# PHYS102Lec6 - Chapter 15 Electric Forces and Electric...

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Chapter 15 Electric Forces and Electric Fields Dr. Armen Kocharian

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First Observations Greeks Observed electric and magnetic phenomena as early as 700 BC Found that amber, when rubbed, became electrified and attracted pieces of straw or feathers Also discovered magnetic forces by observing magnetite attracting iron
Benjamin Franklin 1706 1790 Printer, author, founding father, inventor, diplomat Physical Scientist 1740’s work on electricity changed unrelated observations into coherent science

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Properties of Electric Charges Two types of charges exist They are called positive and negative Named by Benjamin Franklin Like charges repel and unlike charges attract one another Nature’s basic carrier of positive charge is the proton Protons do not move from one material to another because they are held firmly in the nucleus
More Properties of Charge Nature’s basic carrier of negative charge is the electron Gaining or losing electrons is how an object becomes charged Electric charge is always conserved Charge is not created, only exchanged Objects become charged because negative charge is transferred from one object to another

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Properties of Charge, final Charge is quantized All charge is a multiple of a fundamental unit of charge, symbolized by e Quarks are the exception Electrons have a charge of e Protons have a charge of +e The SI unit of charge is the Coulomb (C) e = 1.6 x 10 -19 C
Conductors Conductors are materials in which the electric charges move freely in response to an electric force Copper, aluminum and silver are good conductors When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material

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Insulators Insulators are materials in which electric charges do not move freely Glass and rubber are examples of insulators When insulators are charged by rubbing, only the rubbed area becomes charged There is no tendency for the charge to move into other regions of the material
Semiconductors The characteristics of semiconductors are between those of insulators and conductors Silicon and germanium are examples of semiconductors

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Charging by Conduction A charged object (the rod) is placed in contact with another object (the sphere) Some electrons on the rod can move to the sphere When the rod is removed, the sphere is left with a charge The object being charged is always left with a charge having the same sign as the object doing the charging
Charging by Induction When an object is connected to a conducting wire or pipe buried in the earth, it is said to be grounded A neutral sphere has equal number of electrons and protons

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Charging by Induction, 2 A negatively charged rubber rod is brought near an uncharged sphere The charges in the sphere are redistributed Some of the electrons in the sphere are repelled from the electrons in the rod
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## This note was uploaded on 03/07/2011 for the course PHYS 102 taught by Professor Armenn.kocharian during the Winter '11 term at California State University Los Angeles .

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PHYS102Lec6 - Chapter 15 Electric Forces and Electric...

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