Concept of an Electric Charge

      The matter on the earth which occupies the space may be solid, liquid or gaseous. The matter is made up of one or more elements. Each element is made up of many atoms which are of similar nature. Now a days, scientists are successful in breaking atoms and studying the resulting products.

      According to modern electron theory, atom is composed of the three fundamental particles, which are invisible to bare eyes. These are the neutron, the proton and the electron. The proton is positively charged while the electron is negatively charged. Te neutron is electrically neutral i.e. possessing no charge. The mass of neutron and proton is same which is 1.675 x 10^-27 kg while the mass of electron is 9.107 x 10^-31 kg. the magnitude of positive charge on proton is same as the magnitude of negative charge on electron. Under normal conditions, number of protons is equal to number of electrons hence, the atom as a whole is electrically neutral. All the protons and neutrons are bound together into a compact nucleus. Nucleus may be thought as a central sun, about which, the electron revolve in a particular fashion. The electrons are arranged in different orbits i.e. levels. The orbits are also called shells.

      The orbits are more or less elliptical. The electrons revolving in various orbits are held by force of attraction exerted by nucleus. The orbit which is closest to the nucleus is under tremendous force of attraction while orbit which is farthest is under very weak force of attraction. Hence, electrons revolving in farthest orbit are loosely held to the nucleus. Such a shell is called valence shell and the electrons in this shell are valence electrons. In some atoms, at room temperature only, these valence electrons gain an additional energy and they escape from the shell. Such electrons exist in an atom as free electrons. Now, if such electrons are removed from an atom, it will loose negative charge and will become positively charged. Such positively charged atom is called anion. As against this, if excess electrons are added to an atom, it will become negatively charged. Such negatively charged atom is called cation.

Note : This total deficiency or addition of excess electrons in an atom is called as its charge and the atom is said to be charged. The unit of charge is coulomb.

      The deficiency or excess of electrons can be achieved by different methods. One of such methods is to rub two dissimilar materials against each other. When an ebonite rod is rubbed on a fur cloth, then the rod extract electrons from fur cloth and behaves as negatively charged while fur cloth behaves as positively charged. This charged condition of rod cannot be sensed by eyes or by any sense organs. But, we can observe the effect of it by simple experiment. Such charged ebonite rod, when brought near light pieces. This attraction is nothing but the effect of static charge present on the rod. This the basic principle of the static electricity.

       Such phenomena due to static charge are governed by some laws called of electrostatics.