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# lecture5 - Physics 2102 Gauss’ law Physics 2102 Gabriela...

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Unformatted text preview: Physics 2102 Gauss’ law Physics 2102 Gabriela González Carl Friedrich Gauss 1777-1855 Electric Flux • Electric Flux A surface integral! • CLOSED surfaces: – define the vector dA as pointing OUTWARDS – Inward E gives negative Φ – Outward E gives positive Φ Φ = ρ E · d r A ò Gauss’ Law • Consider any ARBITRARY CLOSED surface S -- NOTE: this does NOT have to be a “real” physical object! • The TOTAL ELECTRIC FLUX through S is proportional to the TOTAL CHARGE ENCLOSED! • The results of a complicated integral is a very simple formula: it avoids long calculations! ∫ = ⋅ ≡ Φ Surface ε q A d E S (One of Maxwell’s 4 equations) Properties of conductors Inside a conductor in electrostatic equilibrium , the electric field is ZERO . Why? Because if the field is not zero, then charges inside the conductor would feel forces and would be moving: the system is not in equilibrium yet. SO: the electric field pushes charges in a conductor to redistribute themselves until they make the field inside the conductor ZERO. Excess charges are always on the surface of the conductors (but never inside). Example : “Faraday’s Cage” Given a hollow conductor of arbitrary shape. Suppose an excess charge Q is placed on this conductor. Suppose the conductor is placed in an external electric field. How does the charge distribute itself on outer and inner surfaces?...
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## This note was uploaded on 11/18/2011 for the course PHYSICS 2102 taught by Professor Dowling during the Fall '10 term at LSU.

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lecture5 - Physics 2102 Gauss’ law Physics 2102 Gabriela...

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