Volume 6, No. 2, Fall 2002

Probing the Unseen World

Figure 1. In order to support continuing investigation into heat and temperature, we developed an ultra-fast response temperature probe. The sensing element is the very tiny bulb in the wire that is touching the ruler.

Figure 2.Top: A student measures the surface temperature of a finger as it is placed in contact with rigid foam, wood, and aluminum blocks - all at room temperature (24 °C).

Bottom: The graph shows the changes in surface
temperature.

Figure 3. Students hold the black surface of the radiant lollipop near an incandescent lamp to measure heat rise due to absorbed radiation.

Figure 4. The temperature of the radiant lollipop drops two degrees in 40 seconds when pointed at the sky on a clear night.

Figure 5. Top: A student holds the probe over the mouth of the copper tube while her hand warms the tube.

Bottom: the graph shows the temperature changes recorded.

Figure 6. Air temperature inside a sealed plastic bottle rises when the pressure is increased by squeezing the bottle.

Advanced Probe Captures Subtle Changes in Temperature

By Stephen Bannasch

When students use both sensor- and model-based computer visualizations, will they gain a deeper understanding of the difficult concepts of heat and temperature? Can we extend that learning experience to the investigation of conductivity and thermal gradients in different materials? Can we go even further to include the concepts of radiation and convection, the subtleties of which are far more difficult to grasp? These are some of the questions driving our research with middle school students as we work with them to conduct hands-on experiments in temperature and heat flow.

Through our many years of work on models and probeware, we have encouraged students to use these tools for understanding the world around them. Now we are excited about a new development that will help them understand natural phenomena on a much deeper level.

For the Data and Models project, we have created an ultra-fast response temperature probe (Figure 1). This new generation of highly sensitive probeware enables students to easily conduct investigations into heat and temperature phenomena that we only imagined possible before now.

Thermal Conductivity...from our skin's perspective

The first heat and temperature sensor most of us use is our skin. When we make a snowball in the winter or walk over hot beach sand in the summer, our brain uses information relayed by nerve cells under our skin to decide whether something is too hot, too cold, or just right. These qualitative judgments are based on the temperature of the nerve cells and how quickly that temperature is changing.

When you quickly let go of the handle of a pot that is too hot to hold, your brain has interpreted the quick rise in temperature sensed by the nerve cells under the skin as an alarm. In effect, your fingers sense the rate of heat flow into them. A much greater alarm is elicited by an object that raises your skin temperature to 50 degrees in one second than by an object that does the same in 30 seconds.

A classic heat and temperature misconception many students have is that metal is colder than wood. For example, take three similar small blocks of aluminum, wood, and rigid foam. Even though they are the same temperature, the aluminum block will feel cool to the touch while the foam block feels warm.

The graph in Figure 2 shows the surface temperature of a finger (32 °C) as it is placed in contact with rigid foam, wood, and alum-inum blocks at room temperature (24 °C). While touch-ing the foam and wood blocks, skin temperature has reached a warmer equilibrium. However, when touching the aluminum block, skin temperature quickly dropped three degrees and was still cooling after 30 seconds. Heat from the finger was able to flow more quickly throughout the aluminum block because of aluminum's greater thermal conductivity when compared to wood or foam.

The ultra-fast response temperature sensor allows rapid investigations that help students tease apart the concepts of thermal conductivity and heat capacity in different materials. While the blocks are the same temperature, the graph in Figure 2 clearly shows skin temperature getting colder when touching the aluminum block. Yet, from our finger's point of view, the aluminum block is clearly colder because, after the initial warming of the aluminum surface, the heat from our finger continued to flow quickly into the aluminum block.

In order to help students develop more robust mental models, we combined sensor- and model-based visualizations in the software the students used. Students created simple virtual thermodynamic systems on the iPaq handheld computers using blocks of different thermal conductivities along with heaters and coolers. The model shows the temperature gradient within the blocks. You can use the Heat Flow applet on our Web site to experiment with a similar modeling tool. See Monday's Lesson in this issue for an introduction to that tool.

Radiant Lollipops and Thermal Radiation

We also encourage students to investigate the thermal effects of radiation on the objects around them. The surface of the probe's sensing tip reflects both visible and near-IR radiation. The temperature rise when we hold the probe within six inches of a 100 watt incandescent lamp is less than 0.1 °C. The radiant energy from the lamp does not warm the probe as you might expect, because the radiation is reflected rather than absorbed.

In order to experiment with the thermal effects of radiant energy, we devised a simple instrument we call the radiant lollipop (Figure 3). It is, in effect, a broadband radio-meter.

The small piece of rigid foam insulation is covered on one side by regular weight aluminum foil with the shiny side up. The other side is covered by aluminum foil painted flat black with high-temperature stove paint. The sensing tip of the probe is carefully slid behind the aluminum foil. Students find that it is quite difficult to raise the temperature of the shiny side of the lollipop. However, the black side is exquisitely sensitive to incoming radiation.

The equilibrium temperature reached by the black side is affected by two phenomena. These are conductivity with the surrounding air and the balance of emitted and absorbed radiant energy. When the black foil is at the same temperature as nearby emitting surfaces (such as the walls in a room), the radiant energy emitted by the black foil is closely balanced by the energy absorbed. This causes no net change in the temperature of the foil. However, if the black foil is pointed at the sun, a bright lamp, or even a warm hand, the absorbed incoming radiation exceeds the radiation that is emitted. This causes the temperature of the foil to rise until the heat lost (through conduction to the air) balances the additional energy absorbed from radiation.

The radiant lollipop can also be used to indicate the lack of radiant energy. During a clear night the radiant energy coming from the night sky is very small. By taking the radiometer outside and first pointing it at the ground and then at the sky, it indicates that the sky is cooler than the ground (Figure 4).

Hot Hands and Thermal Convection

The ultra-fast response probe can also be used for measuring the small, and often ephemeral, temperature differences associated with convective flow. Convective heat transfer is the movement of a gas or liquid propelled by buoyancy. This buoyancy is associated with density changes caused by temperature differences. The graph in Figure 5 shows the air temperature directly above a copper tube that is held firmly in the student's hand. Within seconds of grasping the copper tube, her hand warmed the tube and the air inside. This lighter air then flowed up the tube and was detected by the probe.

With this probe students can measure the thickness of a film of air falling down a cold window, the extent of a plume of hot air leaving a television, or even the effect of pouring cold air from a chilled bowl into another bowl.

Pressure, Volume, and Temperature

We can also plot the increase in air temperature caused by increasing pressure. We placed the sensor in a sealed plastic soda bottle and squeezed it hard, reducing its volume and increasing the pressure. The graph in Figure 6 shows the air temperature rising 14 °C. As soon as the pressure was removed, the temperature dropped back close to room temperature. In this experiment temperature variation is caused by changes in density, while in convection the density varies due to changes in temperature.

The simple experiments described in this article are either impossible or impractical with the temperature probes currently used in schools. A probe that responds this quickly will allow the kind of deep explorations that help students understand heat and temperature as indicators of physical and chemical processes. Research has shown that students who use data from probes to analyze experimental results gain a deeper understanding of science concepts. We must supply them with the most appropriate probes for the tasks.

For more details of these and other interesting experiments, including infor-mation on how we made this probe, as well as our research with middle school students, visit the Data & Models project Web site.

Stephen Bannash (stephen@concord.org) is Director of Technology at the Concord Consortium

Article Links & Notes

Data & Models - http://www.concord.org/data-models/

To learn about the interface hardware and the open source software (CCProbe) that allows probe visualization on PalmOS, PocketPC, MacOS, Windows, and Linux - http://www.concord.org/ccprobeware/

In the Spring 2001 issue of @concord, we described our thermal conductivity block system for investigating heat flow - http://www.concord.org/newsletter/2001spring/evidence.html

To read about our research in the middle school classroom, see Barbara Buckley's article, "Investigating Heat Flow at the Hot & Cold Club: Looking Closely at Students' Mental Models" - http://www.concord.org/data-models/hot-cold-club.html

This research is supported by National Science Foundation grant #REC99-73179.

The projects described in this newsletter are supported by grants from the National Science Foundation, the U.S. Department of Education, the Noyce Foundation and others. All opinions, findings, and recommendations expressed herein are those of the authors and do not necessarily reflect the views of the funding agencies. Mention of trade names, commercial products or organizations does not imply endorsement.

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