Temperature Ball and Hoop

Transcription

1 Temperature Ball and Hoop Edited by Anne Starace ABSTRACT: We can sense temperature changes through our senses of touch, but what happens on a molecular level as temperature changes? This module demonstrates what happens to molecules as they change temperature. LAB MANUAL: Recipes for Science: Temperature and Kinetic Theory Citation: The Science Theatre, Recipes for Science, ed. by Jennifer Discenna (Michigan State University, Kalamazoo, Ml, 1993). Permission granted by Jennifer Discenna by The Science Theatre; reprinted with permission. Keywords: Thermal Physics; Kinetic Theory; Kinetic Interpretation Of Temperature; Temperature Molecular Model Of Gases; Cooling; Temperature Scales; Kelvin Scale; Celsius Scale Funded by the National Science Foundation and the University of Nebraska 1

2 Content Standards K History & Process Standards K Skills Used/Developed: 2

3 TABLE OF CONTENTS I. OBJECTIVES...4 II. SAFETY...4 III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTSError! Bookmark not defined. IV. LIST OF MATERIALS...Error! Bookmark not defined. V. INTRODUCTION...Error! Bookmark not defined. VI. PROCEDURE...5 VII. FREQUENTLY ASKED QUESTIONS...6 VIII. TROUBLE SHOOTING...6 IX. HANDOUT MASTERS...6 X. REFERENCES:...6 3

4 I. OBJECTIVES Students will understand that when molecules are cold they are closer to each other and move less than when they are warm. II. SAFETY Do not touch liquid nitrogen with bare skin! Be careful with the liquid nitrogen around the younger students; they don't realize how cold it is. Never keep it within touching distance and never leave liquid nitrogen unattended. III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS Level: This module is good for students grades 2-6. Time required: 5-10 minutes Number of participants: Up to about 30. IV. LIST OF MATERIALS Liquid Nitrogen Safety Equipment (see footnote 1) Ball and Hoop (available from catalogs) V. INTRODUCTION The temperature in Kelvin, denoted with a T, equals the degrees Celsius plus 273. T= C Kinetic theory relates the temperature of an object to the motion of its molecules. The main equation of interest in this case, which is derived in many introductory texts (see for example, Cutnell & Johnson) is 1/2 mvrms2 = 3/2 kt. Where k is the boltzmann constant, T is the Kelvin temperature, m is the mass of the particle and vrms is the root-mean-square speed. This equation relates directly the Kelvin temperature and the average translational kinetic energy of an individual particle. As the kinetic energy of the particles increase, so must the Kelvin temperature and vice versa. When the temperature decreases, the equation above tells us that the kinetic energy, the speed of the molecules, must decrease as well. As the speed of the molecules increase, 4

5 the molecules fill up more space. Molecules that are cold, vibrate very little and stay close together. Molecules that are warm, vibrate more rapidly and take up more space. A short temperature scale (Kelvin): Water boils 373 Room temperature 293 Water freezes 273 CO 2 freezes (dry ice) 194 Liquid Nitrogen 77 Looking at the temperature scale you can see that room temperature is pretty high on the temperature scale and the freezing point of water is a little bit lower, dry ice, which some students are familiar with is also on the list, but a little ways down. Liquid Nitrogen, however is all the way at the bottom of the list at 77K!! This is extremely cold. It is colder than ice, much colder than any winter day! VI. PROCEDURE Show the students that the ball fits nicely through the hoop. Ask the students what they think will happen if you put the hoop into the liquid nitrogen. Place the hoop into the liquid nitrogen and let it cool. The cooling will take about 2 minutes. You can tell that it is frozen because the liquid nitrogen will stop bubbling. We suggest that you do another experiment while the hoop cools. When the hoop is cooled, try to put the ball through the hoop. You will see that it no longer fits, but as the hoop warms up, the ball will start to slide through. What to Say: Ask the students what they think happened. Why didn't the ball go through the hoop? It was too small! It shrunk! If you did the experiment with the balloon, ask them what this has to do with the balloon. It is the same idea. The molecules in the metal were moving around quite a bit when they were warm. When the hoop was cooled down, the molecules slowed down and huddled closer together and the hoop shrunk! When the hoop started to warm up, the molecules started moving again and the hoop began to expand and the ball fit through again. A common misconception with this demonstration is that the hole somehow shrinks inward. Instead, you can ask students to imagine that the hoop is made from a long rod. The rod becomes shorter as it cools, and also becomes thinner. When the rod shortens and thins, it decreases the size of the hole. Footnotes: 5

6 1 Liquid Nitrogen can be purchased on most campuses for a modest fee. Liquid Nitrogen is stored in a dewar which will keep it cold enough for long periods of time. When working with liquid nitrogen, you must use proper safety equipment: safety goggles and gloves must be worn. 2 To order contact: DaMert Company, Department T, 2476 Verna Ct. San Leandro, CA For More Information: TiNi Alloy Company th Street Oakland, CA VII. FREQUENTLY ASKED QUESTIONS VIII. TROUBLE SHOOTING IX. HANDOUT MASTERS X. REFERENCES 6