Lect06-Phys172s10-(3.1-3.5-Fundamental_forces,_Ball-spring_model)

# Lect06-Phys172s10-(3.1-3.5-Fundamental_forces,_Ball-spring_model)

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Spring 2010 PHYS 172: Modern Mechanics Lecture 6 – Fundamental Forces, ball-spring model Read 3.6 – 3.10, 4.1-4.7 Electric force: the electric charges Charges: property of an object • Two types: positive (+) and negative (-) Likely charges: repel. • Opposite charges: attract • Net charge of a system: algebraic sum of all the charges • Conservation of charge • The force exerted by one point charge on another acts along the line joining the charges Charge: measured in C (Coulomb) Elementary charge: e = 1.602 10 -19 C Charge of electron is – e , of a proton + e

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The electric force law (Coulomb’s law) q 2 q 1 2 1 elec on 2by1 2 1 2 0 2 1 1 ˆ 4 q q F r r πε - - = & & 2 1 r - & 2 1 ˆ r - on2by1 elec F & on1by2 elec F & Coulomb’s law 2 9 2 0 1 N×m 9 10 4 C = × Electric force versus gravity Gravity (Cavendish, 1798) 2 2 1 r m m G F g = G = 6.67 10 -11 m 3 /(kg . s 2 ) Electric force (Coulomb, 1795) 1 2 2 0 1 4 e q q F r = 1/(4 0 ) = 8.99 10 9 Nm 2 /C 2 Clicker question: A) Gravity Which of the two forces is in general stronger? B) Electrical 3 m kg m m 70 2 1 = = N 000000036 . 0 = g F 3 m C 10 6 . 1 10 9 28 2 1
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## This note was uploaded on 02/07/2010 for the course PHYS 17200 taught by Professor Gaboracsathy during the Spring '09 term at Purdue.

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Lect06-Phys172s10-(3.1-3.5-Fundamental_forces,_Ball-spring_model)

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