p240_ct6_f07 - Physics 240 Fall 2007 Lecture #6 Dr. Dave...

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Physics 240 Fall 2007 Lecture #6 Dr. Dave Winn 2405 Randall Lab winn@umich.edu
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Reminders from last time • In electrostatics, positive charges are ‘sources’ of electric field, negative charges ‘sinks’ • Gauss’ law relates the electric flux through a surface to the total charge contained inside. It can be used to determine electric fields in cases of sufficient symmetry. ∫∫ = = Φ 0 ε inside q A d E G G
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A point charge is placed at the center of a spherical Gaussian surface and the electric flux through the surface is found to be 435 Nm 2 /C. Now the charge is moved off-center by half the radius of the Gaussian sphere. Does the electric flux: 1. Increase by 2x 2. Decrease by ½ 3. Increase by 4x 4. Decrease by ¼ 5. Stay the same
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Another example: charged plate • Consider a plate charged with surface charge density + σ • What is the electric field at a distance r from the plate? • Pick a Gaussian surface which is a cylinder extending on both sides.
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Another example: charged plate • E must be parallel to A on both ends, and constant across A 0 0 0 2 2 ε σ = = = Φ = = Φ ∫∫ E A EA q A d E inside G G
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Two infinite, insulating sheets of charge (densities + σ and +2 σ ) lie parallel to each other, as shown. What are the electric field values E in units of σ / ε 0 at the points shown (define positive towards the right)? + + + + + + + + + + + + + + A BC A B C 1. -3/2 -1/2 +3/2 2. -1/2 +1/2 +1/2 3. -3 +1/2 +3 4. +3/2 +1/2 +3/2
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Gauss’ law and conductors • Inside conductors we know the electric field is zero • Consider any Gaussian surface inside the conductor • Since field is zero, flux must be zero, and charge inside must be zero; all charge must be at the surface of a conductor.
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This note was uploaded on 04/04/2008 for the course PHYSICS 240 taught by Professor Davewinn during the Fall '08 term at University of Michigan.

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p240_ct6_f07 - Physics 240 Fall 2007 Lecture #6 Dr. Dave...

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