lect02 - Physics 227: Lecture 2 Coulombs Law,...

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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 signi±cantly 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 ±rst lecture. Note the force is negative for charges of opposite signs - this indicates the force is attractive. Friday, September 9, 2011
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q 1 q 2 More formally, with vectors, Coulomb’s law is: In using this deFnition, you need to recall the deFnition 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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This note was uploaded on 09/27/2011 for the course PHYSICS 750:227 taught by Professor Ronaldgilman during the Fall '11 term at Rutgers.

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lect02 - Physics 227: Lecture 2 Coulombs Law,...

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