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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.
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