Electric Charge & Properties

Two Kinds of Charge

The story of electric charge begins with friction. The Greeks knew around 600 BC that amber rubbed with wool attracts light objects; indeed the word electricity descends from elektron, Greek for amber. Centuries of careful experiment reduced these observations to a single, sharp conclusion — there are exactly two kinds of electric charge in nature, and no third has ever been found.

Two glass rods, each rubbed with silk, repel one another. Two plastic rods rubbed with fur likewise repel. Yet a charged glass rod attracts a charged plastic rod. The behaviour is governed by one rule: like charges repel and unlike charges attract. The property that distinguishes the two kinds is called the polarity of charge. Benjamin Franklin named them positive and negative — by convention the charge on glass (or cat's fur) is positive, that on plastic (or silk) is negative.

A simple device that detects charge is the gold-leaf electroscope: a charged object touched to its metal knob sends charge down to two thin gold leaves, which then diverge. The greater the divergence, the greater the charge. A body with excess or deficit of charge is said to be electrified; one with none is electrically neutral.

The SI Unit: Coulomb

In the SI system the unit of charge is the coulomb (C). It is defined through electric current: one coulomb is the charge that flows through a wire in one second when the current is one ampere. The coulomb is an enormous unit for electrostatics — a charge of $-1\,\text{C}$ corresponds to roughly $6 \times 10^{18}$ electrons, a magnitude almost never met in static-electricity problems. For that reason practical charges are quoted in submultiples.

Charging by Friction (Electron Transfer)

All matter is built from atoms that are normally neutral — their positive and negative charges exactly balance. To electrify a body we must add or remove charge. In solids the mobile players are the loosely bound electrons; the heavy positive nuclei stay put. When two surfaces are rubbed together, some electrons are dragged from one to the other.

When a glass rod is rubbed with silk, electrons leave the glass and settle on the silk. The rod, now deficient in electrons, becomes positively charged; the silk, with its excess electrons, becomes equally negatively charged. No new charge is created — it is merely transferred. The number of electrons moved is a minute fraction of those present in the material.

The Point Charge Idealisation

Before stating the properties, NCERT introduces a working simplification. When the sizes of charged bodies are very small compared with the distances between them, we treat each as a point charge — all the charge content of the body assumed concentrated at a single point in space. This idealisation strips away geometry and lets us treat charges as numbers located at positions, which is exactly what makes Coulomb's law and the superposition of forces tractable.

The Three Basic Properties

Beyond polarity, electric charge obeys three properties that NCERT lays out in Section 1.4 and NIOS echoes in Section 15.1. They are best held side by side, because NEET questions routinely test whether you can keep them distinct.

Additivity of charge

Charge is a scalar: it has magnitude but no direction, just like mass. If a system contains charges $q_1, q_2, \dots, q_n$, the total is simply $q_1 + q_2 + \dots + q_n$, added like ordinary real numbers. The one difference from mass is sign — mass is always positive, but charge may be positive or negative, so signs must be carried through the sum.

Conservation of charge

When two bodies are rubbed, what one body gains the other loses — the charging-by-friction picture already shows this. More generally, within an isolated system of many charged bodies, charges may get redistributed through interactions, but the total charge of the system is always conserved. This has been established experimentally and holds at every scale.

Conservation survives even when charged particles are themselves created or destroyed. When a neutron decays it produces a proton and an electron — equal and opposite charges, so the total is zero before and after. It is the net charge of an isolated system that cannot change, not the existence of the carriers.

Quantisation of charge

All free charges are integral multiples of a basic unit $e$. The charge $q$ on any body is therefore $q = ne$, where $n$ is an integer (positive or negative) and $e = 1.6 \times 10^{-19}\ \text{C}$ is the charge carried by a proton; an electron carries $-e$. This fact — that charge comes only in whole lumps of $e$ — is the quantisation of charge. It was first hinted at by Faraday's laws of electrolysis and demonstrated directly by Millikan in 1912.

If quantisation is real, why does charge seem to vary smoothly in the laboratory? Because $e$ is fantastically small. A charge of just $1\ \mu\text{C}$ already contains about $10^{13}$ elementary charges, so adding or removing one $e$ shifts the total by a part in ten trillion — utterly invisible. The grainy nature is lost and charge appears continuous, much as a dotted line looks solid from a distance. Quantisation matters only at the microscopic scale, where charges are a countable few tens or hundreds of $e$.