Physics 1161 Lecture 3 Electric Flux and Shielding

1
Physics 1161 Lecture 3 Electric Flux and
Shielding
2
Checkpoint 3

• 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!

3
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.

4
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

5
charge accumulates, and field is strongest, on
pointy parts of the conductor

• Charge tends to accumulate in greater numbers at
  locations of greatest curvature.

6
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.

7
Checkpoint 3-2

• 1) The magnitude of the electric field inside the
  charged sphere is ...
•  Zero
•  Negative
•  Positive

8
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.

9
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.

10
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

11
Electric Flux
? is the angle between the normal to the
surface and the field
SI units N m2/C
http//webphysics.davidson.edu/physlet_resources/b
u_semester2/c03_flux.html
12
Electric Flux
13
Gauss Law
Flux through surface of sphere which encloses a
charge q
In general, flux through a closed surface depends
only on enclosed charge.
14
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?

1. ?2?R2E
2. ??R2E
3. ?0
4. ?(2?RL2?R2)E

15
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?R2E
2. ??R2E
3. ?0
4. ?(2?RL2?R2)E