Electric Charge

Electric charge is a fundamental property of the elementary particles which are the constituents of all matter. The 'static' that you often feel on your clothes on a dry day, is electric charge. Lightning is a flow of an enormous amount of electric charge through the air.

Through early experiments with charge, people realized that electric charges exert force: one bit of charge could attract or repel another. Study of the nature of this force also showed that there were two kinds of charge, positive, and negative.

Like charges repel each other, while unlike charges attract each other.

For example, two positive charges repel each other, whereas a positive and a negative charge attract each other.

Electric charge is a new physical quantity, which uses the unit Coulomb (C). The Coulomb is a fundamental SI unit. It cannot be derived from or written in terms of the SI units we have seen so far (m, kg, s). We define later it in terms of Ampere (A), the unit of the new fundamental quantity electric current, which is a flow of charges :

1 C = (1 A) (1 s) .

As far as we know, charge comes in multiples of a fixed quantity e, which is the smallest possible charge. This observation is called Quantization of charge.

e = 1.6 $\cdot$ 10^-19 C

Elementary particles can carry charge. The main constituents of the atom are electrons, protons, and neutrons.

Electron charge =	-e
Proton charge =	+e
Neutron charge =	0

Why the charges of electron and proton are exactly equal and opposite is not known, but if they were different even in very small way, it would be a profoundly different world.

The net charge of any object, q, is simply the difference between the number of protons, N_p, and the number of electrons, N_e, contained in it, times the magnitude of the charge of each, e:

q = e $\cdot$ (N_p - N_e)

Experiments have also shown that net charge in an isolated system always remains the same. This is the:

Law of conservation of net charge:

The net charge of an isolated system remains constant.

This conservation law is as important as the laws of conservation of energy (see also: 1st law of thermodynamics), momentum, and angular momentum which we have been introduced to earlier.

Note that ordinary matter is usually close to neutral or uncharged, because it contains an equal number of protons and electrons. To charge a body positively, we remove electrons from it. To charge a body negatively, we add electrons to it.