Phys2212_L02 - Physics 2212 Electricity and Magnetism...

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Physics 2212 Electricity and Magnetism Lecture 2 Coulomb’s Law and the Electric Field
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10/06/09 Physics 2212 - Lecture 2 2 Recall : Coulomb’s Law Like charges repel. Opposite charges attract. 1 2 1 on 2 2 on 1 2 q q F F K r = = Charles Augustine de Coulomb (1736-1806). Coulomb’s Law: ( Magnitude of force )
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10/06/09 Physics 2212 - Lecture 2 3 Recall : Units of Charge 9 2 2 9 2 2 8.99 10 N m /C 9.0 10 N m /C K = × 2245 × -12 2 2 0 1 8.85 10 C /(N m ) permittivity constant 4 K ε π = = × = 1 2 1 2 2 2 0 1 4 q q q q F K r r πε = = -9 coulomb SI unit of charge; 1.0 1.0 10 C nC C = = = × (Note that in Newton’s law of gravitation, G , which plays a role similar to K , has the value G = 6.67 × 10 -11 N m 2 /kg 2 .)
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10/06/09 Physics 2212 - Lecture 2 4 Recall : Using Coulomb’s Law 1. Coulomb’s Law applies only to point charges . (This is particularly important because charge are free to move around on conductors.) 1. Strictly speaking, Coulomb’s Law applies only to electrostatics (non-moving charges). (However, it is usually OK provided v<<c). 1. Electrostatic forces (vectors!) can be superposed. Linear superposition !!!! net 1 on j 2 on j 3 on j F F F F = + + + r r r r L
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10/06/09 Physics 2212 - Lecture 2 5 Two +10 nC charged particles are 2 cm apart on the x axis. (1) What is the net force on a +1.0 nC particle midway between them? (2) What is the net force if the charged particle on the right is replaced by a -10 nC charge? Example : Sum of Two Forces ( ) 1 on 3 2 on 3 ˆ ˆ 0 net F F F F i F i + = + = - = r r r 1 2 2 8 8 9 2 2 2 2 -4 (1.0 10 )(1.0 10 ) (9.0 10 N m /C ) (1.0 10 ) 9.0 10 N q q F K r C C m - - - = × × = × × = × ( ) 1 on 3 2 on 3 -3 ˆ ˆ ˆ 2 1.8 10 N net F F F F i F i F i +- = + = + = = × r r r 8 8 9 2 2 1 2 2 2 2 -4 (1.0 10 )(1.0 10 ) (9.0 10 N m /C ) (1.0 10 ) 9.0 10 N q q C C F K r m - - - × × = = × × = × 9 -9
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10/06/09 Physics 2212 - Lecture 2 6 Example : Point of Zero Force Two positively charged particles q 1 and q 2 =3q 1 are placed 10. 0 cm apart. Where (other than infinity) could another charge q 3 be placed so as to experience no net force? Need force vectors to be co-linear, so location must be on x axis.
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This note was uploaded on 06/07/2009 for the course PHYSICS 2212 taught by Professor Geist during the Fall '09 term at Georgia Perimeter.

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Phys2212_L02 - Physics 2212 Electricity and Magnetism...

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