Materials can be charged by three ways; by friction, by electrostatic induction and by contact.

1. Charging by friction

This means making the material to either gain or loses electrons by rubbing it with dry, soft materials like cotton, wool, cloth, hair, etc. If a glass rod is rubbed with dry cloth for example, the glass rod loses electrons and becomes positively charged while the cloth gains electrons and becomes negatively charged.

The following table shows how some insulating materials become charged when rubbed with cloth, for example.

Conducting materials, if properly insulated, can be charged by electrostatic induction.

Figures below explain some ways in which this can be done without proper insulation, any charge that appears on the conductor will leak to the earth.

Insulators cannot be charged by induction because they do not contain free mobile electrons.

• Charging an isolated conductor by induction

Note that to charge the conductor positively, a negatively charged rod is used. This time, positive charges are induced near the rod (step 1) while electrons flow to the earth in step 2.

• Charging a pair of conductors by induction

a) Both conductors acquiring equal charges of same sign

Notice that the final charges on the conductors have opposite sign to that in the rod.

To charge the conductors positively, a negatively charged rod is used and In Step 4, positive charge is shared between the two conductors.

b) Both conductors acquiring equal charges of opposite signs

                    Figure: giving a pair of conductor equal and opposite charges by induction

1. Charging by contact

A neutral conductor can be charged by placing it in contact with another charged conductor so that charge sharing should occur between the two.

Note that for a conductor to be charged by contact, it must be properly insulated to prevent charge leakage to the earth.

The following points concerning the sharing of charge should be noted.

1. Sharing of charge does not occur between two insulators in contact since electrons do not flow in them. However a conductor can collect charges from the surface of a charged insulator and become charged.
2. If two objects with same amount of different charges are placed in contact, they become neutral. H the positive object carries more charge, and then they end up positive when placed in contact. However if the negative carries more charge, they end up negative. It is impossible to have opposite charges when two charged conductors are placed in contact.