Applications & Explore Online സ്ഥിതവൈദ്യുതി
A copper strip on a temple tower saves it from lightning, the green earth wire in your house saves your life, a photocopier prints with powder and charge, a chimney filter catches smoke, a spray gun paints the back of a pipe it cannot see, a car is the safest place in a thunderstorm, your phone screen feels your finger, and a fuel tanker is clamped to the ground before it unloads. Every lab below uses only the ideas of this chapter – charges, conductors, induction, Coulomb’s law, electric fields and Gauss’s law.
- There are two kinds of electric charge (വൈദ്യുത ചാർജ്): like charges repel, unlike charges attract; rubbing transfers electrons – static electricity (സ്ഥിതവൈദ്യുതി) – page 4.
- Charges move freely through a conductor (ചാലകം) – metals, the human body, the earth – but not through an insulator (ഇൻസുലേറ്റർ (കുചാലകം)); earthing (എർത്തിങ്) lets extra charge flow away into the ground; a charged body near a conductor causes charging by induction (പ്രേരണം വഴി ചാർജ് ചെയ്യൽ (വൈദ്യുതപ്രേരണം)) – page 5.
- Charge is quantised, q = ne with e = 1.6 × 10−19 C, and conserved – page 6. The force between charges follows Coulomb’s law (കൂളോം നിയമം) – page 7.
- A charge in an electric field (വൈദ്യുത മണ്ഡലം) feels F = qE (a negative charge is pushed against E) – page 9; electric field lines (വൈദ്യുത മണ്ഡലരേഖകൾ) start on + and end on − and meet a conductor at right angles – page 10.
- Inside a conductor, and inside an empty cavity in it, E = 0; all excess charge sits on the outer surface – Gauss’s law (ഗോസ് നിയമം), page 13 and Exercise 1.28 on page 16.
Labs marked go further go beyond the textbook – enjoy them, but in the exam stay with the textbook ideas. Values marked “approx.” are typical values, not from the textbook.
1. Lightning and lightning conductors
Inside a thunder cloud, ice crystals and water drops collide and rub; charges separate, and the base of the cloud usually becomes strongly negative. By induction, the ground and the buildings below become positive. The field between cloud and ground grows until the air breaks down (about 3 × 106 N/C in dry air, approx.) and a huge spark – lightning – carries the charge across. A lightning conductor is a pointed metal rod fixed above the highest part of a building, joined by a thick copper strip to a metal plate buried deep in moist earth. Field lines crowd at its sharp tip, so the field there is strongest: the rod is struck first, and the charge runs safely down the copper (a good conductor) into the earth instead of through the walls.
Purple arrows are field lines: they start on the induced + charges on the ground and end on the − charges of the cloud. A real flash carries roughly 5–30 C (approx.) in a fraction of a second.
📝 In the exam this looks like: “A lightning flash transfers 16 C of negative charge to the earth. How many electrons flow?” and “Why is a lightning conductor pointed at the top and connected to the earth?”
Given: q = 16 C, e = 1.6 × 10−19 C. Formula: q = ne ⇒ n = q/e. Substitution: n = 16 / (1.6 × 10−19). Answer: n = 1.0 × 1020 electrons.
Why pointed and earthed? (1) Field lines crowd at a sharp point, so the field at the tip is very strong – the rod is struck first (and it also lets charge leak slowly into the air). (2) The thick copper strip is a good conductor joined to a plate in moist earth, so the charge flows safely into the ground instead of through the building.
2. Earthing in house wiring
Mains electricity reaches every room through three wires: the live wire (red or brown insulation), the neutral wire (black or blue) and the earth wire (green or green-yellow). The earth wire is joined to a metal pipe or plate buried deep in moist soil near the house (the earth pit), and to the metal body of every heavy appliance – iron box, refrigerator, washing machine, water heater. The human body and the earth are both conductors. If a loose live wire touches the metal body, the earth wire gives the charge an easy, low-resistance path into the ground, so a large current flows, the fuse or MCB cuts the supply at once go further, and a person who touches the appliance is safe. Without earthing, the body itself becomes “live” and the current passes through the person.
Moving dots show the direction of the current in the fault (the conventional current). Colours of insulation differ between old and new wiring – never trust colours, trust an electrician.
📝 In the exam this looks like: “What is earthing? Why are the metal bodies of electrical appliances earthed?” (2 marks) or “Name a conductor and an insulator used in house wiring.”
Earthing is connecting a charged body (or the metal body of an appliance) to the earth by a conductor, so that extra charge flows into the earth – the earth is a huge conductor that can take or give any amount of charge.
If the live wire touches the metal body, the charge flows through the earth wire into the ground instead of through the person who touches it – this prevents an electric shock (and the large current blows the fuse). Conductor: copper wire; insulator: PVC (plastic) covering, porcelain or bakelite in switches.
3. Photocopier and laser printer
The heart of a photocopier is a drum coated with a photoconductor – a semiconductor (അർധചാലകം) that is an insulator in the dark but conducts when light falls on it. The drum is first given a uniform charge; light reflected from the white parts of the page makes those parts conducting, and their charge leaks away. A charge “picture” of the dark letters stays behind. Toner powder with the opposite charge sticks only to that picture (unlike charges attract), is pulled onto a charged sheet of paper, and is melted in by hot rollers. A laser printer works the same way, but a laser beam “writes” the picture on the drum.
Red + signs: positive charge on the drum. Black dots: toner grains carrying negative charge. Real copiers may use the opposite signs – the idea is the same.
📝 In the exam this looks like: “Mention two applications of electrostatics in daily life” or “Why does toner stick only to some parts of the drum?”
Applications: photocopiers and laser printers, electrostatic precipitators in chimneys, electrostatic spray painting, lightning conductors, shielding of sensitive instruments (any two).
Toner grains are negatively charged; they are attracted only to the parts of the drum that are still positively charged (the image of the dark letters). Where light fell, the photoconductor conducted and the charge leaked away, so no toner sticks there.
4. Electrostatic precipitator – cleaning chimney smoke
Thermal power plants and cement factories send huge amounts of ash and dust up their chimneys. In an electrostatic precipitator, the smoke passes between earthed metal plates with a thin wire in the middle kept at a high negative voltage. Near the wire the field is very strong; the air there ionises and the smoke particles pick up electrons – they become negatively charged. The field points from the plates towards the wire, so the force on a negative particle, F = qE, is opposite to E: towards the plates. The particles stick to the plates, and are shaken down into hoppers. Clean gas leaves the top.
A simple model: the sideways drift speed of a charged particle grows with the voltage (stronger field, larger force qE). Real precipitators remove more than 99 % of the ash (approx.).
📝 In the exam this looks like: “A dust particle carries a charge equal to that of 500 electrons and is in a field of 2 × 105 N/C. Find the force on it and its direction.”
Given: q = −500e = −500 × 1.6 × 10−19 C = −8 × 10−17 C, E = 2 × 105 N/C. Formula: F = qE. Substitution: |F| = 8 × 10−17 × 2 × 105. Answer: |F| = 1.6 × 10−11 N, directed opposite to E (the charge is negative) – towards the collecting plates.
5. Electrostatic spray painting
A spray gun gives every paint droplet a negative charge and the object (a car body, a cycle frame, a pipe) is earthed. By induction, positive charge gathers on the object, and field lines run from the object to the gun. The negative droplets move along these lines (against E) – and since field lines meet a conductor at right angles all over its surface, they curve round and reach even the back of the object (“wrap-around”). Because like charges repel, the droplets also spread into a fine, even mist. Less paint is wasted and the coat is uniform.
The curved lines are true field lines of a point charge (the nozzle) near an earthed conducting sphere, worked out on this page. In a real booth the droplets also have some forward speed.
📝 In the exam this looks like: “Electric field lines are always normal to the surface of a conductor. How does this help in spray painting?” or “Why do charged paint droplets spread out evenly?”
The charged droplets travel along the field lines between the gun and the earthed object. The lines end on the object at right angles all over its surface – including the sides and back – so paint reaches every part, even the hidden side. Like charges on the droplets repel each other, so they spread out into a fine, even spray instead of forming lumps.
6. Faraday cage – why a car is safe in lightning
Inside a conductor the electrostatic field is zero, and if the conductor has a hollow with no charge in it, the field inside the hollow is also zero, however strong the field outside (Exercise 1.28). The charges on the outer surface rearrange themselves – by induction – so that their field exactly cancels the outside field everywhere inside. This is electrostatic shielding (സ്ഥിതവൈദ്യുത കവചനം). A metal car or bus, an aeroplane, or a room lined with metal mesh (a Faraday cage) protects whatever is inside. When lightning strikes a car, the charge flows over the outer metal body and jumps to the ground – the people inside are safe (keep the windows closed and do not touch the metal frame). Hospitals line MRI rooms with copper sheet so that outside radio signals cannot disturb the scanner go further.
The plates make a uniform field from left to right. Lines end on induced − charges on the near side of the metal body and start again from the induced + charges on the far side; they meet the surface at right angles.
📝 In the exam this looks like: “Why is it safer to be inside a car than under a tree during a thunderstorm?” or “Suggest a way to shield a sensitive instrument from strong electrostatic fields.”
Take a Gaussian surface (ഗോസിയൻ ഉപരിതലം) inside the metal of the body: E = 0 there, so the enclosed charge is zero – all the charge stays on the outer surface, and the field in the empty space inside is zero. The car body acts as a Faraday cage: the lightning current flows over its outer surface to the ground and not through the people inside. To shield an instrument, enclose it in a closed metal box (or a fine metal mesh).
7. Touch screens and anti-static wrist straps go further
A phone screen has a grid of transparent conducting strips under the glass, with a small charge on them. Your finger is a conductor joined to a large body: when it comes close, it changes the charge and field at the strips just below it, and the chip finds the row and column where the change happened. That is why the screen ignores a woollen glove or a plastic pen – they are insulators. In electronics factories, walking on a carpet charges a worker’s body by friction (electrification by friction, ഘർഷണം മൂലമുള്ള വൈദ്യുതീകരണം); one spark from a finger can destroy a chip. So workers wear a wrist strap joined to earth through a large resistor (about 1 MΩ, approx.) – the charge leaks away slowly and safely.
Phone screens actually sense a tiny change in capacitance (2nd PUC Chapter 2) many times a second; the picture shows only the idea.
📝 In the exam this looks like: “By walking on a carpet a person gains a charge of −0.8 μC. How many electrons has he gained? Has his mass changed?”
Given: q = 0.8 μC = 0.8 × 10−6 C (magnitude), e = 1.6 × 10−19 C. Formula: n = q/e. Substitution: n = 0.8 × 10−6 / 1.6 × 10−19. Answer: n = 5 × 1012 electrons gained. Yes, the mass increases, but only by 5 × 1012 × 9.1 × 10−31 kg ≈ 4.6 × 10−18 kg – far too small to notice (compare Exercise 1.11).
8. Static electricity in fuel tankers
When petrol or diesel flows fast through pipes and hoses, it rubs against them and charges separate. The tanker stands on rubber tyres (insulators), so the charge cannot escape and keeps building up on its metal body. A spark near the nozzle, where petrol vapour mixes with air, could start a fire. So before unloading, the driver clamps a thick earthing (bonding) cable from the tanker to an earth pin at the fuel station; older tankers dragged a metal chain on the road. Aeroplanes are earthed the same way while refuelling. At petrol pumps: switch off the engine, do not use phones, and touch a metal part of the vehicle away from the nozzle before you touch the fuel cap.
Red + signs: charge collecting on the tanker body. With the cable clamped, the charge flows away into the ground as fast as it is produced.
📝 In the exam this looks like: “Why are fuel tankers connected to the earth before unloading?” or “Why do rubber tyres of a tanker allow charge to build up?”
Friction between the moving fuel and the pipes separates charges, so the tanker body becomes charged. Rubber tyres are insulators, so the charge cannot flow to the ground and builds up until a spark may jump – which can ignite the fuel vapour. A metal cable (or chain) to the earth is a conductor: the charge flows away into the earth as soon as it is produced, so no dangerous charge collects (charge is conserved – it just moves to the earth).
✅ Check yourself
🌐 Explore online – resources hub
Everything in this guide works offline. The links below are extras – simulations and articles from trusted sources, grouped by topic. They need internet and open in a new tab, so you will not lose your place here. Ask a parent or teacher before you go online.
Electric charge, charging and its properties
- 🧪 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).
Coulomb's law
- 🧪 Coulomb's Law (PhET simulation) PhET, University of Colorado Boulder
Change the charges and the distance and read the force. - 📄 Coulomb's Law The Physics Classroom
Worked examples of the inverse-square law. - 📄 Coulomb's law Wikipedia
The law, its vector form and the superposition principle. - 📄 Charles-Augustin de Coulomb Wikipedia
The French physicist and his torsion balance.
Electric field and field lines
- 🧪 Charges and Fields (PhET simulation) PhET, University of Colorado Boulder
Place charges and see the electric field vectors and field of a dipole. - 🧪 Electric Field Hockey (PhET simulation) PhET, University of Colorado Boulder
A game: steer a charged puck using the forces of other charges. - 📄 Electric Field Lines The Physics Classroom
Rules for drawing field lines, with diagrams. - 📄 Electric field Wikipedia
Definition, field of a point charge and superposition.
Electric dipole
- 📄 Electric dipole moment Wikipedia
Dipoles, polar molecules and torque in a field.
Electric flux and Gauss's law
- 📄 Electric flux Wikipedia
Flux of a field through a surface. - 📄 Gauss's law Wikipedia
Gauss's law and its applications to symmetric charge distributions. - 📄 Carl Friedrich Gauss Wikipedia
The "prince of mathematicians" behind Gauss's law. - 📄 Faraday cage Wikipedia
Why the field inside a closed conductor is zero – electrostatic shielding (Exercise 1.28).
Electrostatics in daily life and technology
- 📄 Lightning rod Wikipedia
Earthing protects buildings from lightning. - 📄 Ground (electricity) Wikipedia
Earthing in house wiring – live, neutral and earth. - 📄 Electrostatic precipitator Wikipedia
Removing smoke particles using electric charges. - 📄 Photocopier Wikipedia
Xerography – copying with static charge.
More to explore
- 📄 MIT OpenCourseWare 8.02: Electricity and Magnetism MIT OpenCourseWare
Free college course notes for students who want to go further.
Now pull the whole chapter together: Summary, Points to Ponder & Quick Quiz ▶