Insulators and Conductors
Some materials do not allow electric charge to move freely; these are known as insulators. Examples are glass, rubber, wood. Electrons in these insulating materials are tied to the atoms.
Some materials -- primarily metals -- allow electrons to move about freely; these are known as conductors. That is a primary characteristic of metals.
When we talked about heat we also talked about insulators and conductors -- of heat. Heat conduction and electrical conduction are closely connected for heat is transferred by the random thermal motion of the free electrons. Good heat conductors are good electrical conductors and good thermal insulators are good electrical insulators.
We can transfer charge by conduction or by induction. Transfer of electric charge by conduction simply means placing a charged object in contact with another object.
Charge by induction is more subtle -- or more interesting:
If a charged object is brought near an uncharged object, charges will rearrange themselves as shown above. If the "uncharged object" is then connected to ground, charges will flow off of it (or onto it) so that it then has a net charge. When the initially charged object -- which never touched the initially uncharged object -- is removed, the object still has a net charge. This is known as "charging by induction" or "inducing a charge".
Uncharged objects can be attracted by a charged object by this rearrangement of electrical charge. This may be called "induction" or "charge polarization":
(c) Doug Davis, 2002; all rights reserved
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