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

cosE A 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

cos

cos(0)

E A

E A

E A

cos

cos(90)

0

E A

E A

Gauss’ Law

EA 2

24

kqr

r

4 k q 0

1

4k

0

q

Flux through surface of sphere

which encloses a charge q

In general, flux through a closed surfacedepends 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?

1 2 3 4

28%

9%

43%

20%1. =2R2E2. =R2E3. =04. =(2RL+2R2)E

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. =2R2E2. =R2E3. =04. =(2RL+2R2)E