Electric Charge and Electroscopes വൈദ്യുത ചാർജ്
Why does a polyester saree or a synthetic sweater crackle when you take it off on a dry winter evening? Why do you sometimes get a small shock from a car door or the iron bar of a bus? This page starts the chapter where the textbook does: with rubbed rods, pith balls and paper strips. By the end you will know that there are two kinds of charge, that like charges repel and unlike charges attract, how an electroscope detects charge, and that rubbing only moves electrons – it never creates charge.
- Atoms have a positive nucleus (protons + neutrons) and negative electrons around it; a normal atom is neutral.
- Force is a push or a pull (page 3 of this guide). Nothing else – the rest is built here.
1. Sparks, crackles and lightning (1.1)
All the everyday experiences above have one cause: electric charges that piled up when insulating surfaces were rubbed together suddenly discharge – often through your body. A spark is a tiny version of the giant discharge we call lightning. You may have heard this called static electricity. "Static" means anything that does not move or change with time.
Electrostatics (സ്ഥിതവൈദ്യുതി) is the study of forces, fields and potentials arising from static charges (charges at rest). Chapter 1 deals with forces and fields; Chapter 2 with potentials.
- Dry weather sparks – crackles from synthetic clothes, blankets and sweaters (worse in dry winter air, rare in the humid monsoon because moist air slowly lets the charge leak away).
- Car-door and bus-rail shocks – sliding over a seat charges your body; touching metal lets it discharge.
- Lightning – charges separated inside thunderclouds discharge to the ground or to another cloud (page 17 explains lightning conductors).
- Photocopiers and laser printers use charged drums to pick up toner powder (page 17).
2. Two kinds of charge – rods and pith balls (1.2, Fig 1.1)
Around 600 BC, Thales of Miletus (Greece) noticed that amber rubbed with wool or silk attracts light objects such as straw. The word electricity comes from the Greek elektron, meaning amber. Many other pairs of materials behave the same way when rubbed: they then attract light objects like straw, pith balls (today, polystyrene balls) and bits of paper. This charging by rubbing is called electrification by friction (ഘർഷണം മൂലമുള്ള വൈദ്യുതീകരണം).
Cut long, thin strips of white paper and iron them lightly. Hold them near a TV screen or computer monitor: they are attracted to the screen and may stay stuck to it for a while. (Old picture-tube TVs worked best; a freshly wiped flat screen also works on a dry day.)
The textbook then describes six careful observations, Fig 1.1 (a)–(f). Click each panel and watch.
Glass rods are rubbed with silk; plastic rods with cat's fur. The suspended rod hangs from a silk thread so that it can turn freely. Pith balls hang from silk or nylon threads (insulators) so that their charge cannot escape. Signs shown follow Franklin's convention (section 3).
- There are only two kinds of electric charge (വൈദ്യുത ചാർജ്).
- Like charges repel and unlike charges attract each other.
- Charge is transferred from the rod to a pith ball on contact – the ball is charged by contact.
- The property that tells the two kinds apart is called the polarity of charge. A body with charge is electrified or charged; one with no charge is electrically neutral.
Try every combination the textbook lists: glass rod and silk; plastic rod and fur; plastic rod and glass rod; plastic rod and silk; glass rod and fur. Then try a charged object with an uncharged pith ball – you will learn why it is attracted on page 5.
3. Positive and negative – Franklin's convention
When a glass rod is rubbed with silk, the rod gets one kind of charge and the silk gets the other kind. If the charged rod is put back in contact with the silk, the two no longer attract each other, and neither attracts light objects any more: the two kinds of charge have neutralised each other. Adding a positive number to a negative number of the same size gives zero – so the American scientist Benjamin Franklin named the two kinds positive (+) and negative (−).
| Rubbed pair | Becomes positive (+) | Becomes negative (−) |
|---|---|---|
| glass rod with silk | glass rod | silk |
| plastic rod with cat's fur | cat's fur | plastic rod |
This choice is only a convention: physics would work just as well with the names swapped. What matters is that the two kinds cancel each other.
📦 Box (textbook page 3) · Unification of electricity and magnetism
Electricity (charges on rods, cat's fur, batteries, lightning) and magnetism (magnets, iron filings, compass needles) were once separate subjects. In 1820 the Danish scientist Oersted found that an electric current deflects a compass needle. Ampère and Faraday showed that moving charges produce magnetic fields and moving magnets generate electricity. The Scottish physicist Maxwell and the Dutch physicist Lorentz then built one theory joining the two – electromagnetism. Maxwell's four equations play the same role in electromagnetism as Newton's laws do in mechanics; he also showed that light is an electromagnetic wave whose speed follows from purely electric and magnetic measurements.
Almost every force around us – friction, the chemical forces holding atoms together, forces inside living cells – is electromagnetic in origin. When all charges are at rest, the forces are purely electric: that is our chapter. Like gravity, the electric force is long-range and falls off as 1/r², but it is stronger by many orders of magnitude (page 7).
4. Detecting charge – the electroscope (Fig 1.2)
A simple apparatus to detect charge is the gold-leaf electroscope (ഇലക്ട്രോസ്കോപ്പ്) [Fig 1.2(a)]. A vertical metal rod passes through a rubber stopper into a box with glass windows; two thin gold leaves hang from its bottom end, and a metal knob sits on top. When a charged object touches the knob, charge flows down the rod to the leaves. Both leaves get the same kind of charge, so they repel and diverge. The degree of divergence indicates the amount of charge.
One "unit" here is just a cartoon amount of charge. Watch three things: (1) more charge → wider divergence; (2) a finger connects the electroscope to the earth through your body, so the charge leaks away; (3) a charged rod held near the knob makes the leaves diverge without touching – the rod pulls electrons up into the knob. The last trick is induction (page 5).
Make your own electroscope [Fig 1.2(b)]
Why do the leaves diverge? The paper-strip experiment (Fig 1.3)
The ironed strip has picked up charge (by friction with the iron). Folded in half, both halves carry the same kind of charge, so they repel. The aluminium-foil leaves of the home-made electroscope behave in exactly the same way: charge given to the ball end flows down the curtain rod, both halves of the foil get similar charge, and they move apart – the more charge, the wider the gap. The paper scale in the bottle gives a rough measure of that gap.
5. Why do bodies get charged on rubbing?
All matter is made of atoms and molecules. Normally a body is electrically neutral – it does contain charges, but they are exactly balanced. In fact the forces that hold atoms and molecules together, the stickiness of glue and surface tension are all electrical in origin: the electric force is everywhere.
To electrify a neutral body we must add or remove one kind of charge. "A body is charged" always means it has an excess or deficit of charge. In solids the charges that move are some of the less tightly bound electrons:
- A body that loses electrons becomes positively charged.
- A body that gains electrons becomes negatively charged.
Each rub in the cartoon moves one electron. In reality a few rubs move billions of electrons – yet that is a tiny fraction of the roughly 1023 electrons in the rod. Only loosely bound electrons move, which is why only certain pairs of materials charge well.
When a glass rod is rubbed with silk, electrons move from the glass rod to the silk. The rod becomes positive, the silk negative, by equal amounts. No new charge is created by rubbing – charge is only transferred (this is the conservation of charge, page 6).
- 🧪 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).
6. Check yourself
- Electrostatics studies forces, fields and potentials of charges at rest.
- Two kinds of charge: like charges repel, unlike charges attract. Franklin named them + (glass rubbed with silk, fur) and − (plastic rubbed with fur, silk).
- An electroscope detects charge: its leaves get the same charge and diverge; more charge, more divergence.
- Rubbing transfers loosely bound electrons: the loser becomes +, the gainer −, by equal amounts. No charge is created.
Next: Conductors, insulators, earthing and charging by induction ▶