Estimate the energy stored in unit gram of the food in J per gram.

Transcription

1 NAME : F.5 ( ) Marks: /70 FORM FIVE PHYSICS REVISION TEST on HEAT Time 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. Data: Specific heat capacity of water = 4200 J kg -1 o C -1 Specific latent heat of evaporation = J kg -1 Section A (50 marks) 1. Figure 1 shows the food labels of a container of biscuits. The energy per serving of the biscuits is 90 calories and there are 4 servings in a container. (a) If one calorie corresponds to the amount of energy required to raise the temperature of 1 g of water by 1 o C, find the total energy, in J, stored in a container of biscuits. (3 marks) Figure 1 (b) The energy stored in foods can be estimated by igniting the foods and use the energy released to heat up water in a test tube. The following shows the data obtained in the experiment. Mass of foods burnt = 0.01 g Mass of water in test tube = 200 g Temperature rise of water = 0.2 o C Estimate the energy stored in unit gram of the food in J per gram. (3 marks) 2. In an experiment to determine the specific latent heat of fusion of ice, 90 g melting ice cubes were added into a glass of 300 g hot water. Figure 2 shows the temperature-time graph of the water obtained. Temperature / o C 80 P Figure Q S 20 R Time / minute (a) P, Q, R and S are four points on the graph. Explain the shape of the following sections of the graph. (3 marks) (i) PQ 1

2 (ii) QR (iii) RS (b) Use your answer in (a), estimate (i) the temperature of the surroundings. (4 marks) (ii) the specific latent heat of fusion of ice. (c) Explain, in terms of molecular motion, why the temperature of the ice cubes remains unchanged, even though it is being warmed. (d) State TWO precautions in the experiment. Explain briefly for your answer. (4 marks) 3. Figure 3 shows the basic structure of a cloth dryer. Air is first drawn into the body of the dryer through a large hole in the front of the dryer. It is then sucked past the heating element and into the tumbler. It enters the door and is directed down through the lint screen. It finally passes through a duct in the front of the dryer and into the fan which forces it into the duct leading out the back of the dryer. Figure 3 (a) Why does a cloth dryer usually white in colour? (1 mark) (b) Name the process in which clothes can be dried in the dryer. (1 mark) 2

3 (c) Calculate the energy required to change 1 kg of water at 25 o C into steam at 100 o C. (d) Assume the power in operating the dryer is 500 W and the initial temperature of the water in the clothes is 25 o C. If 70 % of the power is used to generate steam from the clothes, calculate the maximum mass of steam that can be generated in one second. 4. Figure 4 shows a solar water heater. It includes a storage tank and solar collectors. The storage tank has outlet and inlet connected to and from the collector. Usually, there is a back-up heater system working together with the solar heater system. (a) Explain why the storage tank is installed on the top part of the solar water heater. (1 mark) (b) Suggest and explain TWO ways to reduce heat loss from hot water stored in the storage tank. (4 marks) Figure 4 (c) Explain your choice of materials building the pipes of the solar water heater in the following areas. (i) Colour: black or white? (ii) Material: metal or plastic? (d) At noon, the water flows into the heater at a constant rate of 50 gs -1. The temperature of water rises by about 5 0 C after passing the heater. (i) Estimate the power of the solar heater. 3

4 (ii) At 5 p.m., the temperature of water rises by 2 0 C only. (1) Suggest a reason for the smaller rise in temperature. (1 mark) (2) The back-up heater is switched on so that the water from the system can be increased back to 5 0 C. Calculate the power of the heater required. (3) If the heater in (d)(ii)(2) is switched on for 10 minutes, and the cost of electricity is $0.3 per kwh, calculate the cost paid due to use of back-up heater. (e) It is suggested to use glass to cover the solar water heater, state TWO reasons for this suggestion. 5. Figure 5 shows an energy label of a heater. Annual Energy Consumption *kwh/yr 364 *Yearly consumption is found based on a consumption of 260 hours per year. Figure 5 (a) Calculate the power of the heater. (b) The heater is used to heat a cup of 500 g of water for 2 minutes and its temperature increases from 20 0 C to 75 0 C. Calculate the rate of heat transfer to the water. (c) Calculate the efficiency of the heater. (d) Suggest a reason why efficiency cannot be 100%. (1 mark) End of Section A 4

5 Section B (20 marks) Write your answers for Section C in the spaces provided below The average internal kinetic energies of molecules of object P are greater than that of object Q. Which of the following statements is/are correct? (1) The temperature of P must be higher than that of Q. (2) The total internal kinetic energies of object P must be greater than that of object Q. (3) The average internal energy of object P must be higher than that of object Q. B. (3) only Question 2 and 3 2. A metal block is first immersed in liquid nitrogen at 170 o C for some time. The block is then transferred to a cup of tap water. After a while, the temperature of the water is measured. The result of the experiment is as follows: Mass of metal block = 0.8 kg Mass of water in the cup = 0.3 kg Initial temperature of water in the cup = 25 o C Final temperature of water in the cup = 2 o C Find the specific heat capacity of the metal (in J kg -1 o C -1 ). (Given: Specific heat capacity of water = 4200 J kg -1 o C -1.) A. 211 B. 381 C. 622 D The result obtained in Q.2 is found to be lower than the true value of the specific heat capacity of the metal. Which of the following is a probable reason? (1) Some liquid nitrogen is still adhered to the metal block when the block is transferred to the tap water. (2) Some energy is gained from the surroundings when the metal block is transferred to the cold water. (3) The temperature of the metal block is still lower than 2 o C when the final temperature of the water in the cup is measured. B. (3) only 4. The figure shows the label of a kilowatt-hour meter connected to a mains supply. When an appliance is switched on for 1 minute, the circular disc rotates through 30 complete revolutions. What is the electric power consumed by the appliance? A. 900 W B W C W D W kwh 220 V 50 Hz 1200 revolutions / kwh Circular disc

6 5. The figure shows the cooling curve of a substance which is initially in the gaseous state. The temperature of the substance remains unchanged during the period PQ. Which of the following statements about the substance during the period PQ is/are correct? (1) The substance is not losing any energy to the surroundings. Temperature (2) A body in contact with the substance during PQ would be warmed up. (3) The average potential energy of the molecules of the substance is decreasing. A. (2) only P Q B. (3) only 0 Time 6. Which of the following statements is/are the similar characteristics for s.l.h. of fusion and s.l.h. of vaporization? (1) Both are related to the change of state without change in temperature. (2) Both quantities have the same unit. (3) Both depend on mass of substance used. B. (2) only 7. It is not advisable to use metal cups instead of paper cups to hold hot drinks because metal (1) conducts well (2) has higher heat capacity (3) is a better radiator than plastic B. (1) and (2) only C. (2) and (3) only D. (1), (2) and (3) 8. We feel cool when our body is wet because (1) energy is carried away from our body to lower the temperature of water. (2) latent heat is absorbed from our body. (3) water lowers the temperature of our body. B. (2) only C. (1) and (3) only 9. Martin used a heater to warm a pan of soup. His mother asked me not to use switch to maximum power. Which of the following statements correctly explain(s) his mother s request? (1) The soup would reach a temperature higher than C and not able to drink. (2) The soup would vaporize too quickly and be dried up. (3) The soup would be too salty because of vaporization of water. B. (1) and (2) only C. (2) and (3) only D. (1), (2) and (3) 10. When a gas is heated, (1) it expands. (2) the average kinetic energy of the gas molecules increases. (3) the average potential energy of the gas molecules increases A. (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) End of Test 6