|Figure 1: Evaporative cooling
|Figure 2: IR imaging
This experiment, which concerns evaporation of water, cannot be simpler: Just pour some room-temperature water into a plastic cup, leave it for a few hours, and then aim an IR camera at it. In stark contrast to the thermal background, the whole cup remains 1-2°C cooler than the room temperature (Figure 2). About how much water evaporation is enough to keep the cup this cool? Let’s do a simple calculation. Our measurement showed that in a typical dry and warm office environment in the winter, a cup of water (10 cm diameter) loses approximately six grams of water in 24 hours. That is to say, the evaporation rate is 7×10-5 g/s or 7×10-11 m3/s. Divided by the surface area of the cup mouth, which is 0.00785 m2, we obtain that the thickness of the layer of water that evaporates in a second is 8.9 nm—that is roughly the length of only 30 water molecules lining up shoulder to shoulder! It is amazing to notice that just the evaporation of this tiny amount of water at such a slow rate (a second is a very long time for molecules) suffices to sustain a temperature difference of 1-2°C for the entire cup.
This simple experiment actually raises more questions than it answers. Based on the latent heat of vaporization of water, which is about 2265 J/g, we estimate that the rate of energy loss through evaporation is only 0.16 J/s. This rate of energy loss should have a negligible effect on the 200 g of water in the cup as the specific heat of water is 4.186 J/(g×°C). So where does this cooling effect come from? How does it persist? Would the temperature of water be even lower if there is less water in the cup? What would the temperature difference be if the room temperature changes? These questions pose great opportunities to engage students to propose their hypotheses and test them with more experiments. It is through the quest to the answers that students learn to think and act like scientists.
IR imaging is an ideal tool for guided inquiry as it eliminates the tedious data collection procedures and focuses students on data analysis. In the practice of inquiry, data analysis is viewed as more important than data collection in helping students develop their thinking skills and conceptual understandings. Although this cooling effect can also be investigated using a thermometer, students’ perception might be quite different. An IR camera immediately shows that the entire cup, not just the water surface, is cooler. Seeing the bulk of the cup in blue color may prompt students to think more deeply and invite new questions, whereas a single temperature reading from a thermometer may not deliver the same experience.