electrons_and_conductors

electrons_and_conductors - Electrons and Conductors Atoms...

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1 Electrons and Conductors Atoms consist primarily of electrons, protons, and neutrons. A modified Bohr model of the atom is shown below. There are two types of charge, labeled positive (+) and negative (-), with the property that “like” charges repel, and “unlike” charges attract. Electrions are negatively charged, protons are positively charged and neutrons carry no charge. In Bohr’s model, the pull of the protons keeps the electrons from flying away. The laws of quantum physics say that electrons orbiting a nucleus can only travel in specific “bands” or “shells”. In addition, each shell has a limit as to how many electrons it can hold. The electrons orbiting in the shells closest to the nucleus have the least energy so those shells are normally filled. Most atoms have outer shells that are not fully filled. The force keeping the electrons in orbit is inversely proportional to the square of the distance to the protons in the nucleus. Thus elec- trons can be added and taken away most easily from the outermost shells. In a conductor , the outer electrons of the atoms are loosely bound and free to move through the material. In fact, the electrons are so mobile, that they are sometimes described as an electron gas, in which the atomic nuclei rest. The best naturally occurring conductor is silver. Copper is almost as good at conducting as sil- ver, and it costs a lot less. Therefore, copper is the most popular material used in electrical wiring. Most metals are good electrical conductors, most nonmetals are not. The resistivity of a metal is a measure of its opposition to the flow of electrons. The electrical resistance of a wire is greater for a longer wire, less for a wire of larger cross sectional area, and depends upon the material the wire is made of. Generally speaking, the resistance of a wire can be expressed as: e e e e e electron proton neutron Modified Bohr’s model of the atom. nucleus orbital electrons composed of a cluster of protons and neutrons e e e outer shell R = L A ρ ρ is resistivity L is the length of the conductor A is the cross sectional area of the conductor where
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2 The factor in the resistance which takes into account the nature of the material is the resistiv- ity. Although it is temperature dependent, it can be used at a given temperature to calculate the
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electrons_and_conductors - Electrons and Conductors Atoms...

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