Walk of Conduction
We want you to examine the reason certain materials feel colder than others, so we offer an experiment that involves a qualitative examination of materials chosen for their conductive properties.
- Aluminum Plate (sold as plates for thawing frozen foods)
- Aquarium thermometer
- Carpet or Astroturf
- Marble Pastry Board
- Piece of Cardboard
- Piece of Plywood
We know that heat energy will flow from hot to cold in an effort to reach the most stable (lowest energy) state. Yet the rate at which heat energy leaves a warmer body (here the feet of a science student) is felt as “coldness” in metals (good conductors) more clearly than it is in “warmer” feeling materials such as carpet and wood (poorer conductors of heat energy). The highlight in this particular experiment is the specially engineered “superconductive” plate made of an alloy (or solution of metals) that allows heat to move quickly through it. Our “Thawing Plate” feels ice cold to a science student’s bare foot because it allows heat energy to move swiftly through it. We use aquarium thermometers to show students that the materials are actually the same temperature, yet feel “warmer” or “cooler” depending on the rate at which heat energy passes from your bare feet through each material in the walk of conduction.
Extension: Quantify the rate at which heat energy moves through the “superconductive” plate by standing a push pin in a drop of melted wax or petroleum jelly. Then gently warm the opposite end of the metal plate over a tea candle and measure how long it takes for the heat of the candle to travel through the plate, melt the wax or petroleum jelly, causing the pin to fall over. Compare this time to that needed for the head energy to pass through the marble slab. For data on the combustible materials, you may want to dip one end of the sample in very hot water and measure the temperature increase with an aquarium thermometer on the far end of the sample. In porous materials, you may substitute a hair dryer in lieu of the hot water, with settings on high heat and low fan, taking care to aim the dryer at the bottom of the sample so as not to warm the thermometer directly.
Want some More?: Carefully glue thermosensitive paper to the top surface of the wood, cardboard and stone surfaces and use the color change to observe the rate at which the added heat energy moves through these materials when they are heated with the hair dryer.
Hidden Variables: Run this experiment in a shaded indoor location to eliminate radiant heat energy from interfering with your results. Students may also suggest painting each material the same color (I have used green, blue, gray) to normalize the rate at which they absorb any ambient infrared radiation.