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# chap19 - PHYS 1302 Introductory Physics II Chapter 19...

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1 PHYS 1302 Introductory Physics II Chapter 19 Electric charges, forces, and fields Lecture pdf for chapter 19 is available on Webct

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7/6/09 LAU/PHYS 1302 Chapter 19 12-2 Topics square6 Electric charge square6 Coulomb’s Law (quantitative) square6 Electric field (quantitative) square6 Electric lines of force square6 Gauss’s Law
7/6/09 LAU/PHYS 1302 Chapter 19 12-3 Static Electricity

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7/6/09 LAU/PHYS 1302 Chapter 19 12-4 The nature of charge SI unit for charge is coulomb (C) e = 1.60 × 10 - 19 C Charge is quantized q = ± ne All building blocks of matter are charged or neutral q e = - e q proton = + e q neutron = 0 Number of electrons in 1 C 1 C 1.6 × 10 - 19 C / electron = 6.25 × 10 18 electrons
7/6/09 LAU/PHYS 1302 Chapter 19 12-5 Electrification Elektron: Greek for Amber. Charge is conserved.

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7/6/09 LAU/PHYS 1302 Chapter 19 12-6 Demo on electrification QuickTime™ and a Cinepak decompressor are needed to see this picture.
7/6/09 LAU/PHYS 1302 Chapter 19 12-7 Electric forces Negative charge: amber (when rubbed against fur) Like charges repel Unlike charges attract Positive charge: glass (when rubbed against silk)

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7/6/09 LAU/PHYS 1302 Chapter 19 12-8 Coulomb’s Law F = k q 1 q 2 r 2 k = 1 4 πε 0 = 8.99 × 10 9 N m 2 / C 2 r F 12 = - r F 21 SI unit for charge: coulomb (C)
7/6/09 LAU/PHYS 1302 Chapter 19 12-9 Electric forces, compared to gravity, are very strong! Electric force between a proton and an electron F q = k q 1 q 2 r 2 = (8.99 × 10 9 N m 2 / C 2 ) × (1.6 × 10 - 19 C ) × (1.6 × 10 - 19 C ) (1.0 m ) 2 = 2.3 × 10 - 28 N The ratio of electric force to gravity = F q F g = 2.3 × 10 - 28 N 1.0 × 10 - 67 N = 2.2 × 10 39 ! Force due to gravity F g = G m 1 m 2 r 2 = (6.67 × 10 - 11 N m 2 / kg 2 ) × (1.67 × 10 - 27 kg ) × (9.10 × 10 - 31 kg ) (1.0 m ) 2 = 1.0 × 10 - 67 N

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7/6/09 LAU/PHYS 1302 Chapter 19 12-10 Principle of Superposition Electric forces are vectors !
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chap19 - PHYS 1302 Introductory Physics II Chapter 19...

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