The Defining Question
NCERT opens the topic with an observation, not a definition: "Some substances readily allow passage of electricity through them, others do not." The substances that allow electricity to pass through them easily are called conductors; those that offer high resistance to its passage are called insulators. Metals, the human and animal bodies, and the earth are conductors. Most non-metals — glass, porcelain, plastic, nylon, wood — are insulators.
The deeper question is why they differ, and the answer is microscopic. A conductor "has electric charges (electrons) that are comparatively free to move inside the material." An insulator does not. Everything else in this topic — surface distribution, earthing, the behaviour of a comb — follows from that single fact about the mobility of charge.
| Property | Conductor | Insulator |
|---|---|---|
| Charge carriers | Electrons comparatively free to move | Charges bound to atoms; no free movement |
| Passage of electricity | Allowed easily | High resistance to passage |
| Typical examples | Metals, human/animal body, earth, electrolytes | Glass, porcelain, plastic, nylon, wood, rubber |
| Charge deposited on it | Spreads over the entire (outer) surface | Stays at the same place — localised |
Free Electrons: The Conductor's Engine
In a metal, the outermost electrons of the atoms are so loosely bound that they detach from their parent atoms and wander throughout the lattice. NCERT introduced this idea one section earlier: "In solids, some of the electrons, being less tightly bound in the atom, are the charges which are transferred from one body to the other." In a conductor those loosely bound electrons are not merely transferable — they are mobile inside the bulk, forming a sea of free charge carriers against a fixed background of positive ion cores.
Because these electrons can move in response to the slightest electric influence, any push redistributes them almost instantly. This mobility is the engine behind every conductor behaviour: it is why a conductor allows current, why it cannot hold an internal field at rest, and why deposited charge cannot stay bunched in one spot. The same mobility extends beyond metals — the human body and the earth conduct because they too contain charges free to move, and electrolytes conduct through mobile ions rather than electrons.
In a conductor (left) electrons roam freely between fixed positive ion cores. In an insulator (right) each electron stays tethered to its parent atom and cannot wander.
Bound Charges in an Insulator
An insulator is not charge-free — every atom in it still contains protons and electrons. The difference is that those charges are bound: the electrons are held tightly to their parent atoms and cannot stray across the material. There is no sea of free carriers, so there is nothing to ferry charge from one region to another. This is exactly why NCERT states that "if some charge is put on an insulator, it stays at the same place."
An insulator used in the context of an applied field is also called a dielectric. The bound charges can shift very slightly within each atom or molecule — a polarisation effect explored in detail in the chapters on capacitors — but they never break free to flow as a current. For the purposes of this topic, the operational rule is simple: charge dropped on glass, plastic or rubber remains a frozen, localised deposit.
"Insulators have no charge" is wrong
An insulator is electrically neutral overall and has plenty of charge inside it — it simply has no free charge to move. The distinction is between mobile and bound charge, never between presence and absence of charge.
Conductor = free charge carriers · Insulator = bound charges only.
Conductor vs Insulator vs Semiconductor
NCERT notes in a footnote that the two-class scheme is not exhaustive: "There is a third category called semiconductors, which offer resistance to the movement of charges which is intermediate between the conductors and insulators." Silicon and germanium sit in this middle band — they conduct far less freely than a metal but far more readily than glass, and that controllable in-between behaviour is what makes them the foundation of electronic devices.
| Class | Mobility of charge | Resistance to current | Examples |
|---|---|---|---|
| Conductor | High — electrons comparatively free | Low | Copper, silver, human body, earth |
| Semiconductor | Intermediate | Intermediate | Silicon, germanium |
| Insulator | None free — charges bound | Very high | Glass, plastic, rubber, nylon, wood |
Why Charge Spreads to the Surface
"When some charge is transferred to a conductor, it readily gets distributed over the entire surface of the conductor." This single NCERT sentence carries a lot of physics. Because the excess charges are free to move and are all of the same sign, they repel one another and push themselves as far apart as the body allows. The only configuration in which the mutual repulsion is fully relieved — the lowest-energy, equilibrium state — places every bit of excess charge on the outer surface, with none left in the interior.
Contrast this with the insulator. With no free carriers, the deposited charge cannot redistribute at all and remains exactly where it was placed. NCERT is explicit that the full justification for the surface result belongs to the next chapter — but the qualitative reason, free charges fleeing one another, is enough to lock in the conclusion for now.
The mobility of free charge is the whole basis of charging by induction, where a nearby charge rearranges a conductor's electrons without any contact.
Charge on a conductor resides on the OUTER surface
A common MCQ tests whether students believe excess charge fills a conductor's volume. It does not — at electrostatic equilibrium the excess charge sits on the outer surface, and the interior carries no net excess charge. This holds whether the conductor is solid or hollow.
Solid or hollow, excess charge on a conductor lives on the outer surface.
Earthing and the Comb-vs-Spoon Test
The earth is an enormous conductor, and connecting a charged conductor to it by a conducting path lets the excess charge drain away — this is earthing (or grounding). NCERT frames the everyday version of it: a nylon or plastic comb gets electrified on combing dry hair, "but a metal article like a spoon does not. The charges on metal leak through our body to the ground as both are conductors of electricity."
The comb keeps its charge because it is an insulator with nowhere for the charge to go. The spoon loses its charge because the hand and body conduct it onward to the earth. NCERT adds a clinching qualifier: "if a metal rod with a wooden or plastic handle is rubbed without touching its metal part, it shows signs of charging." Block the conducting path to ground with an insulating handle, and even a metal object retains charge.
An isolated charged conductor (left) is connected to the earth by a conducting wire. The excess electrons flow into the vast reservoir of the earth, leaving the conductor effectively neutral (right).
Earthing neutralises — it does not "store" charge on the body
Connecting a charged conductor to earth makes its excess charge drain into the ground; the conductor ends up neutral, not "charged through the wire." Treat the earth as an infinite sink whose own potential and charge state are unaffected.
Earth a charged conductor → it goes neutral. Earth is the reference (zero) and an unlimited charge reservoir.
Conductors and Insulators in one screen
- Conductors contain electrons comparatively free to move; they allow electricity to pass easily. Metals, the human/animal body, the earth and electrolytes conduct.
- Insulators have charges bound to atoms with no free flow, so they resist the passage of electricity. Glass, porcelain, plastic, nylon, wood and rubber are insulators (dielectrics).
- Semiconductors are a third class with mobility intermediate between the two — silicon, germanium.
- Charge given to a conductor spreads over its outer surface; charge put on an insulator stays localised at the same place.
- Earthing drains a conductor's excess charge into the earth, leaving it neutral — which is why a metal spoon does not stay charged but an insulating comb does.