Physics 1161 Lecture 4
Electric Flux and Shielding
Preflight 4 -- 1
Which is (are) true?
• When the charge distribution on a conductor
reaches equilibrium,
• a. the electric field within the conductor is zero.
• b. any electric charge deposited on the
conductor resides on the surface.
• c. the electric field at the surface is
perpendicular to the surface.
• ALL 3 are true!
Charged Conductors
• Electrostatic equilibrium -excess charge has
distributed itself so as to
reduce the total amount of
repulsive forces.
• Once a charged conductor
has reached the state of
electrostatic equilibrium,
there is no further motion
of charge about the
surface.
Electrostatic Equilibrium
any excess charge lies only
at the surface of the
conductor
the electric field is zero
within the solid part of the
conductor
the electric field at the
surface of the conductor is
perpendicular to the surface
charge accumulates, and the
field is strongest, on pointy
parts of the conductor
charge accumulates, and field is strongest, on
pointy parts of the conductor
• Charge tends to accumulate in greater numbers
at locations of greatest curvature.
excess charge lies only at surface of conductor
Charges want to get as far away from each other
as possible. To do this they move to the surface of
the conductor.
electric field is zero within the solid part of the
conductor
• Charges distribute themselves so the electric
field inside the conductor is zero. If the field
wasn't zero, any electrons that are free to
move would.
electric field at surface of conductor is
perpendicular to the surface
If there were a component
of the field along the
surface, charges would
move along the surface in
response, which is
inconsistent with the
conductor being in
equilibrium.
Shielding
• A conductor shields its interior from
external electric fields.
• Shielding occurs whether the
conductor is hollow or solid.
• Many electrical devices use this
property to shield sensitive circuit
elements
Electric Flux
  E  A cos 
θ is the angle between the normal
to the surface and the field
SI units: N m2/C
http://webphysics.davidson.edu/physlet_resources/bu_semester2/c03_flux.html
Electric Flux
  E  A cos 
  E  A cos(0)
  EA
  E  A cos 
  E  A cos(90)
 0
Gauss’ Law
  EA
kq
2
  2  4 r
r
1
  4 k  q k 
4 0
q
Flux through surface of sphere

which encloses a charge q
0
In general, flux through a closed surface
depends only on enclosed charge.
An uncharged cylinder of radius R and length L is
immersed in a uniform electric field E. What is the flux
of the electric field through the closed surface?
43%
28%
1.
2.
3.
4.
=2R2E
=R2E
=0
=(2RL+2R2)E
20%
9%
1
2
3
4
A cylinder of radius R and length L is immersed in a
uniform electric field E. What is the flux of the
electric field through the closed surface?
1.
2.
3.
4.
=2R2E
=R2E
=0
=(2RL+2R2)E