Radiation

As mentioned above, the movement of energy from more active (warmer) areas to less active (cooler) areas occurs through radiation, conduction, and convection. The first of these, radiation, occurs when heat flows in electromagnetic waves from hotter surfaces through any medium, even the emptiness of outer space, to detached colder ones. With conduction, heat is transferred by contact directly from the molecules of warmer surfaces to the molecules of cooler surfaces. In convection, molecules of cooler air absorb the heat from warmer surfaces and then expand in volume, rise, and carry away the heat energy. Let's look at radiation first.

The internal energy that sets molecules vibrating sets up electromagnetic waves. Electromagnetic energy comes in many forms, including cosmic-ray photons, ultraviolet (UV) radiation, visible light, radio waves, heat, and electric currents, among others. Infrared (IR) radiation is made up of a range of longer, lower frequency wavelengths between shorter visible light and even longer microwaves. The sun's heat is mostly in wavelengths from the shorter, and hotter, end of IR radiation.

Infrared radiation is an invisible part of the light spectrum, and behaves exactly like light, that is, IR radiation travels in a straight line, doesn't turn corners, and can be instantly blocked by objects in its path. You can visualize whether radiated energy will be spread to or blocked from an object by checking whether the source object can "see" the other object through a medium that is transparent to light (air, a vacuum). Breaking the line of sight breaks the transmission path. For example, you will feel the radiated heat from a fireplace if you are sitting in a big chair facing the fire, but if you are behind the chair, the heat will be blocked (Fig. 15-2).

Your work as an interior designer can have a direct effect on how radiant heat is distributed in a space.

Buildings get heat in the shorter IR wavelengths directly from the sun. Buildings also receive thermal radiation from sun-warmed earth and floors, warm building surfaces, and even contact with human skin, all of which emit irradiation at much lower temperatures and at longer wavelengths. Radiation warms our skin when the sun strikes it, or when we stand near a fire. When we stand near a cold wall or under a cool night sky, radiation cools our skin. A cold window in a room usually has the greatest effect of draining radiated heat away from our bodies, making us feel colder. Closing the drapes blocks the heat transfer, and helps keep us warm.

Infrared electromagnetic waves are what emanate from an object and carry energy to all bodies within a direct line of sight of that object. The electromagnetic waves excite the molecules in the objects they hit, increasing the internal energy, and thereby raising the temperature. All objects give off heat in the form of IR electromagnetic radiation, and they all receive radiation from surrounding objects. When objects are close in temperature, the transfer of heat from the warmer to the cooler will be relatively slower than if there was a great difference in temperature. If two objects are at the same temperature, they will continue to radiate to each other, but no net exchange of heat takes place.

When electromagnetic waves contact an object or a medium, they are either reflected from the surface, absorbed by the material, or transmitted through the material. Different materials transmit some wavelengths of electromagnetic radiation, and reflect or absorb others. Materials that reflect visible radiation (light), such as shiny, silvered, or mirrored surfaces, also reflect radiant heat. Glass is transparent to visible light and to radiant energy from the hot sun, but is opaque to wavelengths of thermal radiation released by objects at normal earth temperatures. That is why the sun warms plants inside a glass greenhouse, but the heat absorbed and reradi-ated by the plants and soil can't escape back out through the glass, a situation known as the greenhouse effect. Radiation is not affected by air motion, so a breeze doesn't blow away the sunlight that pours down on your sunburned back at the beach.

Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment