. The gas thermometer is placed at ice point and then at 50 0 C, the readings are 1.33 and 1.45 units respectively. (i) What temperature does ice point mean? How can it be obtained? (ii) Ice point is 0 0 C () Put sufficient ice cubes into water (1 M) and wait for equilibrium (both exist) (1 M) If the maximum gas pressure the thermometer can withstand is 2.00 units, calculate the maximum temperature that the thermometer can measure. T 0 2 1.33 = 50 0 1.45 1.33 (1 M) T = 280 0 C (1 M) (iii) State the major assumption made in your calculations in (ii). (1 mark) Pressure varies linearly with temperature (1 M) (iv) Find the sensitivity of the thermometer. (Hint: Sensitivity is the change in reading per unit change of temperature.) 1.45 1.33 50 (1 M) = 2.4 x 10-3 unit 0 C -1 (1 M) (v) State one method to increase the sensitivity of the thermometer. (1 mark) larger mass of gas etc. (1 M) (vi) State the thermometer used in measuring the temperature of a furnace. (1 mark) Thermocouple (1 M) 1
2. A student performed an experiment to measure the specific heat capacity of a liquid. She used an immersion heater with specification shown in figure (a) to heat up 2 kg of liquid. Figure (b) shows the results obtained. Heater 1000 220V, 1000W, 50Hz Max 5A Figure (a) Initial temperature: 25 0 C recorded at t = 3 min: 80 0 C recorded at t = 4 min: 95 0 C Figure (b) (a) Use the result for the first 3-minute to find the specific heat capacity of the liquid. 1000 (3 x 60) = 2 (c) (80-25) (1 M) (b) c = 1640 J kg -10 C -1 (1 M) Suggest two reasons why the results obtained from data using t = 3 min and t = 4 min differ. Heat lost to surroundings at higher temperature is more serious. (1 M) (c) Rate of heat transfer is slower at higher temperature etc. (1 M) In order to obtain more accurate results, some precautions should be taken when performing the experiment. State two major precautions. Stirring/ use a lid/ use an insulating cup etc. (any 2 x 1 M) (d) Compare the specific heat capacity of the liquid with that of water. Which is more suitable to be used as coolant? Why? It is much smaller than that of water. (1 M) Water is more suitable to be coolant. (1 M) (e) It can absorb a lot of heat without much change in temperature, as a result, effective in absorbing heat. During (1 M) the experiment, some liquid evaporates. How will this affect the result obtained? Explain your answers. Any correct argument (1 M) + correct prediction (1 M) + explanation (1 M) E.g. mass decreases thus measured temperature increases, thus the specific heat capacity found is smaller. 2
3. In an experiment to find the specific latent heat of fusion, a student dropped 100 g melting ice into a cup of 50 g boiling water as shown in Figure (a). After thermal equilibrium has been reached, the student took out the ice remained in the cup and the mass of water remained in the cup was measured (Figure (b)). 100 g melting ice was dropped into cup After thermal equilibrium Remaining ice was taken out 50 g boiling water Mass of water in cup is measured Figure (a) Figure (b) *(a) Explain why the ice could not melt completely after the mixing process. Energy required to melt all the ice is greater than energy given out by boiling water as it cools down to 0 o C. Com (b) Sketch a graph to show the time variation of the temperature of the 50 g water from the moment before the ice was dropped to the moment that thermal equilibrium was reached. 100 o C Graph 1 M Label 0 (c) If the mass of water remained in the cup was measured as 111.5 g, find the specific latent heat of fusion of ice. Take the specific heat capacity of water as 4200 J o C -1 kg -1. (111.5 50)L = (50)(4200)(100) L = 3.41 10 5 J kg -1 (d) Give two possible precautions of the experiment. Briefly account for your suggestions. (4 marks) - stirring during mixing to ensure uniform temperature. - use crushed ice to ensure better thermal contact. (e) Determine if the experimental result would be higher or lower than the standard value of the specific latent heat of fusion if (i) the ice used is not melting. higher (ii) a cup of significant heat capacity is used. lower 7- (iii) the remaining ice is taken out from the cup before thermal equilibrium is reached. higher 3
4. The figure shows the internal structure of an electrical appliance. Wet air flows into Part A of the appliance and dry air flows out from Part B of it. A liquid called the refrigerant circulates through the coiled tube. The refrigerant absorbs heat from the wet air and evaporates inside the coiled tube in Part A. The vapour of the refrigerant is then pumped to the coiled tube in Part B where it is compressed and condensed into a liquid. The liquid refrigerant then passes back to the coiled tube in Part A and the process is repeated. (a) Suggest the electrical appliance in which the above structure can apply. (1 mark) (b) Dehumidifiers. Wet air in Coiled tube Water tank Part A Air fan Part B In the coil tube in Part A, the refrigerant absorbs heat to evaporate into vapour. State the change of average potential energy of the refrigerant molecules during the process of changing state. (1 mark) Increase. (c) Explain why the coiled tube in Part A is designed in a coiled shape. Metal fins pump Dry air out Increase surface area (d) for energy exchange In the coiled tube in Part B, the vapour of the refrigerant is compressed and condenses into liquid. Explain, in terms of molecular motion, why the coiled tube in Part B gives out heat. Internal potential energy of molecules decreases, Latent heat is released and hence the tube is heated up. (e) State and explain two designs that could prevent the Part B from overheating. Black to enhance heat loss by radiation, (f) Conducting materials to enhance heat loss by conduction. Suppose water vapour in the incoming wet air condenses and 1.8 kg of water is collected in the water tank. (i) Estimate the total energy released by the water vapour. The specific latent heat of vaporization of water is 2.26 10 6 J kg -1. (1.8)(2.26 10 6 ) (ii) = 4.07 10 6 J Using the data and the formula in the following table, estimate the increase in temperature of the air in the room, assuming that all energy released in Part B is used to raise the temperature of air inside the room. Volume of the air in the room = 400 m 3 Density of the air = 1.3 kg m -3 Specific heat capacity of air = 1030 J kg -1 o C -1 Mass = density volume 4.068 10 6 J = (400)(1.3)(1030) T 2M T = 7.60 o C End of Section A 4
Section B (20 marks) Write your answers for Section C in the spaces provided below. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1. A 2-kg substance with specific heat capacity 1000 Jkg -10 C -1 is mixed with 3-kg substance with specific heat capacity 1500 Jkg -10 C -1. Assume no chemical reaction, the specific heat capacity of the mixture is A. 1000 Jkg -10 C -1 B. 1250 Jkg -10 C -1 C. 1300 Jkg -10 C -1 D. 2500 Jkg -10 C -1 2. The solid line shows the variation of temperature when a 1-kg solid is heated using a 500 W heater. Which of the following dotted lines best represents the variation when 2-kg of the same kind of solid is heated by a 1000W heater? A. B. C. D. 3. Equal masses of four different liquids are separately heated at the same rate. The initial temperatures of the liquids are all 20 o C. The boiling points and specific heat capacities of the liquids are shown below. Which one of them will boil first? Liquid Boiling point / o C Specific heat capacity / J kg -1 o C -1 A. P 60 1000 B. Q 70 600 C. R 80 900 D. S 90 700 4. Which of the following explains why solids conduct better than liquids in general? (1) Solid particles are more closely packed. (2) Solids have more free electrons (3) Solids have higher conductivity. A. (1) only B. (1) and (2) only C. (2) and (3) only D. (1), (2) and (3) 5. Figure (a) Figure (b) When gets off from an air-conditioned bus in the summer, glasses become misty (see Figure (a)). After a while, the glasses become clear again (see Figure (b)). Which of the following physical processes are involved in the above phenomena? A. condensation followed by evaporation. B. condensation followed by fusion C. solidification followed by evaporation D. solidification followed by fusion 5
6. An immersion heater is used to heat a cup of water. It takes 10 minutes to bring the water from 20 C to its boiling point 100 C. Find the extra time taken for the heater to vaporize the boiling water completely. (Given : Specific heat capacity of water = 4200 J kg 1 K 1, specific latent heat of vaporization of water = 2.268 10 6 J kg 1.) A. 54 minutes B. 60 minutes C. 67.5 minutes D. 77.5 minutes 7. In the experiment to find the specific latent heat of fusion of ice using a power supply and a joulemeter, the result obtained is lower than the expected one. The main reason could be that A. there is heat gain from the surroundings. B. there is heat loss to the surroundings. C. there is some water remaining in the funnel, not falling into the beaker. D. the temperature of ice is below 0 C. 8. Two solid substances X and Y of equal mass are separately heated by two identical heaters. The graph shows the variation of the temperatures of the substances with time. Which of the following statements is/are correct? (1) The melting point of X is higher than that of Y. (2) The specific heat capacity of X is smaller than that of Y. (3) The specific latent heat of fusion of X is smaller than that of Y. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only 0 X Y 9. Which of the following statements is/are correct? (1) Energy transfers from an object with higher internal energy to an object with lower internal energy. (2) Energy transfers from an object with higher temperature to an object with lower temperature. (3) The internal kinetic energy of an object must increase during change of state. A. (2) only B. (3) only C. (1) and (2) only D. (1) and (3) only 10. A beaker of ether is placed in a dish of water. When air is bubbled through the ether, the water freezes because A. the speed of the molecules of ether is increased by the process of bubbling. B. the specific heat capacity of air is very large. C. the freezing point of ether is very low. D. ether absorbs energy from the water during evaporation. End of Test 6