An Exploration in Heat Transfer

Heat is the measure of the internal energy of a substance, in which the process of increasing the internal energy is heating the substance, and the process of decreasing the internal temperature is cooling the substance. In order for heat energy to flow, a temperature gradient must exist. When energy flows between substances of different temperatures, heat has been transferred from the substance of higher temperature to the substance of lower temperature. Heat can be transferred between substances by the processes of convection, conduction, or radiation. Convection is the movement of heat within a fluid, such as air or water, conduction is the movement of heat between two solid objects through direct contact of the molecules, and radiation is the movement of heat through space via electromagnetic waves (Tillery, Enger, & Ross, 2008).
In exploring heat transfer I was tasked with testing materials which would inhibit or slow the transfer of heat energy via conduction by covering the top of coffee mugs filled with hot water and comparing the temperature difference from zero minutes to 30 minutes. Materials which are effective in slowing the transfer of heat are known as insulators. For this investigation I chose to use the common household items newspaper, a plastic lid, cotton cloth, and cardboard, each with a thickness of 3mm to ensure I was only testing the effectiveness of the material itself since varying thicknesses could affect the results. I also established a control mug with no material covering the top to use as a comparison. To counteract any heat loss through the mug as opposed to the tested materials, I first filled each mug with hot water and placed them in the microwave for one minute to heat the mugs then emptying them prior to filling each mug with 400mL of boiling water. The ambient temperature of the room was 26 degrees Celsius. A starting temperature of 92 degrees Celsius was recorded and each mug was covered with the different materials being tested verifying that there were no spaces between the mug and the material for heat to escape.
My hypothesis was that the plastic lid would be the most effective insulator as it is less porous than the other materials thereby increasing the likelihood of the heat remaining trapped in the coffee mug. Since heat propagates across a gradient from higher temperatures to lower temperatures the heat from the 92 degree Celsius water wants to flow in direction of the 26 degree Celsius air surrounding the mug.
Upon analyzing the results (see data table below), I rejected my hypothesis that the plastic lid would be the best insulator of the materials tested. The data shows that the cardboard was the most effective insulator resulting in a decrease of 24 degrees Celsius over the 30 minute period compared with a 27 degree Celsius decrease in temperature for the plastic lid. My reasoning is that the plastic is a solid material with a dense arrangement of molecules which allow for the easy conduction of heat evenly throughout the material. The cardboard however has spaces of air between the layers which do not allow the heat to transfer as easily as the air molecules in the spaces are less dense making it difficult for the heat to transfer across the material. By comparison, the control mug with no material decreased 38 degrees Celsius allowing the heat to flow freely from the mug into the surrounding air. Overall, there was only a difference of 4 degrees Celsius between all of the materials used.
Perhaps letting the experiment run for an hour would produce a greater disparity between results of the materials thereby making the findings more conclusive. The temperature difference between the control mug and the other mugs with insulators becomes quite evident demonstrating that any material will retard the transfer of heat to some degree; however some materials are more effective than others.
Another variation for this experiment would be to use heated solids as opposed to liquids such as pancakes. Pancakes notoriously lose their heat quickly and an interesting investigation may be to see which materials are able to keep a pancake the hottest over a period of 30 minutes. An initial pancake temperature can be recorded then wrap each pancake with a different material and record a final temperature at the end of 30 minutes. I would expect any material with air pockets embedded in it would be the most effective, and those materials too porous or too solid would create a larger heat gradient for the transfer of the heat energy.
The most challenging thing for me during this investigation was ensuring all of my variables were as controlled as possible so as any temperature difference could be accounted for as a result of the material being used and not an outside factor such as heat loss through the mug, or varying thicknesses of the materials.
In conducting heat transfer experiments in my classroom using a structured inquiry format, students use three Styrofoam cups starting with different temperatures of water. One is the control using room temperature water, the second is ice water, and the third is boiling water. Students record the temperature of each cup every five minutes for an hour. After recording their data, the students create a triple line graph of their data, and they should find that the control cup does not change, the ice water cup should increase, and the boiling water should decrease. When the students analyze the graph they should come to the conclusion that if we were to extend the experiment over several hours or days, that the temperature of all three cups should be the same temperature of the air surrounding the cups due to heat wanting to reach a point of equilibrium between substances.
Temperature experiments are excellent activities in getting students to understand the concept of heat transfer as measurements can be easily recorded and results are fairly clear. These experiments also help reinforce the scientific process in making good observations, recording data, and drawing inferences and conclusions based on the results they achieve. Students have a common misconception that colder substances permeate hotter substances such as when they hold an ice cube in their hands, many will note that the cold from the ice goes into their hands making them feel colder. After conducting these experiments, the students are able to realize that no matter how heat is transferred, it will always flow from a region of higher temperature to a region of lower temperature.

Data Table
Material Temp 0 mins (oC) Temp 30 mins (oC) Temp Difference (oC)
Control (no material) 92 54 -38
Newspaper 3mm 92 67 -25
Plastic lid 3mm 92 65 -27
Cotton cloth 3mm 92 66 -26
Cardboard 3mm 92 68 -24
Ambient temperature surrounding the mugs is 26 oC.