What is the Difference between Heat Capacity and Emissivity?

Heat Capacity: Let’s take water as the most familiar example of a material that has a relatively high heat capacity.

When we heat water, we’re pumping calories of heat into it; its temperature will therefore rise. Temperature is a measure of how fast the molecules are moving. Because water molecules stick quite tenaciously to one another (by dipole-dipole attraction and hydrogen bonding), it’s relatively difficult to goose them into moving faster.

We have to add a whole (nutritional) calorie of heat in order to raise the temperature of a kilogram (a liter) of water by a single degree Celsius. (That is, the specific heat of water is one kilocalorie per kilogram per degree C.) Conversely, when water cools, it has to lose a lot of heat, that same one nutritional calorie per kilogram, for its temperature to be reduced by a single Celsius degree.

A couple of consequences of these facts are that (i) it takes “forever” for a heated pot of water to come to a boil, and (ii) a body of water, such as a large lake or an ocean, moderates the surrounding climate by refusing to heat up or cool down as easily as the land does.

• Emissivity: In any environment above absolute zero in temperature, and that includes all environments, there is infrared radiation flying through the space.

When such radiation strikes a surface, the molecules in that surface absorb some of it. They exhibit the fact that they now contain more energy by moving more agitatedly: twisting, rotating, and tumbling like a hyperactive kindergarten class during a Ritalin shortage. Each kind of molecule has its own unique ways of rotating and tumbling, corresponding to the unique, characteristic sets of energies that it is capable of absorbing. (That is, different molecules have different inf rared absorption spectra.)

After absorbing the radiant energy, the excited molecules “calm down” by re-emitting some of it. Some kinds of molecules re-emit virtually all the energy they had absorbed, while others retain some, converting it into different forms of energy. A substance that re-emits 100 percent of the energy it absorbs is said to have an emissivity of 1.00. (In Techspeak, it behaves like a black-body radiator.)

In general, metals have very low emissivities because their loose electrons can soak up the energy like a sponge.

Aluminum, for example, re-emits only 5 percent of the infrared radiation that strikes it; copper, only 2 percent. In contrast, materials such as stone and brick re-emit virtually all of the radiation they absorb: 90 percent for dark brick, 93 percent for marble, 97 percent for tile; that is, their emissivities are 0.90, 0.93, and 0.97, respectively.

That’s because the molecules in these substances are fixed rigidly in place, and can’t retain the energy by oscillating and tumbling. In these materials, very little infrared energy is wasted; almost all of the infrared radiation that strikes these stone-like surfaces is re-emitted toward the food.