1. Electric Charge

Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric or magnetic field. All matter is built from atoms, and atoms in turn are made of three basic particles: protons (positively charged), electrons (negatively charged), and neutrons (electrically neutral, carrying no charge). In a normal, unexcited atom, the number of protons equals the number of electrons, so the atom as a whole is electrically neutral. When electrons are transferred from one object to another — for example, by rubbing two materials together — one object ends up with an excess of electrons (it becomes negatively charged) and the other ends up with a deficit of electrons (it becomes positively charged). This transfer of charge without any accompanying flow of matter in bulk is the basis of what we call static electricity.

Properties of Electric Charge

• Charge is of two kinds only: positive and negative. This was established by Benjamin Franklin, who also introduced the naming convention still used today.
• Like charges repel each other; unlike charges attract each other. This is the most basic qualitative law of electrostatics.
• Charge is quantised — it always occurs in integral multiples of the charge on a single electron, e = 1.6 × 10⁻¹⁹ coulomb. You cannot have half an electron's worth of charge.
• Charge is conserved — the total electric charge of an isolated system remains constant; charge can be transferred from one body to another but can never be created or destroyed.
• Charge is additive — the total charge of a system is the algebraic (scalar, with sign) sum of all the individual charges present.
• The SI unit of electric charge is the coulomb (C), named after the French physicist Charles-Augustin de Coulomb.
• A body can be charged by three methods: friction (rubbing two neutral bodies so electrons transfer from one to the other), conduction (direct contact with an already charged body), and induction (bringing a charged body near a neutral one without contact, causing charge to redistribute).

Conductors and Insulators

Materials differ enormously in how easily charge can move through them, and this distinction is central to nearly all practical uses of electricity.

Conductors: Materials that allow electric charge to flow through them easily because they contain a large number of free (loosely bound) electrons that can move about within the material. Metals such as copper, silver, aluminium, and gold are excellent conductors. Silver is the best conductor of electricity, followed closely by copper, which is used for household wiring because it is far cheaper than silver while still being an excellent conductor. The human body, moist earth, and graphite (a non-metal) also conduct electricity.

Insulators (or dielectrics): Materials that do not allow charge to flow through them because their electrons are tightly bound to their atoms and are not free to move. Rubber, glass, dry wood, plastic, mica, and dry air are common insulators. Insulating materials are used to coat electric wires (as plastic or rubber sheathing) precisely so that current stays confined to the conducting wire and does not leak out to give a person a shock.

Semiconductors: Materials such as silicon and germanium whose conductivity lies between that of conductors and insulators, and which can be modified by adding impurities (doping). Semiconductors are the basis of transistors, diodes, and modern electronics, though this topic is generally treated separately from basic electricity.

Static Electricity — Everyday Examples

Static electricity is the accumulation of electric charge on the surface of an object, as opposed to current electricity where charge flows continuously. Common examples frequently cited in exams include: a plastic comb run through dry hair attracts small bits of paper because friction transfers electrons between the comb and the hair, leaving the comb charged; a woollen cloth rubbed on a glass rod or an ebonite rod charges both objects oppositely; lightning is a large-scale natural discharge of static electricity built up between storm clouds and the earth (or between two clouds); and the mild shock one sometimes feels after walking on a synthetic carpet and then touching a metal door handle is the sudden discharge of static charge that had built up on the body.