Charging by rubbing

This is a familiar electric phenomenon in which friction transfers charged particles from one body to another.

If two objects are rubbed together, especially if the objects are insulators and surrounding air is dry, the objects acquire equal and opposite charges and an attractive force develops between them.

• The object that loses electrons becomes positively charged.
• The other that gains electrons becomes negatively charged.
• The force is simply the attraction between charges of opposite sign.

Types of electric charges

Each type of charge attracts the opposite type but repels the same type. This leads to the basic law of electrostatics: Unlike charges attract, like charges repel.

• SI unit of electric charge is the coulomb (C). It is a scalar quantity.
• Every electron has a charge of -1.6 x 10^-19 C, and every proton has a charge of +1.6 x 10^-19 C.

Examples:

• When a piece of amber, plastic, polythene, or hard rubber is rubbed with fur, electrons are transferred from fur to the other material. Fur acquires net positive charge, since it has fewer electrons than protons. Similarly, the amber, plastic, or hard rubber acquires a net negative charge since they have excess electrons.
• Combing hair charges the comb in the same way.
• Rubbing glass (perspex) with silk causes the glass to acquire a net positive charge.