A very pointed conductor has a large charge concentration at the point. The electric field is very strong at the point and can exert a force large enough to transfer charge on or off the conductor. Lightning rods are used to prevent the buildup of large excess charges on structures and, thus, are pointed.(a) A lightning rod is pointed to facilitate the transfer of charge. (credit: Romaine, Wikimedia Commons) (b) This Van de Graaff generator has a smooth surface with a large radius of curvature to prevent the transfer of charge and allow a large voltage to be generated. The mutual repulsion of like charges is evident in the person’s hair while touching the metal sphere. (credit: Jon ‘ShakataGaNai’ Davis/Wikimedia Commons).
Section summary
A conductor allows free charges to move about within it.
The electrical forces around a conductor will cause free charges to move around inside the conductor until static equilibrium is reached.
Any excess charge will collect along the surface of a conductor.
Conductors with sharp corners or points will collect more charge at those points.
A lightning rod is a conductor with sharply pointed ends that collect excess charge on the building caused by an electrical storm and allow it to dissipate back into the air.
Electrical storms result when the electrical field of Earth’s surface in certain locations becomes more strongly charged, due to changes in the insulating effect of the air.
A Faraday cage acts like a shield around an object, preventing electric charge from penetrating inside.
Conceptual questions
Is the object in
[link] a conductor or an insulator? Justify your answer.
If the electric field lines in the figure above were perpendicular to the object, would it necessarily be a conductor? Explain.
The discussion of the electric field between two parallel conducting plates, in this module states that edge effects are less important if the plates are close together. What does close mean? That is, is the actual plate separation crucial, or is the ratio of plate separation to plate area crucial?
Would the self-created electric field at the end of a pointed conductor, such as a lightning rod, remove positive or negative charge from the conductor? Would the same sign charge be removed from a neutral pointed conductor by the application of a similar externally created electric field? (The answers to both questions have implications for charge transfer utilizing points.)
Why is a golfer with a metal club over her shoulder vulnerable to lightning in an open fairway? Would she be any safer under a tree?
Can the belt of a Van de Graaff accelerator be a conductor? Explain.
Are you relatively safe from lightning inside an automobile? Give two reasons.
Discuss pros and cons of a lightning rod being grounded versus simply being attached to a building.
Using the symmetry of the arrangement, show that the net Coulomb force on the charge
at the center of the square below (
[link] ) is zero if the charges on the four corners are exactly equal.
Four point charges
,
,
, and
lie on the corners of a square and
is located at its center.
