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# Lect03 - Electric Potential Capacitance capacitors Charges...

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Electric Potential - Capacitance, capacitors Chapter18: Electric field and conductors Capacitance Potential energy in capacitors Capacitors in series and in parallel Physics 104, Lecture 4 Sep 14, 2011 Charges in a conductor The charges move apart until net force on each vanishes So… excess charge moves to the surface The amount of charge per unit area is greater at the flat end Physics 104, Fall 2011 2 E inside conductor Conductor means electrons free to move electron in electric field feels a force electrons will not move when E=0 electrons will move around until E=0 everywhere inside a conductor Physics 104, Fall 2011 3 F = qE = ma Conductors in Equilibrium A conductor with an excess of positive charge All of the charge resides at the surface Electric field: E = 0 inside the conductor The electric field just outside the conductor is perpendicular to the surface Potential: The potential is a constant everywhere on the surface of the conductor The potential everywhere inside the conductor is constant and equal to its value at the surface Physics 104, Fall 2011 4 Shielding the Electric Field A region can be designed where the electric field is zero Go inside a metal cavity Example: Your car acts like the piece of metal to shield you from a lightning strike Section 18.2 E outside a conductor On an irregularly shaped conductor, the charge accumulates at locations where the radius of curvature of the surface is smallest (that is, at sharp points) Physics 104, Fall 2011 6 Consider what would happen it this was not true The component along the surface would cause the charges to move It would not be in equilibrium Charges will move until they are in equilibrium = fieldlines are perpendicular to surface

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Question A conductor is placed as shown in the figure. Now, the electric potential at point A is _________ the electric potential at point B. greater than equal to less than Electric Field and potential near a Metal Consider a solid metal sphere carrying an excess positive charge, q: The excess charge resides on the surface The field inside the metal is zero, outside it is the same as for a point charge Section 18.2 E = kq r 2 for r > r
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Lect03 - Electric Potential Capacitance capacitors Charges...

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