Physics · Electric Charges And Fields · NEET
| Free charge carriers | Present (free electrons in metals; ions in electrolytes) | Almost none; electrons are tightly bound |
| Movement of charge | Charge flows through easily | Charge cannot flow; it stays put |
| Where added charge goes | Spreads over the outer surface, can flow to earth | Stays only at the spot where placed |
| Resistivity | Very low (about 10^-8 to 10^-6 ohm-metre for metals) | Very high (about 10^11 to 10^19 ohm-metre) |
| Examples | Copper, aluminium, human body, earth | Glass, plastic, porcelain, wood, rubber |
In a metal, the outer (valence) electrons leave their atoms and become free to move through the whole material. These free electrons act like a moving gas of charge, so when charge is added it spreads out at once. In plastic, glass or wood the electrons stay tightly bound to their atoms, so there are almost no free charges to move. That is the one real difference: free electrons present (conductor) versus free electrons absent (insulator).
The human body is a conductor. NCERT lists metals, human and animal bodies, and the earth as conductors. This is why a person standing on the ground who touches a live wire gets a shock: charge flows through the body to the earth. This exact idea appears in NCERT Example 2.7.
On a conductor, added charge spreads over the whole surface almost instantly and can flow away to earth if the conductor is connected to the ground (earthing). On an insulator, charge stays only at the spot where you put it and does not spread. That is why you can charge one end of a plastic rod by friction and the charge stays there.
A semiconductor is a third category. Its ability to conduct is in between a conductor and an insulator. Silicon and germanium are examples. Its resistivity lies between a metal (about 10^-8 to 10^-6 ohm-metre) and an insulator (about 10^11 to 10^19 ohm-metre). For this chapter you mainly need conductor versus insulator; semiconductors are studied in detail later.
Yes, the earth is treated as a very large conductor. Because it is so large, giving it extra charge or taking charge from it does not change its condition. Earthing (grounding) means connecting a conductor to the earth by a wire so unwanted charge flows away safely. This keeps metal appliance bodies safe to touch.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A conductor has free (mobile) charges, usually electrons, so electric charge moves through it easily. An insulator has no free charges, so charge cannot move and stays where it is placed.
Conductors: metals (like copper, aluminium), the human body, and the earth. Insulators: glass, plastic, porcelain, nylon, wood and rubber.
Free electrons repel each other and move until they are as far apart as possible. In the static state the charge sits on the outer surface and the electric field inside the conductor becomes zero.
Yes, insulators have electrons, but the electrons are bound tightly to their atoms and are not free to move through the material, so the substance does not conduct.
Pure water is a poor conductor, but ordinary water has dissolved salts that provide mobile ions, so it conducts. This is why a rainy day or wet hair changes electrostatic experiments, as noted in NCERT.
