Transcript The Electric Field

Electric Forces and
Fields
The Electric Field
Electric Field Strength
• Electric field – the region in space around
a charged object in which a stationary
charged object experiences an electric
force because of its electric charge
– Any charged particle in that electric field will
experience an electric force
Electric Field Strength
• Electric field strength is a vector
• Measured in Newtons/Coulomb (N/C)
• The direction of the electric field depends on the
sign of the charge producing the field
• The strength of an electric field depends on
charge and distance
– Positive charges produce an outward electric field
– Negative charges produce an inward electric field
Electric Field Strength
Electric Field Strength
• Electric field strength is measured by placing a
very small test charge near a larger charge
– The test charge (q0) must be very small compared to
the main charge (Q)
• If not, is will cause a redistribution of charge on Q, which will
effect electric field strength
• Electric field strength = coulomb constant *
charge producing the field/distance2
• E = kCq/r2
• Electric fields exist near any charged body, even
in the absence of a test charge
Electric Field Strength
Electric Field Strength
Electric Field Lines
• Show electric fields with electric field lines
– Lines that represent both the magnitude and
the direction of the electric field
– The stronger the field, the more lines are
present
– Point out from a positive charge
– Point in toward a negative charge
Electric Field Lines
Electric Field Lines
• Rules for drawing electric field lines
– The lines must begin on positive charges or at
infinity and terminate on negative charges or
at infinity
– The number of lines drawn leaving a positive
charge or approaching a negative charge is
proportional to the magnitude of the charge
– Electric field lines from the same field never
cross
Electric Field Lines
• Electric dipole – the electric field produced
between to objects of equal and opposite
charge
– The number of electric field lines leaving the
positive charge must be equal to the number
of electric field lines entering the negative
charge
• If one charge or field is twice as strong as
another, it should have twice as many field
lines
Electric Field Lines
Electric Field Lines
Electric Field Lines
Conductors in Electrostatic
Equilibrium
• Good conductors contain charges
(electrons) that are free to move about
within the material
• When there is no net motion of charge
within a conductor, it is said to be at
electrostatic equilibrium
Conductors in Electrostatic
Equilibrium
• Electrostatic equilibrium has the following
conditions
– The electric field is zero everywhere inside the
conductor
– Any excess charge on an isolated conductor resides
entirely on the conductor’s outer surface
– The electric field just outside a charged conductor is
perpendicular to the conductor’s surface
– On an irregularly shaped conductor, charge tends to
accumulate where the radius of curvature of the
surface is smallest, that is, at sharp points
Conductors in Electrostatic
Equilibrium
• The air surrounding a very strong electric
field can become ionized
– A bluish glow can be seen as charges leap
from the surface to the air
• Called a corona or St. Elmo’s fire
• Van de Graaff generators collect electric
charge
– Uses static electricity to build up negative
charge in the belt and ball and positive charge
on the pulleys moving the belt
Conductors in Electrostatic
Equilibrium
Conductors in Electrostatic
Equilibrium