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3.1 | |
Copper Media | |
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Resistance and impedance |
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The materials through which current flows
offer varying amounts of opposition, or resistance to the movement of
the electrons. The materials that offer very little, or no,
resistance, are called conductors. Those materials that do not allow
the current to flow, or severely restrict its flow, are called
insulators. The amount of resistance depends on the chemical
composition of the materials.
All materials that conduct electricity have a measure of resistance to the flow of electrons through them. These materials also have other effects called capacitance and inductance associated with the flow of electrons. The three characteristics comprise impedance, which is similar to and includes resistance. The term attenuation is important when learning about networks. Attenuation refers to the resistance to the flow of electrons and why a signal becomes degraded as it travels along the conduit. The letter R represents resistance. The
unit of measurement for resistance is the ohm ( Electrical insulators, or insulators, are materials that allow electrons to flow through them with great difficulty, or not at all. Examples of electrical insulators include plastic, glass, air, dry wood, paper, rubber, and helium gas. These materials have very stable chemical structures, with orbiting electrons tightly bound within the atoms. Electrical conductors, usually just called conductors, are materials that allow electrons to flow through them with great ease. They flow easily because the outermost electrons are bound very loosely to the nucleus, and are easily freed. At room temperature, these materials have a large number of free electrons that can provide conduction. The introduction of voltage causes the free electrons to move, resulting in a current flow. The periodic table categorizes some groups of atoms by listing them in the form of columns. The atoms in each column belong to particular chemical families. Although they may have different numbers of protons, neutrons, and electrons, their outermost electrons have similar orbits and behave similarly when interacting with other atoms and molecules. The best conductors are metals, such as copper (Cu), silver (Ag), and gold (Au), because they have electrons that are easily freed. Other conductors include solder, a mixture of lead (Pb) and tin (Sn), and water with ions. An ion is an atom that has more electrons, or fewer electrons, than the number of protons in the nucleus of the atom. The human body is made of approximately 70% water with ions, which means that the human body is a conductor. Semiconductors are materials where the amount of electricity they conduct can be precisely controlled. These materials are listed together in one column of the periodic chart. Examples include carbon (C), germanium (Ge), and the alloy, gallium arsenide (GaAs). The most important semiconductor which makes the best microscopic-sized electronic circuits is silicon (Si). Silicon is very common and can be found
in sand, glass, and many types of rocks. The region around San Jose,
California is known as Silicon Valley because the computer industry,
which depends on silicon microchips, started in that area.
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). The symbol comes
from the Greek letter
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