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# lect02 - Physics 227 Lecture 2 Coulombs Law Superposition...

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Physics 227: Lecture 2 Coulomb’s Law, Superposition, Electric Fields, Field Lines Lecture 1 review: All your questions are answered on class web pages: Sakai: 01:750:227 or http://www.physics.rutgers.edu/ugrad/227 Separate charges by rubbing (appropriate materials). There are two types of charges: + and -. Opposites signs attract, same sign repel. Conductors conduct, insulators do not. Charged objects ``polarize’’ uncharged insulators or conductors, leading to an attractive force. Friday, September 9, 2011

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Coulomb’s Law - force between two charges q 1 q 2 F 1 on 2 = k q 1 q 2 / r 2 Consider two point charges q 1 and q 2 which have the same sign. The force is along the line connecting the charges. F 2 on 1 = k q 1 q 2 / r 2 k = 8.99x10 9 N . m 2 /C 2 or k = 1/4 π ε 0 with ε 0 = 8.854x10 -12 C 2 /N . m 2 permittivity of free space q 1 q 2 F 1 on 2 = k q 1 q 2 / r 2 If q 1 , q 2 have opposite signs: F 2 on 1 = k q 1 q 2 / r 2 Note: until further notice, we are dealing with point charges, or with charge distributions that we assume are not significantly affected by the presence of other external charges. We ignore polarization effects such as we saw between a charged rod and uncharged insulators or conductors in the first lecture. Note the force is negative for charges of opposite signs - this indicates the force is attractive. Friday, September 9, 2011
q 1 q 2 More formally, with vectors, Coulomb’s law is: In using this definition, you need to recall the definition of the direction of the unit vector as going from q 1 to q 2 . Thus the force of q 1 on q 2 is in the direction from q 1 to q 2 , if q 1 and q 2 have the same sign. Coulomb’s Law - force between two charges Consider two point charges q 1 and q 2 which have the same sign. The force is along the line connecting the charges.

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lect02 - Physics 227 Lecture 2 Coulombs Law Superposition...

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