Introduction: charges at rest
On a dry January morning in Delhi you pull a nylon kurta or a polyester sweater over your head and hear a faint crackle.
In a dark room you may even see tiny blue sparks.
Step out of an autorickshaw, touch the metal frame of the door, and a small shock stings your fingertip.
A few months later, in April or May, a kalbaishakhi storm rolls over Kolkata and a lightning flash joins a cloud to the ground with a crack you can hear across the city.
These are the same physics on very different scales.
In each case electric charge has piled up somewhere it does not normally sit, and then suddenly found a path to escape.
The crackle, the shock and the lightning flash are all discharges of charge that had been building up quietly.
That charge came from rubbing: cloth on skin, a seat cover against your clothes, ice crystals and water drops colliding inside a thundercloud.
This unit and the next one study charges in exactly this "built up and sitting still" state.
That is why the subject is called electrostatics: "static" means not moving and not changing with time.
What electrostatics studies
Electrostatics asks three questions about charges at rest.
- Forces. How hard do two charges push or pull on each other, and how does that depend on their sizes and their distance apart? The answer is Coulomb's law, which you meet later in this unit.
- Fields. A charge affects the space around it even when nothing else is there. The electric field describes that effect at every point, so you can find the force on any charge you place there.
- Potentials. The energy side of the story: how much work is needed to move a charge from one place to another. Potential is the main topic of the next chapter.
Everything in this chapter is built from two ideas: Coulomb's law for a pair of charges, and the rule that forces from several charges simply add as vectors.
Keep that in mind and the later results (fields of rings, sheets and spheres, the dipole, Gauss's law) stop looking like separate formulas to memorise.
Why "at rest" makes things simpler
Charges that move do more than charges that sit still.
A moving charge also produces a magnetic field, and a changing current sets up induced effects.
You will study those in the units on magnetism, induction and alternating current.
By keeping the charges still, electrostatics deals with the electric force alone.
This is not a toy case: most of the charge on a rubbed comb, a charged cloud base or the plates of a capacitor is effectively at rest for the time we care about.
"At rest" does not mean that nothing moved to create the charge.
When you rub a balloon on dry hair, electrons do move from your hair to the balloon.
Electrostatics begins once that transfer is over and the charges have settled.
Why dry weather makes it worse
The shock from a car door or a metal railing is much more common in a dry north Indian winter than in the Mumbai monsoon.
The reason tells you something useful about charge.
Ordinary air is a very good insulator, so charge on your body or your clothes has no easy way to leave.
In humid weather, however, a very thin film of moisture forms on surfaces such as cloth, skin and furniture.
That film contains dissolved ions and lets charge leak away slowly and harmlessly as fast as it is produced.
In dry air there is no such film, so charge keeps building up with every step you take on a carpet or every time you slide across a seat.
When your hand finally gets close to a metal object, the charge escapes all at once as a spark.
What a spark actually is
Air will not normally carry charge.
But when a large amount of charge builds up and a conductor comes very close, the electric push across the small gap becomes enormous.
Dry air breaks down when the field across it reaches roughly 3 × 10⁶ V m⁻¹, which is why the spark jumps only when your finger is a few millimetres from the handle.
The air in the gap is torn into ions, it conducts for a moment, the charge rushes across, and the heated air glows and snaps.
Lightning is the same process across hundreds of metres or more of air, with charges separated inside the cloud by colliding ice and water particles.
The photograph shows such a discharge: a single flash carries the charge that has built up at the base of the storm cloud down to the ground in a fraction of a second.
You will meet the electric field and its unit properly later in this unit; for now the point is that a discharge needs both a large build-up of charge and a short path.
Electric forces are everywhere, not only in sparks
Static electricity can look like a curiosity, but the electric force is behind most of the forces you meet every day.
Matter is made of positively charged nuclei and negatively charged electrons, normally in exact balance.
The forces that hold atoms together in a molecule, molecules together in a solid, the stickiness of glue, friction between surfaces and the push of a table on your book are all electrical in origin.
Gravity is far too weak to explain any of them: between two electrons the electric repulsion is more than 10⁴² times their gravitational attraction, as you will check when you reach Coulomb's law.
So electrostatics is not a side topic.
It is the start of the physics of how matter holds together, and it leads directly to capacitors, electric current, and electromagnetic waves later in the course.
Connecting it
- Class XI: you will use vectors (adding, resolving into components), Newton's laws and the kinematics of uniform acceleration throughout this chapter.
- Next units: forces and fields between still charges come first; then field lines, flux and Gauss's law; then potential and capacitors; then charges in motion as current.
- Technology: photocopiers and laser printers, electrostatic spray painting of car bodies, the dust precipitators on thermal power station chimneys and the touch screen of your phone all use the ideas in this chapter.
How electrostatics is examined
In the board paper, the two electrostatics chapters and current electricity share one block of marks, and questions on this chapter come mainly as 1-mark MCQs and Assertion-Reason items, short 2- and 3-mark answers, and as one alternative of a long answer.
Many 1-mark items test whether you can explain everyday static effects with the right physics, so the reasons in this section are worth learning properly.
- Answers earn value points of ½ or 1 mark. Write each point as a separate, clear sentence or line.
- In a numerical, the formula earns ½, the substitution ½, and the answer with its unit ½. The unit belongs to that final value point, so always write it.
- A derivation earns marks line by line: draw the diagram and write every step, even the obvious ones.
- Give a direction with every force and field (towards the charge, along +x, away from B).
- In an internal choice ("(a) ... OR (b) ..."), answer one alternative only, fully.
- Correct physics in your own words earns the marks; you do not need a memorised sentence, except for definitions and laws, where the textbook wording is safest.
Where an entry gives a sentence in italics, that sentence is the textbook wording to learn for a definition question.
- Static: Static means anything that does not move or change with time.
- Electrostatics: Electrostatics deals with the study of forces, fields and potentials arising from static charges.
- Discharge: the sudden flow of built-up charge to another body or to the earth, often through air as a spark or a lightning flash.
- Static electricity: charge that has built up on a body (usually by rubbing) and stays in place until it is discharged.
Give reasons: (a) a crackle is heard when a polyester sweater is pulled off in a dry winter; (b) the same sweater hardly crackles in the humid monsoon. [2]
1. Credit line: reason (a) 1; reason (b) 1.
2. (a) Rubbing between the sweater and the shirt or skin transfers charge, so both become charged. Pulling them apart makes the charge jump across small air gaps as tiny sparks, which we hear as a crackle (1).
3. (b) In humid air a thin film of moisture on the cloth lets the charge leak away as it forms, so not enough builds up to spark (1).
Answer: as above.
The key words are "rubbing transfers charge" and "moisture lets the charge leak away".
Answer the following giving reasons. (a) After sliding off the seat of a car on a dry day, a passenger feels a shock on touching the metal door.
(b) The spark jumps only when the finger is very close to the metal.
(c) The topic is called electrostatics although electrons move during rubbing. [3]
1. Credit line: 1 + 1 + 1.
2. (a) Sliding over the seat cover charges the passenger's body by rubbing; touching the metal door lets the charge flow suddenly to the car body and the earth through the passenger's hand, which is felt as a shock (1).
3. (b) Air is an insulator. Only when the gap is very small does the electric field across it become large enough to break the air down and let charge cross as a spark (1).
4. (c) Electrostatics deals with the charges after they have been transferred, when they are at rest, and with the forces, fields and potentials they produce; the brief transfer is not what is being studied (1).
Answer: as above.
Each part needs the cause, not just a description of what is seen.
Differentiate between electrostatics and current electricity, giving two points of difference. [2]
Electrostatics
Current electricity
Deals with charges at rest (½)
Deals with charges in steady motion (½)
Studies forces, fields and potentials of the still charges, for example the force between a rubbed comb and bits of paper (½)
Studies the flow of charge round a circuit, for example the current through a bulb (½)
Answer: two matched pairs, ½ + ½ each.
A "differentiate" answer needs the two sides of each point side by side; listing two facts about electrostatics alone earns at most 1.
When a question asks "why" about a spark, a shock or lightning, name the full chain in one line each: charge builds up by rubbing, it cannot leak away (dry air, insulating surfaces), then it escapes suddenly through a short conducting path.
Each link is usually a ½-mark value point.
- "Static" read as "no charge moves at all". Charges move while a body is being charged; electrostatics studies them once they have come to rest. Say "charges at rest", not "charges that never move".
- "Rubbing creates charge." Rubbing only moves electrons from one body to the other. Both bodies end up charged, with equal and opposite charges. You will see why in the section on conservation of charge.
- "Dry air conducts better, so the shock is bigger." The opposite: dry air and dry surfaces are better insulators, so charge cannot leak away and more of it builds up before it sparks.
- Thinking electric forces matter only in sparks. Contact forces, friction and chemical bonds are all electrical in origin; gravity between everyday objects is tiny by comparison.
Check your understanding
1. Which one of the following is not an electrostatic effect?
- (A) A balloon rubbed on dry hair sticks to a wall.
- (B) Small bits of paper jump towards a plastic scale rubbed on a woollen sweater.
- (C) The coil of an electric heater glows red when it is switched on.
- (D) A crackle is heard when a synthetic shirt is pulled off on a dry day. [1]
2. Electrostatics is the study of
- (A) forces, fields and potentials arising from charges at rest
- (B) the steady flow of charge through a conductor
- (C) the magnetic effects of moving charges
- (D) the heating effect of electric current [1]
3. Two statements are given, one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) as given below.
Assertion (A): Shocks from car doors and metal railings are more common in dry winter weather than in humid weather.
Reason (R): Dry air allows charge on the body to leak away quickly.
- (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- (C) Assertion (A) is true, but Reason (R) is false.
- (D) Both Assertion (A) and Reason (R) are false. [1]
4. A student says: "Lightning and the crackle of a woollen sweater have nothing in common, because one is huge and the other is tiny." Explain what the two have in common, naming the two conditions needed for each to happen. [2]
5. Answer the following giving reasons.
(a) Electric forces, not gravitational forces, decide how atoms hold together in a solid. (1)
(b) A plastic comb run through dry hair can pick up small bits of paper, but the same comb works poorly after the hair has been washed and is still damp. (1)
(c) A spark from a charged finger to a metal tap is seen only when the finger is brought very close to the tap. (1) [3]
6. State two reasons why physicists study charges at rest separately before studying charges in motion. [2]
Answers
1. (C) The coil of an electric heater glows red when it is switched on. (1) This is the heating effect of a current, which needs charges in continuous motion; the other three are charges built up by rubbing and held at rest.
2. (A) forces, fields and potentials arising from charges at rest (1)
3. (C) Assertion (A) is true, but Reason (R) is false. (1) Shocks are indeed more common in dry weather, but the reason is the opposite of R: dry air and dry surfaces do not let the charge leak away, so it builds up until it sparks.
4. Credit line: common process 1; two conditions 1.
- Both are discharges: charge that has built up suddenly flows through air to another body or to the earth (1).
- Condition 1: a large amount of charge must build up, which happens by rubbing (sweater on skin; colliding ice and water particles in the cloud) (½).
- Condition 2: the charge must find a path; the air breaks down and conducts only when the electric field across the gap is very large (½).
5. Credit line: 1 + 1 + 1.
(a) Atoms are made of charged particles, and the electric force between charges is enormously stronger than gravity between such small masses (more than 10⁴² times for two electrons), so gravity has a negligible share (1).
(b) The damp hair has a moisture film that lets charge leak away, so the comb does not stay charged enough to attract the paper (1).
(c) Air is an insulator; only across a very short gap does the electric field from the charge become large enough to break the air down and let the charge cross (1).
6. (any two, 1 each)
- With charges at rest only the electric force acts; moving charges also produce magnetic forces, which are added later.
- Forces, fields and potentials of still charges can be found from Coulomb's law and superposition alone, which then underlie current electricity.
- Many real charges (on rubbed objects, cloud bases, capacitor plates) are effectively at rest, so the results are directly useful. or any other relevant point.