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 / r 2 E = F/q = kQq/r 2 /q E = kQ/r 2

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)