zhang15 - 1/10/2010 Electric Forces and Electric Fields Two...

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1/10/2010 1 Electric Forces and Electric Fields Two types of charges exist They are called positive and negative Named by Benjamin Franklin The SI unit of charge is the Coulomb (C) Like charges repel; unlike charges attract Demo + - + + + -
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1/10/2010 2 Charge is quantized: all charge is a multiple of a fundamental unit of charge, symbolized by e Quarks are the exception Nature’s basic carrier of positive charge is the proton; nature’s basic carrier of negative charge is the electron Atoms are normally neutral, i.e., consist of equal amount of positive and negative changes If an electron is knocked out, what is left will be positively charged. In most cases, objects becomes charged because negative charges (i.e., electrons) are transferred from one object to another An atom Why most matter appear uncharged? How does an object get charged?
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1/10/2010 3 Don’t get confused with this conductor. Conductors: materials in which the electric charges move freely in response to an electric force Copper, aluminum, silver (most metals) are good conductors Some water solutions are also good conductors Stay away from water near downed electric lines! When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material All electric wires are made of good conductors
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1/10/2010 4 Insulators: materials in which electric charges do not move freely Example: Glass, rubber, dry wood Most plastics and ceramics When insulators are charged (e.g., by rubbing), only the rubbed area becomes charged There is no tendency for the charge to move into other
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zhang15 - 1/10/2010 Electric Forces and Electric Fields Two...

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