Lecture28_Kim

# Lecture28_Kim - Physics 241 Lecture 28 Y E Kim December 8...

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Physics 241 Lecture 28 Y. E. Kim December 8, 2011 Review 1

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2 Chapter 21 Electric Field I: Discrete Charge Distributions (1) The electric force F between two charges, q 1 and q 2 , separated by a distance r is given by Coulomb’s Law: 1/r 2 dependence The constant k is called Coulomb’s constant and is given by: F k q 1 q 2 r 2 Opposite charges: F is attractive (-) Like charges: F is repulsive (+) k 8.99 10 9 Nm 2 C 2 or k 1 4  0 where 0 8.85 10 12 C 2 2 0 is called the permittivity constant .
The electric field lines from a point charge extend out radially For a positive point charge, the field lines point outward Terminate at infinity For a negative charge, the field lines point inward Originate at infinity 3D 2 () kq E x r r 2D Chapter 21 Electric Field I : Discrete Charge Distributions (2) 3

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Gauss’s Law: The net electric flux through any closed surface equals the net charge enclosed by that surface divided by 0 . How do we use this equation?? The above equation is TRUE always but it doesn’t look easy to use. BUT - It is very useful in finding E when the physical situation exhibits a lot of SYMMETRY . 0 enclosed E Q E ndA    Chapter 22 Electric Field II : Continuous Charge Distributions (1) 4
Charges and fields of a conductor The electric field must be perpendicular to the surface just outside a conductor, since, otherwise, there would be currents flowing along the surface. 0 E ( ) ( ) () E E end cap side end cap EA EA         Gauss’s Law 00 enc QA  0 enc E Q  0 E Chapter 22 Electric Field II Continuous Charge Distributions (2) 5 E 2 0

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Chapter 23 Electric Potential Change in the elecric potential energy: The electric potential: V= U/q For a point charge: V=kq/r Others Change in electric potential energy = -Work done by electric field is the initial electric potential energy is the final electric potential energy f i e i f U U U W U U   i V E ds “potential energy per unit charge of a test particle” 1 n i i VV 12 , , , x y z kq q V V V E E E U x y z r       6
Chapter 24 Capacitors Capacitance C: Parallel plate capacitor: Capacitance of a cylindrical capacitor of length L: Capacitance of a spherical capacitor: Capacitors in parallel: Capacitors in series: Capacitor filled with dielectric of dielectric constant : q CV 0 q E A 0 A C d   0 21 2 ln / L C rr  C 4 0 r 1 r 2 r 2 r 1 C eq C i i 1 n 1 11 n i eq i CC 2 0 1 2 uE air 7

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8 Chapter 25 Electric Currents and Direct Current Circuits (1) Current: Current density: For electrons: Resistivity: Resistance: Temperature dependence: dq i dt i J dA  i J A d J
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## This note was uploaded on 12/07/2011 for the course PHYS 241 taught by Professor Wei during the Fall '08 term at Purdue.

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Lecture28_Kim - Physics 241 Lecture 28 Y E Kim December 8...

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