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Transcript
Electric Fields What is an Electric Field? • One charged object can influence another charged object without any direct contact. • We say a charged object is surrounded by an electric field, a region of influence. Any other charged object in that space will interact with that field and experience an electrical force. Electric Field Strength • The electric field strength at a given location is defined as the amount of force per unit of charge at that location. E=F/q (units: N/C) Electric Field Strength • Consider a sphere with a charge of Q. A second charge q nearby would feel a force from the interaction with Q. q Q r From Coulombs Law… F = kQq / r2 E = F/q = kQq/r2 /q E = kQ/r2 Electric field Strength • E is a vector quantity • The direction of E is defined as the direction of force that would be felt by a positive test charge at the location in question Electric Field Lines • In a diagram, the vector nature of E can be represented by arrows that represent “field lines” Field around a positive charge Field around a negative charge Electric Field Lines • When drawing, or interpreting, electric field lines keep in mind the following… – Filed lines originate at + charge or infinity – Field lines terminate at – charge or infinity – Filed lines are always perpendicular to the surface of a charged object – Line density is an indicator of field strength – The number of lines leaving a + and terminating at a – is proportional to the magnitude of charge – Field lines NEVER cross each other (Why?) Field around two charges (same size) Field around two charges of different size Conductors in Electrostatic Equilibrium • The electric field is ZERO everywhere inside the conductor • Excess charge resides on the conductors outer surface • The electric field just outside the conductor is perpendicular to the conductor’s surface • For irregular shapes, charge tends to accumulate where radius of curvature is smaller (ie. sharp points)