Period 5: Thermal Energy, the Microscopic Picture

Transcription

1 Name Section Period 5: Thermal Energy, the Microscopic Picture 5.1 How Is Temperature Related to Molecular Motion? 1) Temperature Your instructor will discuss molecular motion and temperature. a) At a particular temperature, do all of the molecules move at the same speed? b) How does the average speed of molecules at a higher temperature differ from their average speed at a lower temperature? c) Watch the demonstration of diffusion of food coloring in beakers of warm and cold water. Explain the differences in the diffusion rates. 2) Measuring Temperature a) Thermometers Examine the five types of thermometers and explain what changing property each type of thermometer relies upon. 1) alcohol thermometers 2) bimetallic strip thermometers 3) liquid crystal thermometers 4) infrared thermometer 5) Galileo s thermometer b) 1

2 c) Temperature Scales Your instructor will discuss Fahrenheit, Celsius and Kelvin temperature scales. 1) Examine a thermometer with both Fahrenheit and Celsius scales. On the Celsius scale, how many degrees are between the freezing point and the boiling point of water? 2) On the Fahrenheit scale, how many degrees are between the freezing point and the boiling point of water? 3) Use the number of degrees between the freezing and boiling points of water to make a ratio of the number of Celsius degrees per Fahrenheit degrees. 4) Write an equation to convert degrees Fahrenheit into degrees Celsius. Use your ratio from part 3 plus the fact that the freezing temperature of water in the Celsius scale is 32 degrees lower than in the Fahrenheit scale. 5) Use your equation to convert 70 degrees Fahrenheit into Celsius degrees. 6) In the Kelvin scale, water boils at 373 Kelvin and freezes at 273 Kelvin. A change in how many degrees Celsius equals how much of a change in Kelvin? 7) Write an equation to convert degrees Celsius to Kelvin. d) Group Discussion Question: Which temperature scale gives the greatest distinction between temperature degrees Fahrenheit, Celsius, or Kelvin? 2

3 3) Evaporative Cooling What happens to the temperature of a liquid as it evaporates? Your instructor will demonstrate a cool-tube (a cryophorus tube) that contains water. a) What happens when one end of the tube is cooled with liquid nitrogen? b) The middle of the tube remains near room temperature. Explain why the water in the tube s bulb freezes. c) Group Discussion Question: When you put a pot of water on the stove to heat to boiling, why does the water come to a boil faster if you put a lid on the pot? 4) The Dippy Duck The dippy duck contains liquid freon, which evaporates easily at room temperature. Wet the head of the duck and place the cup of water in front of the duck s head. Explain what happens to the dippy duck in terms of evaporative cooling and the center of mass of the duck. 3

4 5) Rates of Evaporation a) Note the temperature of each thermometer while it is immersed in liquid. Remove the thermometers from the test tubes and place them in an empty glass beaker. Wait one minute. Then check and record their temperatures. Initial Temp Final Temp Temp Change Alcohol Oil Water b) Explain the differences in temperature change. c) Group Discussion Question: What change could you make to the dippy duck experiment to make the duck s head dip faster? 5.2 Temperature and Phase Changes 6) Volume and Temperature a) What happens to the volume of most substances when they are heated? b) Do all substances expand when heated? At the beginning of class, your instructor placed a beaker full of ice water on your table. What has happened to the volume of the ice as it melted? 7) Pressure and Phase Changes a) Your instructor will demonstrate weights on a wire supported by an ice cube. What happens? What causes the change of phase of the ice from solid to liquid water? b) Would this demonstration work as well with a wide wire? Why or why not? (Hint: pressure = force/area of the wire) c) Group Discussion Question: When the temperature is well below freezing, ice skaters find it more difficult to skate. Why is this? 4

5 Name Section Period 5 Exercises: Thermal Energy, the Microscopic Picture 1) Explaining transfer of thermal energy Explain how the kinetic energy of gas molecules is related to the temperature of the gas. 2) Understanding evaporative cooling In class you saw an example of liquid water turning into ice in the glass cryophorus tube (cool tube). Liquid water was in a hollow glass sphere at one end of the tube. Your instructor placed the other end of the tube into very cold liquid nitrogen. Although the glass sphere was not cooled below freezing, the water in the sphere froze into ice. The following questions ask you to explain this. a) As the air in the tube cooled, what happened to the average motion of the air molecules in the tube? b) As the air in the tube cooled, what happened to the temperature of the air molecules? c) As the air in the tube cooled, what happened to the pressure exerted by these air molecules on the walls of the tube and on the surface of the liquid water? d) As the pressure in the tube dropped, some water molecules were able to leave the surface of the liquid water and evaporate into molecules of water vapor. Which water molecules evaporated first: fast moving molecules, slow moving molecules, or did molecules of all speeds evaporate equally? e) As water molecules evaporated, what happened to the average kinetic energy of the remaining water molecules? f) As water molecules evaporated, what happened to the temperature of the remaining water molecules? g) Why did the water in the glass sphere freeze? 5