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The Science of Heat Transfer: What Is Conduction?
Heat is an interesting form of energy. Not only does it sustain life, make us comfortable and help us prepare our food, but understanding its properties is key to many fields of scientific research. For example, knowing how heat is transferred and the degree to which different materials can exchange thermal energy governs everything from building heaters and understanding seasonal change to sending ships into space.
Heat can only be transferred through three means: conduction, convection and radiation. Of these, conduction is perhaps the most common, and occurs regularly in nature. In short, it is the transfer of heat through physical contact. It occurs when you press your hand onto a window pane, when you place a pot of water on an active element, and when you place an iron in the fire.
This transfer occurs at the molecular level — from one body to another — when heat energy is absorbed by a surface and causes the molecules of that surface to move more quickly. In the process, they bump into their neighbors and transfer the energy to them, a process which continues as long as heat is still being added.
The process of heat conduction depends on four basic factors: the temperature gradient, the cross section of the materials involved, their path length, and the properties of those materials.
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes in a specific location. Temperature always flows from the hottest to coldest source, due to the fact that cold is nothing but the absence of heat energy. This transfer between bodies continues until the temperature difference decays, and a state known as thermal equilibrium occurs.
Cross-section and path length are also important factors. The greater the size of the material involved in the transfer, the more heat is needed to warm it. Also, the more surface area that is exposed to open air, the greater likelihood for heat loss. So shorter objects with a smaller cross-section are the best means of minimizing the loss of heat energy.
Last, but certainly not least, is the physical properties of the materials involved. Basically, when it comes to conducting heat, not all substances are created equal. Metals and stone are considered good conductors since they can speedily transfer heat, whereas materials like wood, paper, air, and cloth are poor conductors of heat.
These conductive properties are rated based on a “coefficient” which is measured relative to silver. In this respect, silver has a coefficient of heat conduction of 100, whereas other materials are ranked lower. These include copper (92), iron (11), water (0.12), and wood (0.03). At the opposite end of the spectrum is a perfect vacuum, which is incapable of conducting heat, and is therefore ranked at zero.