18.7 Conductors and Electric Fields in Static Equilibrium

18.7 Conductors and Electric Fields in Static Equilibrium

• The figure shows the effect of an electric field on free charges.
• The free charges move until the field is in line with the conductor's surface.
• The field can't be parallel to the surface in equilibrium since it would produce more movement of charge.
• Free charges can be positive or negative in metals, but a positive free charge is shown.
• The motion of a positive charge is the same as the motion of a negative charge in the opposite direction.

• When an electric field is applied to a conductor, free charges inside the conductor move until the field is parallel to the surface.

• The free charge has left the conductor's surface with equilibrium forces.

• The field becomes stronger near the conductor.

• The spherical conductor is in static equilibrium with the electric field.
• When the electric field lines are parallel to the surface, free charges move within the conductor.
• The field lines end on the negative side of the surface and begin again on the positive side.
• The conductor has no electric field since free charges in the conductor would keep moving until it was eliminated.

• When the field inside the conductor is zero, excess charge is forced to the surface.
• The field is the same as if the conductor were replaced by a point charge at its center.

• There is a mutual repulsion of excess positive charges on a spherical conductor.
• The electric field is zero inside and parallel to the surface.
• The field is the same as the point charge at the center and the excess charge.

• The conductor has no electric field.

• The electric field lines are just outside a conductor.

• The properties of a conductor can be used to analyze any conductor in an equilibrium situation.
• This can lead to new insights.

• The electric field between the plates will be uniform in strength and direction according to the conductors' properties.
• The field lines that are produced by the excess charges are uniform in strength and direction since the plates are flat.
• The edge effects are not important when the plates are close together.

• The field is uniform in strength and direction.
• The electron gun of a TV tube can be used to produce uniform acceleration of charges between the plates.

• A near uniform electric field of approximately 150 N/C, directed downward, surrounds Earth, with the magnitude increasing slightly as we get closer to the surface.
• The electric field surrounding Earth is caused by the ionosphere.
• In fair weather the ionosphere is positive and the Earth is mostly negative.

• In storm conditions clouds can form and the electric fields can be reversed.

• If the electric field is large, the material surrounding it becomes conductors.
• This happens at N/C for air.
• We get discharge in the form of lightning sparks and corona discharge when the air ionizes.

• Earth and the ionosphere are conductors.
• The electric field is about 150 N/C.
• The local electric fields can be larger in the presence of storm clouds.
• The air can break down and lightning can occur in high fields.
• The conductor surface has excess charges on it.
• Excess charges on a conductor become concentrated at certain points.
• Excess charge can move on or off the conductor.

• The best way to move them apart on the flattest surface is to use the repulsion of like charges.
• The forces at either end of the conductor are the same, but the components of the forces parallel to the surfaces are different.
• The component parallel to the surface is more effective in moving the charge on the flattest surface.

• More of the field lines are focused on the most curved parts.

• The location of greatest curvature is where the excess charge on a conductor becomes most concentrated.
• The charges move apart once they reach the surface.

• When pointed, lightning rods work best.

• The more dramatic lightning strike is prevented by the bleeding away of the induced charge.

• We sometimes wish to prevent the transfer of charge.
• In that case, the conductor should be very smooth and large.
• Smooth surfaces are used on high-voltage transmission lines to keep charge out of the air.

• A metal shield surrounds a volume.
• There will be no electrical field in this shield, and all electrical charges will reside on the outside surface.
• The electrical signals inside a nerve cell can be interfered with by stray electrical fields in the environment.

• If you are driving a car during an electrical storm, it is best to stay inside the car as its metal body acts as a Faraday cage with zero electrical field inside.
• If there is a lightning strike in the vicinity, the effect on the outside of the car is felt, but the inside is unaffected.
• If an electrical wire broke in a storm or an accident and fell on your car, this is also true.

• A conductor has a large charge concentration.
• The electric field is very strong at the point and can exert a force large enough to transfer charge on or off the conductor.
• The point of a lightning rod is to prevent the build up of large excess charges on structures.