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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)