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Phys 4, Section 2 



Electric charge [from Greek word for amber - ελεκτρον (electron)]:

There are two kinds (positive and negative) of charge; opposite signed charges attract each other, while charges of the same sign repel each other.

Total charge is constant, so any increase in negative charge is balanced somewhere by a corresponding increase in positive charge.

When two neutral objects of different materials are rubbed together, one of them will become negatively charged and the other positively charged. Benjamin Franklin decided (arbitrarily) that if a rubber rod is rubbed with animal fur, the charge on the rod is to be considered negative.

Atoms are built up from a positive nucleus and negative surrounding electrons, with the total charge balanced out. If the charge of an atom is not balanced out, the atom is called an ion.

If the atoms of a material are close together, so that the electrons can move around easily, the material is called a conductor; otherwise, the material is an insulator or dielectric.

Charging materials

Although most materials are neutral (positive and negative charges balance out), they can be charged by using a second charged object in one of two ways:

            Conduction: physical contact between the material and the charged 
                object, get material charged with same sign as charge of the 
                original charged object.

            Induction: no physical contact between the material and the charged
             object, get material charged oppositely from sign of charge of the 
             original charged object.



Last modified on January 31, 2002