Physics 1161 Lecture 4 Electric Flux and Shielding.
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Transcript of Physics 1161 Lecture 4 Electric Flux and Shielding.
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