Electric Flux and Shielding

ALL 3 are true! Preflight 3 -- 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 is the condition established by charged conductors in which the excess charge has optimally distanced 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 At equilibrium, the charge and electric field follow these guidelines: the 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

excess charge lies only at surface of conductor Consider a negatively-charged conductor; in other words, a conductor with an excess of electrons. The excess electrons repel each other, so they 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 They also 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. There are plenty of free electrons inside the conductor (they're the ones that are canceling out the positive charge from all the protons) and they don't move, so the field must be zero.

electric field at surface of conductor is perpendicular to the surface If field at the surface of the conductor weren’t perpendicular to the surface, there would be a component of the field along the surface. A charge experiencing that field would move along the surface in response to that field, which is inconsistent with the conductor being in equilibrium.

charge accumulates, and field is strongest, on pointy parts of the conductor Charge tends to accumulate in greater numbers at locations of greatest curvature.

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 θ is the angle between the normal to the surface and the field SI units: N m2/C

Electric Flux

Gauss’ Law Flux through surface of sphere which encloses a charge q In general, flux through a closed surface depends only on enclosed charge.

=2R2E =R2E =0 =(2RL+2R2)E 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? =2R2E =R2E =0 =(2RL+2R2)E

=2R2E =R2E =0 =(2RL+2R2)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? =2R2E =R2E =0 =(2RL+2R2)E