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Do Not Foil Your Teaching Career, Know This For The Cset

May 13, 2008
Teacher candidates studying for the CSET should be able to describe the physics of aluminum foil. This type of question will appear in the Subtest II Science portion of the CSET.

A question asking you to describe the physics of aluminum foil has a very high chance of being on your CSET. Understanding the physics of foil involves knowing about many different types of scientific principles such as: conduction, convection, radiation, emissivity, and absorption.

Depending on which version of the CSET test you get, you may not have the aluminum foil question. But I can guarantee you will have a question on heat transfer no matter which version of the CSET exam you receive. Heat transfer is a core knowledge requirement for all science exams.

There are three modes of heat transfer: conduction, convection, and radiation. Of the three, radiation is the primary mode; conduction and convection are secondary and come into play only as matter interrupts or interferes with radiant heat transfer. As the matter absorbs radiant energy, it is heated, develops a difference in temperature, and results in molecular motion (conduction in solids) or mass motion (convection in liquids and gas).

All substances, including air spaces, building materials, such as wood, glass and plaster, and insulation, obey the same laws of nature, and transfer heat. Solid materials differ only in the rate of heat transfer which is mainly affected by differences in: density, weight, shape, permeability and molecular structure, Materials which transfer heat slowly can be said to resist heat flow.

Conduction is direct heat flow through matter (molecular motion). It results from actual physical contact of one body with another. For example, if one end of an iron rod is heated, the heat travels by conduction through the metal to the other end; it also travels to the surface and is conducted to the surrounding air which is another, but less dense, body. An example of conduction through contact between two solids is a cooking pot on the solid surface of a hot stove. The greatest flow of heat possible between materials is where there is direct conduction between solids. Heat is always conducted from warm to cold; never from cold to warm, and always moves via the shortest path of least resistance.

Convection is the transport of heat within a gas or liquid, caused by the actual flow of material itself (mass motion). In building spaces, natural convection heat flow is mostly upward, not downwards. This is called "free convection".

For example, a warm stove, person, floor, wall, etc., loses heat by conduction to the cooler air in contact with it. This added heat activated (warms) the molecules of the air which expand, becoming less dense, and rise. Cooler, heavier air rushes in from the side and below to replace it. Notice the smoke that rises from a cigarette. The motion is upward, with a component of sideways motion. Convection may also be mechanically induced by a fan. This is called "forced convection".

Radiation is the transmission of electromagnetic rays through space. Radiation is invisible. Infrared rays occur between light and radar waves. Thus, when we speak of radiation, we refer only to infrared rays. Each material whose temperature is above absolute zero emits infrared radiation, including: the sun, icebergs, stoves or radiators, humans, animals, furniture, ceilings, walls, floors, etc.

All objects radiate infrared rays from their surfaces in all directions, until they are reflected or absorbed by another object. Traveling at the speed of light, these rays are invisible, and they have no temperature, only energy. Heating an object excites the surface molecules, causing them to give off infrared radiation. When these infrared rays strike the surface of another object, the rays are absorbed, and only then is heat produces in the object. This heat spreads throughout the mass by conduction. The heated object then transmits infrared rays from exposed surfaces by radiation, if these surfaces are exposed directly to an air space.

The amount of radiation emitted is a function of the emissivity factor of the source's surface. Emissivity is the rate at which radiation (emission) is given off. Absorption of radiation of an object is proportional to the absorptivity factor of its surface which is reciprocal of its emissivity.

The surface of aluminum has the ability not to absorb, but reflect, 95% of the infrared rays which strike it. Since aluminum has such a low mass to air ration, very little conduction can take place, particularly when only 5% of the rays are absorbed.

Heat control with aluminum foil is made possible by taking advantage of its low thermal emissivity and the low thermal conductivity of the air. It is possible with layered foil and air to practically eliminate heat transfer by radiation and convection: a fact employed regularly by the NASA space program. In the space vehicle Columbia, ceramic tiles are imbedded with aluminum bits which reflect heat before it can be absorbed. "Moon suits" are made of reflective foil surfaces surrounding trapped air for major temperature modification.

Aluminum Foil, with its reflective surface can block the flow of radiation. Most aluminum insulation has only a 5% absorption and emissivity ratio. It is impervious to water vapor and convection currents, and reflects 95% of all radiant energy which strikes its surface.
About the Author
Peter Lorison is a CSET coach who writes for Ace the CSET, because everyone (even Tiger Woods) needs a coach.
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