241-22_lec-5 - Physics 241 Lecture 5 Y. E. Kim September 7,...

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Physics 241 Lecture 5 Y. E. Kim September 7, 2010 Chaper 22 September 7, 2010 University Physics, Chapter 22 1
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September 7, 2010 University Physics, Chapter 22 2 Review - Gauss’s Law Gauss’s Law (named for German mathematician and scientist Johann Carl Friedrich Gauss, 1777-1855) states (q = net charge enclosed by Gaussian surface S). If we add the definition of the electric flux we get another expression for Gauss’ Law Gauss’s Law: the electric field flux through S is proportional to the net charge enclosed by S   q 0 0 q E dA   E dA  
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Review - Charges and fields of a conductor In electrostatic equilibrium, free charges inside a conductor do not move. Thus, E = 0 everywhere in the interior of a conductor. • Since E = 0 inside, there are no net charges anywhere in the interior by Gauss’s Law. Net charges can only be on the surface(s). The electric field is perpendicular to the surface(s). E
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Reviw - Cavity in a spherical conductor q Total induced charge -q • With +q in the cavity of a conductor, a net charge –q is induced on the inside surface of the cavity. • If +q is at the center of the spherical cavity, E is radial by symmetry. Uniform charge distributions. As long as the E contribution due to all the interior charges, measured at the outer surface is zero, then moving the +q charge does not affect the exterior surface charge distribution. Electrostatic shielding
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Quiz Are you here ? a) Yes b) No
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September 7, 2010 University Physics, Chapter 22 6 Pointed Surfaces Now consider a sharp point . The field lines are much closer together, i.e., the field is much stronger and looks much more like the field of a point charge. Efield is always perpendicular to the surface of a charged conductor
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September 7, 2010 University Physics, Chapter 22 7 Demo - Electrostatic Wind The strong electric field at the sharp tip polarizes atmospheric molecules. If a molecule is ionized, the ion will be repelled from the point. The recoil force causes the whirl to spin.
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Electrostatic Shielding (1) + + + + + + Conducting shell electrostatically shields its interior from changes on the outside, too. If you now add charge Q’ to the conductor and/or Q’’ on the outside of the conductor, the interior electric fields do not change. Add Q’ Q’’
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September 7, 2010 University Physics, Chapter 22 9 Electrostatic Shielding (2) Shielding Illustration Start with a hollow conductor Add charge to the conductor The charge will move to the outer surface We can define a Gaussian surface that encloses zero charge Flux is 0 No electric field!
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Electrostatic Shielding (3) If you move charge q in the cavity, the exterior electric fields and the extreior charge distribution are not affected. Conducting shell electrostatically shields its exterior from changes on the inside.
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This note was uploaded on 07/15/2011 for the course PHYS 241 taught by Professor Wei during the Summer '08 term at Purdue University.

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241-22_lec-5 - Physics 241 Lecture 5 Y. E. Kim September 7,...

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