Charge polarization can occur in insulators that are near a charged object.
Charging by induction is not restricted to conductors. Charge polarization can occur in insulators that are near a charged object.
When a charged rod is brought near an insulator, there are no free electrons to migrate throughout the insulating material.
Instead, there is a rearrangement of the positions of charges within the atoms and molecules themselves.
One side of the atom or molecule is induced to be slightly more positive (or negative) than the opposite side.
The atom or molecule is said to be electrically polarized.
a. When an external negative charge is brought closer from the left, the charges within a neutral atom or molecule rearrange.
b. All the atoms or molecules near the surface of the insulator become electrically polarized.
Polarization explains why electrically neutral bits of paper are attracted to a charged object, such as a charged comb.
Molecules are polarized in the paper, with the oppositely charged sides of molecules closest to the charged object.
The bits of paper experience a net attraction.
Sometimes they will cling to the charged object and suddenly fly off.
Charging by contact has occurred; the paper bits have acquired the same sign of charge as the charged object and are then repelled.
A charged comb attracts an uncharged piece of paper because the force of attraction for the closer charge is greater than the force of repulsion for the farther charge.
Rub an inflated balloon on your hair and it becomes charged.
Place the balloon against the wall and it sticks.
The charge on the balloon induces an opposite surface charge on the wall. The charge on the balloon is slightly closer to the opposite induced charge than to the charge of the same sign.
Electric Dipoles
Many molecules—H2O, for example—are electrically polarized in their normal states.
The distribution of electric charge is not perfectly even.
There is a little more negative charge on one side of the molecule than on the other.
Such molecules are said to be electric dipoles.
In summary, objects are electrically charged in three ways.
By friction, when electrons are transferred by friction from one object to another.
By contact, when electrons are transferred from one object to another by direct contact without rubbing.
By induction, when electrons are caused to gather or disperse by the presence of nearby charge without physical contact.
If the object is an insulator, on the other hand, then a realignment of charge rather than a migration of charge occurs.
This is charge polarization, in which the surface near the charged object becomes oppositely charged.
