13.4

Applications

To keep heating and air-conditioning bills to a minimum, it pays to use good thermal insulation in your home. Insulation inhibits convection between inner and outer walls and minimizes heat transfer by conduction. With respect to conduction, the logic behind home insulation ratings comes directly from Equation 13.1. According to this equation, the heat per unit time Q/t flowing through a thickness of material is . Keeping the value for Q/t to a minimum means using materials that have small thermal conductivities k and large thicknesses L. Construction engineers, however, prefer to use Equation 13.1 in the slightly different form shown below:

The term L/k in the denominator is called the R value of the insulation. For a building material, it is convenient to talk about an R value because it expresses in a single number the combined effects of thermal conductivity and thickness. Larger R values reduce the heat per unit time flowing through the material and, therefore, mean better insulation. It is also convenient to use R values to describe layered slabs formed by sandwiching together a number of materials with different thermal conductivities and different thicknesses. The R values for the individual layers can be added to give a single R value for the entire slab. It should be noted, however, that R values are expressed using units of feet, hours, F°, and BTU for thickness, time, temperature, and heat, respectively.

The physics of rating thermal insulation by R values.

The physics of regulating the temperature of an orbiting satellite.

When it is in the earth’s shadow, an orbiting satellite is shielded from the intense electromagnetic waves emitted by the sun. But when a satellite moves out of the earth’s shadow, the satellite experiences the full effect of these waves. As a result, the temperature within a satellite would decrease and increase sharply during an orbital period and sensitive electronic circuitry would suffer, unless precautions are taken. To minimize temperature fluctuations, satellites are often covered with a highly reflecting and, hence, poorly absorbing metal foil, as Figure 13-17 shows. By reflecting much of the sunlight, the foil minimizes temperature rises. Being a poor absorber, the foil is also a poor emitter and reduces radiant energy losses. Reducing these losses keeps the temperature from falling excessively when the satellite is in the earth’s shadow.

Figure 13-17 Highly reflective metal foil covering this satellite (the Hubble Space Telescope) minimizes temperature changes. (Courtesy NASA)

The physics of a thermos bottle.

The physics of a halogen cooktop stove.

A thermos bottle, sometimes referred to as a Dewar flask, reduces the rate at which hot liquids cool down or cold liquids warm up. A thermos usually consists of a doublewalled glass vessel with silvered inner walls (see Figure 13-18) and accomplishes its job by minimizing heat transfer via convection, conduction, and radiation. The space between the walls is evacuated to minimize energy losses due to conduction and convection. The silvered surfaces reflect most of the radiant energy that would otherwise enter or leave the liquid in the thermos. Finally, little heat is lost through the glass or the rubberlike gaskets and stopper, since these materials have relatively small thermal conductivities.

Figure 13-18 A thermos bottle minimizes energy transfer due to convection, conduction, and radiation.

Halogen cooktops use radiant energy to heat pots and pans. A halogen cooktop uses several quartz–iodine lamps, like the ones used for ultrabright automobile headlights. These lamps are electrically powered and are mounted below a ceramic top. (See Figure 13-19.) They radiate a great deal of electromagnetic energy, which passes through the ceramic top and is absorbed directly by the bottom of the pot. Consequently, the pot heats up very quickly, rivaling the time of a pot on an open gas burner.

Figure 13-19 In a halogen cooktop, quartz–iodine lamps emit a large amount of electromagnetic energy that is absorbed directly by a pot or pan.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.