1. Thermal energy is transferred through the glass windows of a house mainly by. D. radiation and convection. (1)

Transcription

1 1. Thermal energy is transferred through the glass windows of a house mainly by A. conduction. B. radiation. C. conduction and convection. D. radiation and convection. 2. The specific latent heat of vaporization of a substance is the quantity of energy required to A. raise the temperature of a unit mass of a substance by one degree Celsius. B. convert a unit mass of liquid to vapour at constant temperature and pressure. C. convert a unit mass of solid to vapour at constant temperature and pressure. D. convert a unit mass of liquid to vapour at a temperature of 100 C and a pressure of one atmosphere. 3. Two different objects are in thermal contact with one another. The objects are at different temperatures. The temperatures of the two objects determine A. the process by which thermal energy is transferred. B. the heat capacity of each object. C. the direction of transfer of thermal energy between the objects. D. the amount of internal energy in each object. 1

2 4. Which two values of temperature are equivalent to the nearest degree when measured on the Kelvin and on the Celsius scales of temperature? Kelvin scale Celsius scale A B C D A substance changes from solid to liquid at its normal melting temperature. What change, if any, occurs in the average kinetic energy and the average potential energy of its molecules? Average kinetic energy Average potential energy A. constant constant B. increases constant C. increases decreases D. constant increases - 6. The specific latent heat of vaporization of a substance is greater than its specific latent heat of fusion because constant A. boiling takes place at a higher temperature than melting. B. thermal energy is required to raise the temperature from the melting point to the boiling point. C. the volume of the substance decreases on freezing but increases when boiling. 2

3 D. the increase in potential energy of the molecules is greater on boiling than on melting. A. B. increases constant C. increases decreases D. constant increases 7. The kelvin temperature of an object is a measure of A. the total energy of the molecules of the object. B. the total kinetic energy of the molecules of the object. C. the maximum energy of the molecules of the object. D. the average kinetic energy of the molecules of the object. 8. The specific latent heat of fusion of a substance is defined as the amount of thermal energy required to change the phase of A. the substance at constant temperature. B. unit mass of the substance to liquid at constant temperature. C. unit mass of the substance at constant temperature. D. the substance to gas at constant temperature. 3

4 9. The distance between the 0 C and 100 C marks on a mercury-in-glass thermometer is 20 cm. When the thermometer bulb is placed in a mixture of ice and salt, the mercury level is 4 cm below the 0 C mark. The temperature of the mixture is A. +20 C. B. +5 C. C. 5 C. D. 20 C. 10. Some liquid is contained in a shallow dish that is open to the atmosphere. The rate of evaporation of the liquid does not depend on A. the temperature of the liquid. B. the temperature of the atmosphere. C. the depth of the liquid. D. the pressure of the atmosphere. 11. As part of an experiment to determine the latent heat of vaporisation of water, a student boils some water in a beaker using an electric heater as shown below. beaker heater water The student notes two sources of error. Error 1: thermal energy is lost from the sides of the beaker Error 2: as the water is boiling, water splashes out of the beaker 4

5 Which of the following gives the correct effect of these two errors on the calculated value for the specific latent heat? Error 1 Error 2 A. Increase Decrease B. Increase No change C. Decrease Increase D. Decrease No change 12. Which of the following is the internal energy of a system? A. The total thermal energy gained by the system during melting and boiling. B. The sum of the potential and the kinetic energies of the particles of the system. C. The total external work done on the system during melting and boiling. D. The change in the potential energy of the system that occurs during melting and boiling. 13. During an experiment, a solid is heated from 285 K to 298 K. Which one of the following gives the rise in temperature, in deg C, and the final temperature, in C, of the solid? Rise in temperature in deg C Final temperature in C A B C D

6 14. A liquid is contained in a dish open to the atmosphere. Which one of the following contains three factors that affect rate of evaporation of the liquid? A. Temperature of the liquid Surface area Specific latent heat of vaporization B. Temperature of the liquid Mass of liquid Specific latent heat of vaporization C. Surface area Mass of liquid Temperature of the liquid D. Mass of liquid Surface area Specific latent heat of vaporization 15. A liquid is evaporating, causing the liquid to cool. The temperature of the liquid decreases because A. the number of liquid molecules is decreasing. B. the mean kinetic energy of the liquid molecules is decreasing. C. the pressure above the liquid surface is increasing. D. the rate of evaporation is increasing. 16. Some students were asked to design and carry out an experiment to determine the specific latent heat of vaporization of water. They set up the apparatus shown below. d.c. supply V A Water Heater g Top-pan balance 6

7 The current was switched on and maintained constant using the variable resistor. The readings of the voltmeter and the ammeter were noted. When the water was boiling steadily, the reading of the top-pan balance was taken and, simultaneously, a stopwatch was started. The reading of the top-pan balance was taken again after 200 seconds and then after a further 200 seconds. The change in reading of the top-pan balance during each 200 second interval was calculated and an average found. The power of the heater was calculated by multiplying together the readings of the voltmeter and the ammeter. (a) Suggest how the students would know when the water was boiling steadily (b) Explain why a reading of the mass lost in the first 200 seconds and then a reading of the mass lost in the next 200 second interval were taken, rather than one single reading of the mass lost in 400 seconds (2) 7

8 The students repeated the experiment for different powers supplied to the heater. A graph of the power of the heater against the mass of water lost (the change in balance reading) in 200 seconds was plotted. The results are shown below. (Error bars showing the uncertainties in the measurements are not shown.) power / W mass / g (c) (i) On the graph above, draw the best-fit straight line for the data points. (ii) Determine the gradient of the line you have drawn. 8

9 In order to find a value for the specific latent heat of vaporization L, the students used the equation P = ml, where P is the power of the heater and m is the mass of water evaporated per second. (d) Use your answer for the gradient of the graph to determine a value for the specific latent heat of vaporization of water (e) The theory of the experiment would suggest that the graph line should pass through the origin. Explain briefly why the graph does not pass through the origin (2) (Total 12 marks) 9

10 17. This question is about the change of phase (state) of ice. A quantity of crushed ice is removed from a freezer and placed in a calorimeter. Thermal energy is supplied to the ice at a constant rate. To ensure that all the ice is at the same temperature, it is continually stirred. The temperature of the contents of the calorimeter is recorded every 15 seconds. The graph below shows the variation with time t of the temperature θ of the contents of the calorimeter. (Uncertainties in the measured quantities are not shown.) / C t / s (a) (b) On the graph above, mark with an X, the data point on the graph at which all the ice has just melted. Explain, with reference to the energy of the molecules, the constant temperature region of the graph

11 The mass of the ice is 0.25 kg and the specific heat capacity of water is 4200 J kg 1 K 1. (c) Use these data and data from the graph to (i) deduce that energy is supplied to the ice at the rate of about 530 W. (ii) determine the specific heat capacity of ice. (iii) determine the specific latent heat of fusion of ice. (2) (Total 12 marks) 11

12 18. This question is about thermal physics. (a) Explain why, when a liquid evaporates, the liquid cools unless thermal energy is supplied to it (b) State two factors that cause an increase in the rate of evaporation of a liquid (2) (c) Some data for ice and for water are given below. Specific heat capacity of ice = J kg 1 K 1 Specific heat capacity of water = J kg 1 K 1 Specific latent heat of fusion of ice = J kg 1 A mass of 350 g of water at a temperature of 25 C is placed in a refrigerator that extracts thermal energy from the water at a rate of 86 W. Calculate the time taken for the water to become ice at 5.0 C (4) (Total 9 marks) 12

13 19. This question is about specific heat capacity and specific latent heat. (a) Define specific heat capacity (b) Explain briefly why the specific heat capacity of different substances such as aluminium and water are not equal in value (2) A quantity of water at temperature θ is placed in a pan and heated at a constant rate until some of the water has turned into steam. The boiling point of the water is 100 C. (c) (i) Using the axes below, draw a sketch-graph to show the variation with time t of the temperature θ of the water. (Note: this is a sketch-graph; you do not need to add any values to the axes.) 100 C C 0 time at which heating starts time at which water starts to boil t 13

14 (ii) Describe in terms of energy changes, the molecular behaviour of water and steam during the heating process. (5) Thermal energy is supplied to the water in the pan for 10 minutes at a constant rate of 400 W. The thermal capacity of the pan is negligible. (d) (i) Deduce that the total energy supplied in 10 minutes is J. (ii) Using the data below, estimate the mass of water turned into steam as a result of this heating process. initial mass of water = 0.30 kg initial temperature of the water θ = 20 C specific heat capacity of water = J kg 1 K 1 specific latent heat of vaporization of water = Jkg 1 14

15 (iii) Suggest one reason why this mass is an estimate. (Total 14 marks) 20. This question is about modelling the thermal processes involved when a person is running. When running, a person generates thermal energy but maintains approximately constant temperature. (a) Explain what thermal energy and temperature mean. Distinguish between the two concepts (4) The following simple model may be used to estimate the rise in temperature of a runner assuming no thermal energy is lost. A closed container holds 70 kg of water, representing the mass of the runner. The water is heated at a rate of 1200 W for 30 minutes. This represents the energy generation in the runner. (b) (i) Show that the thermal energy generated by the heater is J. (2) 15

16 (ii) Calculate the temperature rise of the water, assuming no energy losses from the water. The specific heat capacity of water is 4200 J kg 1 K 1. (c) The temperature rise calculated in (b) would be dangerous for the runner. Outline three mechanisms, other than evaporation, by which the container in the model would transfer energy to its surroundings (6) A further process by which energy is lost from the runner is the evaporation of sweat. (d) (i) Describe, in terms of molecular behaviour, why evaporation causes cooling. 16

17 (ii) Percentage of generated energy lost by sweating: 50% Specific latent heat of vaporization of sweat: J kg 1 Using the information above, and your answer to (b) (i), estimate the mass of sweat evaporated from the runner. (iii) State and explain two factors that affect the rate of evaporation of sweat from the skin of the runner. (4) (Total 25 marks) 17