Conductors, Insulators, Earthing and Induction ചാലകം · ഇൻസുലേറ്റർ · എർത്തിങ്
Why does a plastic comb charge when you run it through dry hair, but a steel spoon does not? Why is the metal body of your iron box connected to a green earth wire? And how can a charged rod charge a metal sphere without touching it – and without losing any of its own charge? This page answers all three, and finally explains why a charged comb picks up bits of paper.
- Two kinds of electric charge (വൈദ്യുത ചാർജ്); like charges repel, unlike charges attract.
- In solids, only electrons move; losing electrons → positive, gaining electrons → negative.
1. Conductors and insulators (1.3)
The textbook starts with two puzzles. First: a metal rod held in the hand and rubbed with wool shows no sign of charge, but the same rod with a wooden or plastic handle, rubbed without touching the metal, does get charged.
Second: connect one end of a copper wire to a neutral pith ball and the other end to a negatively charged plastic rod – the pith ball becomes negative. Repeat with a nylon thread or a rubber band – no charge reaches the ball. Why?
- Conductors (ചാലകം) allow electricity to pass through them easily: they have electric charges (electrons) that are comparatively free to move inside the material. Examples: metals, human and animal bodies, the earth.
- Insulators (ഇൻസുലേറ്റർ (കുചാലകം)) offer high resistance to the passage of electricity. Examples: most non-metals – glass, porcelain, plastic, nylon, wood.
- Semiconductors (അർധചാലകം) – a third category – offer resistance intermediate between conductors and insulators (silicon, germanium; you meet them in Chapter 14).
So the puzzles are solved: a metal rod is charged by rubbing, but the charge leaks away through your body (a conductor) to the ground. A plastic comb or nylon thread is an insulator – charge stays on it, and cannot travel along it.
Where does the charge sit?
When charge is given to a conductor, it quickly spreads over the entire surface. On an insulator it stays where it was put. (Why charge sits only on the outer surface of a conductor is explained in Chapter 2.)
2. Earthing and the wiring in your house
When a charged body is connected to the earth, all its excess charge disappears: a momentary current flows to the ground through the connecting conductor (which may be your own body). This process of sharing the charge with the earth is called grounding or earthing (എർത്തിങ്). The earth is so huge that it can accept or supply any amount of charge without its own state changing noticeably.
Earthing is a safety measure for circuits and appliances. A thick metal plate is buried deep in the ground near the mains supply, and thick wires from it run through the building. House wiring has three wires: live and neutral carry current from the power station; the third, earth, is connected to the buried plate. The metal bodies of appliances – electric iron, refrigerator, TV – are connected to the earth wire.
Colours used here: live red, neutral blue (in India often black), earth green. When the fault current rushes to the earth through the thick earth wire, it is so large that the fuse blows or the MCB trips (going further – you study fuses in Class 10 and Chapter 3) and the supply is cut off. Without the earth wire, the metal body stays "live" – and the next person who touches it becomes the path to the earth.
3. Charging by induction (1.4, Fig 1.4)
Touching a pith ball with a charged plastic rod gives the ball some of the rod's charge: it is charged by contact. There is a cleverer way that needs no contact at all. Bring two metal spheres A and B on insulating stands into contact, and follow the textbook's steps (i)–(iv). The rod is a positively charged glass rod.
- The two spheres end up with equal and opposite charges.
- The charged rod does not lose any of its charge – unlike charging by contact.
- The redistribution of charge in (b) is called induction of charge; it happens almost instantly and lasts only while the rod is held near.
Example 1.1 · Charge a metal sphere positively without touching it
Q. How can you charge a metal sphere positively without touching it? (textbook Example 1.1, Fig 1.5)
Use a negatively charged rod. Think about where the free electrons of the sphere go when the rod comes near, and then give those electrons an escape route to the earth while the rod is still there.
- (a) Take an uncharged metal sphere on an insulating stand.
- (b) Bring a negatively charged rod close to it (no contact). Free electrons are repelled to the far side; the near side becomes positive (deficit of electrons).
- (c) Connect the sphere to the ground with a wire: the repelled electrons flow to the ground; the + charge at the near end is held by the attraction of the rod.
- (d) Disconnect the ground wire, keeping the rod in place – the + charge stays.
- (e) Remove the rod: the + charge spreads uniformly over the sphere. The sphere is charged positively by induction; the rod loses no charge.
4. Why does a charged rod attract light objects?
A charged rod brought near a light object (bits of paper, a pith ball) induces opposite charge on the near surface and leaves similar charge on the far side. (This happens even if the object is an insulator – see Sections 1.10 and 2.10.) The centres of the two kinds of charge are slightly separated. The near, opposite charge is attracted; the far, similar charge is repelled. Because the force weakens with distance, the attraction (nearer) outweighs the repulsion (farther), and the light object is pulled to the rod.
Cartoon model: the induced charges are taken as ±q, separated by 0.3 cm inside the paper, and each force falls off as 1/(distance)² (Coulomb's law, page 7). The closer the comb, the bigger the difference – which is why paper bits jump up only when the comb is quite near.
A balloon rubbed on your hair sticks to a wall the same way: it induces opposite charge on the wall's surface. Dust sticks to a TV screen or a plastic CD for the same reason. On page 17 you will see how photocopiers and electrostatic precipitators (in thermal power stations) use these forces.
- 🧪 Balloons and Static Electricity (PhET simulation) PhET, University of Colorado Boulder
Rub a balloon on a sweater and watch electrons move; see induction on the wall. - 🧪 John Travoltage (PhET simulation) PhET, University of Colorado Boulder
Rub a foot on a carpet and get a spark from a door knob. - 📄 Static Electricity tutorial The Physics Classroom
Charge, charging methods, induction and electroscopes explained step by step. - 📄 Charging by Induction The Physics Classroom
Animated steps of charging two spheres by induction. - 📄 Triboelectric effect Wikipedia
Why some materials give electrons and others take them when rubbed. - 📄 Electroscope Wikipedia
The gold-leaf electroscope and how it detects charge. - 📄 Oil drop experiment Wikipedia
How Millikan showed that charge is quantised (Exercise 1.25).
5. Check yourself
- Conductors (metals, human body, earth) have free electrons; insulators (glass, porcelain, plastic, nylon, wood) do not; semiconductors are in between.
- Charge given to a conductor spreads over its surface; on an insulator it stays put.
- Earthing shares a body's charge with the earth; the earth wire protects us when a live wire touches an appliance's metal body.
- Induction: a charged rod separates charges in nearby conductors; separating the spheres (or earthing) leaves them charged – equal and opposite, and the rod loses nothing.
- A charged rod attracts neutral light objects because the induced opposite charge is nearer than the induced similar charge.
Next: Basic properties of charge: additivity, conservation, quantisation ▶