Electric Charges & Charging By Rubbing


In electrostatics, we study the electromagnetic phenomena that occur when there are no moving charges (After a static equilibrium has been established). Charges reach their equilibrium positions rapidly because the electric force is extremely strong.

Electric Charges

  • All substances are made up of atoms, which consist of a nucleus (containing protons and neutrons) and orbiting electrons. Protons are positively charged and electrons are negatively charged. Neutrons are so named because they are neutral; they have no net electric charge.
  • In each atom of a material, the number of protons and electrons are equal. The positive and negative charges cancel each other to a large extent and there is no net electrical charges.
  • Electrons can be removed from atoms or added to atoms and the material is left with a net charge, and electrical effects result. The study of these net stationary charges and their effects on each other constitutes the subject of static electricity or electrostatics.
  • Atoms with net charges are called ions.

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 Of Charging By Rubbing

  • 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.

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