Figure 1(a) The maximum mass of ice melted (m) for difference mass of liquid X (M) are obtained and recorded in Table 1.

Transcription

1 NAME : F.4 ( ) Marks: /70 FORM FOUR 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. Take g = 10 ms -2 if necessary. Section A (50 marks) 1. A student performs an experiment with the setup in Figure 1(a) to measure the specific heat capacity of a liquid X. Lumps of melting ice are added to liquid X at 25 o C until no further ice can be melted. The mass of ice added is measured and the experiment is repeated for different mass of liquid X. melting ice liquid X thermometer Figure 1(a) The maximum mass of ice melted (m) for difference mass of liquid X (M) are obtained and recorded in Table 1. m / kg M / kg Table 1 (a) (i) Suggest a method to measure the mass of ice melted. (ii) Plot a graph of m against M in Figure 1(b). (4 marks) Figure 1(b) (iii) Given specific latent heat of fusion of ice = J kg -1. Using the graph plotted in (a)(ii), find the specific heat capacity of liquid X. Express your answer in scientific notation and with 2 significant figures. (3 marks) (iv) The graph does not pass through the origin but cuts the y-axis at a positive value. Why? 1

2 (v) If the experiment is repeated with liquid Y with a larger specific heat capacity than liquid X and its initial temperature is 30 o C, sketch a graph of m against M you would expect to obtain in Figure 1(b), and label it as L. (vi) Suggest one way to improve the accuracy of the experiment. 2. Figure 2(a) and Figure 2(b) show respectively an electric steam iron and an electric steamer which produces steam by different mechanisms. steamer `jet steam button Iron plate Water tank Figure 2(a) Figure 2(b)\ Given: heat capacity of the ironing plate is 637 J o C -1 specific heat capacity of water = 4200 J kg -1 o C -1 specific latent heat of vaporization of water = J kg -1 (a) The electric steam iron produces steam by first heating up the iron plate (refer to Figure 1) at constant rate of 1 kw until its temperature reaches 250 o C and the iron would be automatically switched off. When the `jet steam button is pressed once, 20 g of water at 25 o C will be pumped onto the heated plate at 250 o C and the water is converted into steam at 100 o C instantly. (i) Find the time required to heat up the iron plate from 25 o C to 250 o C. (ii) What is the amount of energy required to produce 20 g steam from 25 o C? (iii) Estimate the temperature of the plate just after producing 20 g of steam. (iv) Can the iron convert 40 g of water at 25 o C to steam when the button is pressed twice? Explain why. (b) The electric steamer produces steam by heating up water from 25 o C in a water tank (refer to Figure 2) at a constant rate of 1 kw. (i) Assume the water tank contains 200 g water. Estimate the time required to produce 20 g of steam from the tank. (3 marks) 2

3 (ii) Suggest a method to speed up the steam-producing process. (c) An electric steamer is usually more efficient in generating steam with less power loss. Give two reasons for this fact. 3. Ken constructed a thermometer using a liquid. He put 500 ml of the liquid into a narrow glass tube and measure the lengths of the liquid column when it is put into pure melting ice and pure boiling water respectively. Before he started his measurement, he tried to measure the room temperature using his thermometer. Figure 3(a) shows the reading of his measurement. Figure 3(a) cm (a) Which of the records should Ken took as measured value? Circle your answer cm / cm / cm (b) Table 3 shows the readings for some of his measurements. Complete the table and explain the advantage of repeating the measurements a few times for an average value. (5 marks) Temperature/ 0 C Reading 1/cm Reading 2/cm Reading 3/cm Average/cm Melting ice Boiling water (c) Using the information shown in figure 3(a), calculate the percentage error in measurement when the reading is 19.8 cm. (d) Ken suggested that using more liquid will increase the accuracy of the measurement. Do you agree? Why? (e) Using the information in table 3 above, if the thermometer reads 25.0 cm, what temperature does it show? (f) Suggest THREE possible reasons for the deviation of the calculated value in (e) from the true value. (3 marks) 3

4 4. Matthew used a heater working at 70 W to heat 500 ml of liquid X in an insulated container. It takes 2 minutes to heat the liquid from 20 0 C to 40 0 C. He then repeated the experiment with 1L of the same liquid and it takes 3.8 minutes. (a) Explain why the time taken to heat 1 L of liquid X does not double that used to heat 500 ml of the same liquid? (Neglect heat loss to surroundings.) (b) Hence, find the specific heat capacity of liquid X. (4 marks) (c) Suggest THREE different methods to reduce heat loss to surroundings in the situation. (3 marks) (d) If the cost of electricity is $0.3 per kwh, calculate the electricity cost for Matthew s two experiments. End of Section A Section B (20 marks) Write your answers for Section C in the spaces provided below In which of the following cases will heat always flow away from a body X? (1) X is changed from its liquid state to a solid at freezing point. (2) X is in thermal contact with another system at a lower temperature. (3) X is in thermal contact with another system which has a smaller amount of internal energy. 2. Heat energy must be supplied to a pan of water at 100 o C to keep it boiling. This energy replaces the heat energy lost to the surroundings, and also supplies energy to (1) increase the average kinetic energy of the molecules. (2) increase the average potential energy of the molecules. (3) allow the water vapour to expand against atmospheric pressure. 4

5 3. A candidate places an electrical heater in the middle of a lump of ice and measures the time it takes to just melt all of the ice. The power of the electric heater and the initial mass of ice were measured and found to be 12 W and 0.10 kg, respectively. From these measurements, he calculates the specific latent heat of fusion of ice, but finds that his answer is lower than expected. This result is possibly due to the fact that (1) energy has been lost to the surroundings. (2) the mass of ice is actually more than the measured value. (3) the power of the heater is actually more than the measured value. 4. In an experiment, a kilowatt-hour meter is used to measure the energy supplied to an immersion heater. An electronic balance is used to measure the change of the mass. The data obtained is used to estimate the specific latent heat of vaporization of water (l). Immersion heater kilowatt-hour meter to mains water polystyrene cup Electronic balance If the following experimental errors arise: (I) The data are taken before water starts to boil. (II) The polystyrene cup is replaced by metal cup. How would the calculated value of l be affected? (I) (II) A. increases decreases B. increases increases C. decreases decreases D. decreases increases 5. Which of the following statements is/are correct? (1) The boiling point of pure water increases with atmospheric pressure. (2) The water molecules in 100 o C steam moves with a higher speed than those in pure water boiling at 100 o C. (3) When pure water is boiling, the supplied energy becomes the intermolecular potential energy and the kinetic energy of the water molecules. 6. Which of the following statements about heat capacity is/are correct? (1) Heat capacity depends on mass of the object. (2) Heat capacity increases when energy absorbed by the object increases. (3) Heat capacity relates to the ability of an object to absorb heat. C. (1) and (3) only 5

6 7. When 3 kg of liquid P at 70 0 C mixes with 2 kg of the same kind of liquid at 20 0 C, the final temperature of the mixture will be A C B C C C D. unable to be determined because of unknown specific heat capacity. 8. Line I in the figure shows the time variation of temperature of a beaker of water when heated by a heater of constant power, which of the following reasons can be used to explain the change of variation to line II? Temperature (1) A more powerful heater is used. II (2) A beaker of smaller heat capacity is used. (3) Heat loss to surroundings is reduced. I Time 9. An empty boiling tube is put near a light bulb, with its opening covered with cotton wool. When the light bulb is switched on, (1) the temperature inside the tube increases but with decreasing rate. (2) both the total kinetic and potential energy of the air molecules inside the tube increases. (3) heat is transferred through radiation. B. (2) only C. (1) and (3) only 10. If it takes 2 minutes to boil off 4 kg of a liquid from its boiling point with a heater of 100 W, how long will it take to boil off 16 kg of the same kind of liquid with a heater of 200 W from its boiling point? A. 1 minute B. 2 minutes C. 4 minutes D. 8 minutes End of Test 6