Thermal Transfer Rates for Different Materials

Differential Heating of Materials

Lab Report in Geography

The heating of materials varies considerable depending on material composition and atmospheric conditions. A considerable amount of the sun's energy can be reflected back into the atmosphere, in a process called albedo (Lutgens & Tarbuck, 1998, p. 36). The average albedo rates for sand, mud, asphalt, and water are approximately 20-30%, 10%, 5-10%, and 3-80%, respectively, depending on the position of the sun relative to the surface of water (p. 40).

The process of heating materials will vary as well. For example, asphalt and dry sand would primarily use conduction for thermal transfer, while bodies of water or air would use convection (p. 30). On the other hand, thermal transfer for wet soil or mud has the added complexity of latent heating (p. 76). Latent heating or evaporation has a strong cooling effect on the liquid moisture remaining, a loss of 600 calories for every gram of water turned into vapor. Based on the above information, the materials that would heat the quickest when exposed to a heat source would be asphalt > sand > water > mud. This hypothesis assumes that the air is not saturated with water vapor.

Procedure

Containers filled with sand, water, asphalt, and wet garden soil (mud) were placed at the front fo the classroom (Jones, 2015, p. 35). Initial temperatures were recorded at 0 minutes, representing the ambient temperature. The heat lamps were then turned on and a countdown timer set for 30 minutes. Once the 30 minute period had ended the heat lamps were turned off. Air, surface, and internal temperatures were taken every 10 minutes for a total period of 50 minutes.

Results

The surface temperature of asphalt (Fig. 1) increased the greatest of the four materials analyzed. When exposed to the heat source the temperature increased from 30° to 65° C. within 50 minutes, but the internal temperature of the asphalt remained relatively stable and increased by only 8° C. The surface of sand (Fig. 2) and water (Fig. 3) increased at near equivalent rates, reaching a final temperature of 30° and 31° C, respectively, within the same time frame. Sand appears to have started at a lower temperature compared to water, but based on subsequent readings, sand and water probably started at almost the same temperature. The overall increase of the surface of sand and water during the 50 minute period of heating was about 6-7° C, while the internal temperature increased by 4° C. For both materials. The air temperature reading for sand at 20 minutes appears to be erroneous and therefore an outlier. By comparison, wet garden soil (mud) increased by only 4° C. within the same time frame and reached a final temperature of 24° C (Fig. 4). The internal temperature of the mud remained stable for the first 30 minutes of heat exposure and then increased by only 2° C. during the next 20 minutes.

Figure 1. Heating pattern of the surface of asphalt when exposed to a heat source. The temperature of the air, along with the internal temperature of the asphalt, was also recorded.

Figure 2. Heating pattern of the surface of sand when exposed to a heat source.

Figure 3. Heating pattern of the surface of water when exposed to a heat source.

Figure 4. Heating pattern of the surface of mud when exposed to a heat source.

Discussion

As predicted, mud was the least susceptible of the four materials tested to heating. This was probably due the role of latent heating, which transfers thermal energy from the liquid remaining in the material to the water transitioning into vapor. Exposure to heat has both a heating and cooling effect on wet soil, because the heat increases the rate of latent heating. The mechanisms of thermal transfer acting upon mud exposed to a heat source would include latent heating, convection of the liquid remaining in the soil, and conduction of heat by the soil itself. The results presented here support this conclusion.

The thermal transfer rate of asphalt was the fastest of the four materials, but most of the heat remained at the surface. The relatively cool internal temperature of asphalt suggests this material is not a good thermal conductor, when compared to iron for example, but conduction would have been the primary method of heating.