Activities

 

Introduction

Life depends on an essentially continuous exchange of mass and energy between living organisms and their environment.  Human impact on this vital exchange has occurred on a global or macroclimate scale.  Understanding the physical principles involved in heat transfer and absorption in the atmosphere is critical to understanding how these physical factors affect living organisms.  The specific objectives of this section are to explain the properties of heat transfer, and to describe laboratory activities that can be used at a variety of academic levels with only slight modification.   Described below are three series of experiments performed in the laboratory to address questions that emphasize the underlying principles of heat transfer.  These hands-on experiments focused on principles that relate to conduction and convection.    The object was to identify the method of heat transfer through solids,  liquids, gases, and between boundaries.  Understanding these concepts gave us a better understanding of how heat is transferred between our environment and living organisms.  These experiments were used as an integral part of the workshop, which consisted of reflections on redesigning or modifying lab exercises to fit personal needs of workshop teachers.   These exercises could be adapted for middle school, high school, and college level courses.   The methods utilized for the three experiments involved increasing or decreasing the temperature of a solid or liquid, and where applicable, observing the motion of a dye caused by the changes in temperature and density of the medium.      Modes of Heat Transfer  Lab 1:  Heating From Below:  Convection  Lab 2:  Conduction                              Lab 3:  Cooling From Above

Modes of Heat Transfer: Conduction:  heat transfer resulting from direct contact between substances of different temperatures; heat is transferred from the high-temperature substance to the low by direct molecular interaction. Convection:  heat transport by a moving fluid (gas of liquid).  The heat is first transferred to the fluid by conduction, but the fluid motion carries the heat away. Radiative exchange:  heat transfer via electromagnetic waves, the amount of radiant energy emitted, transmitted, or absorbed.

(Figure from Microsoft Encarta) Return to Top

Laboratory Apparatus for Labs 1-3

Lab 1:  Heating from Below:  Convection Introduction In this experiment, water was heated from below to produce convection.  Although the atmosphere is composed of air, this experiment was relevant to atmospheric motion as well.  The lower atmosphere (troposphere) is mostly heated from below because the oceans and continents absorb radiation from the sun and then transfer some of the resulting heat energy to the lower atmosphere.   Procedure  In Lab 1, a beaker was heated (see figure below). Thermometers were placed in 1/2 cm below water surface and 1/2 cm above the bottom of the beaker.  The temperature was recorded at 30 second intervals.  Drops of dye were added to the bottom of the beaker between intervals.  After three minutes the beaker was removed from the hot plate and temperature reading recorded for another five minutes.  Convection was visualized by observing the motion of the dye.   Applications The motion of the dye was circular from bottom to top and returning to the bottom of the beaker.  The energy from heating created a less dense liquid at the bottom, thus causing the upward motion of the dye.  Upon reaching the surface, the dye was now in the denser medium and therefore returned to the bottom.  This motion is an example of convection.  This phenomenon is evident in the motion of wind.  The difference in densities and kinetic movement of the water molecules driven by temperature change resulted in the movement of air molecules.  This lab can be used at lower levels to demonstrate simple properties of heat transfer and convection.  At higher levels, this lab illustrates these basic principles, and could be extended to address more complex applications related to convection such as the Coriolis effect.  Sample Questions 1.  Explain the process by which the water is heated.   2.  Describe the motion of the water as made visible by the dye.   3.  Why does convection occur?   4.  Did convection cease?  When?  Why?

Environmental Applications of Principles of Radiative Exchange, Conduction and Convection  (Figure from E. Zerba, Princeton University;  ezerba@princeton.edu)   Return to Top

Lab 2:  Conduction Introduction Comparison of this experiment with the first illustrated the difference between the rate of heat transfer by conduction and that of convection.  It also illustrated the difference in heat capacities between water and the solid materials of the earth.   Procedure Lab 2 was configured similarly to Lab 1, but looked at the effect of heating and cooling temperature difference using sand of equal weight as water used in experiment 1.  No dye was used in this experiment, as convection was not a factor.       Applications The temperature difference between the top and bottom layers of sand indicated that sand heats and cools at a faster rate compared to water.  When the beaker was removed from the heat, the temperature continued to increase via conduction from the bottom of the beaker.  This lab exercise is useful for demonstrating the concept of conduction to lower level students.  Upper level students can use this lab to make the connections between conduction and heat capacity of various substances related to heat transfer that occurs between the earth's surfaces and the surface of living organisms.   Sample Questions 1.  Is there any convection in the sand?  Explain.    2.  Why did the temperature recorded by the lower thermometer continue to rise dramatically after the heating ceased?   3.  On the basis of heat capacity, explain why the temperature changes for the sand and water were different.   4.  Using what you have observed in the two experiments, predict whether a cold front will lower temperatures more at inland locations or on the coast.  Explain your answer.   Return to Top

Lab 3:  Cooling From  Above Introduction In lakes and oceans, convection is generally the result of cooling from above rather than heating from below.  This was demonstrated by adding ice to the water.   Procedure Using an experimental setup that allowed measurement of temperature at the top and the bottom of a beaker of water, ice was added to the top of the beaker.  This experiment illustrated the concept that at 4 °C,  water has higher density and sinks.  Convection was visualized by the movement of dye added to the bottom of the beaker which was displaced by the cooler more dense water.     Applications This lab demonstrates several physical principles associated with heat transfer, including density, kinetic molecular theory, and convection.  On a larger scale, this laboratory exercise demonstrates the process by which seasonal turnovers occur in ponds and lakes.   At lower levels, teachers may choose to discuss physical principles of heat transfer only, while at upper levels, teachers may choose to integrate this small-scale investigation with the study of climate processes and lake nutrient stratification and mixing.   Sample Questions 1.  Why does ice float?   2.  Is there any evidence of convection?  Why does or does it not occur?   3.  Draw a diagram to explain how seasonal turnover occurs in a pond.   Return to Top