Electrostatics
FoundationFriction never creates charge — it only transfers electrons from one object to another: gaining electrons makes an object negative, losing electrons positive. Like charges repel and unlike charges attract; a charged object is surrounded by an electric field. Point discharge underlies lightning conductors, while earthing lets tanker trucks safely drain charge.
- Charging by friction transfers electrons — it never creates charge; gainer negative, loser positive, total charge conserved
- Like charges repel, unlike charges attract; an electroscope's leaves spread apart by repulsion
- A charged body attracts small uncharged objects through electrostatic induction (the rod lifting paper)
文體小知識：科學知識說明文——由摩擦起電的現象入手，講解電荷、電子轉移、感應起電與電場，再以避雷針和油罐車的拖地鏈說明靜電原理怎樣守護日常生活。
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1Rub a plastic rod with a dry cloth and the rod picks up small pieces of paper — charging by friction. Friction never creates charge; it only transfers electrons from one object to the other: the rod gains electrons and becomes negative, while the cloth loses electrons and becomes positive. Nature has only two kinds of charge: protons in the nucleus are positive, electrons around it negative; with equal numbers an object is neutral, and once electrons transfer, the object is charged.
2Charges interact by a clear rule: like charges repel, unlike charges attract. Hold two rods rubbed in the same way near each other — both carry the same negative charge and push apart; a rubbed plastic rod and a rubbed glass rod attract each other. An electroscope works on repulsion: touch a charged object to its metal cap and the two metal leaves, carrying like charges, spread apart — the wider the angle, the greater the charge.
3A charged body attracts not only unlike charges but also small uncharged objects. As the rod approaches a scrap of paper, charge inside the paper redistributes by induction: the near side gathers unlike charge, the far side like charge; since the unlike side is nearer, attraction beats repulsion and the paper is pulled towards the rod. This is electrostatic induction. The crackle when pulling off a sweater in winter, or the sting of a door handle, are everyday versions of induction and discharge.
4Around every charged object there is an electric field: another charge entering the field feels an electric force, so the charged body acts on distant charges through its field. Field lines visualise the field: starting on positive charge and ending on negative charge, with the direction at each point giving the force on a positive charge there; the denser the lines, the stronger the field. Between two parallel plates of opposite charge the field is uniform — parallel, evenly spaced lines — an important model for circuits ahead.
5When too much static charge builds up, discharge can occur, and engineering has two classic countermeasures. First, point discharge: charge crowds at sharp points and leaks away gently into the air, and a lightning conductor uses this to drain cloud charge and the induced charge on a building, reducing strikes. Second, earthing: connect the charged object to the ground with a conductor and the charge flows away. A metal chain dragging from an oil tanker earths the charge generated by friction on the road, preventing sparks from igniting petrol vapour.
1Summary 重點摘要
Friction never creates charge — it only transfers electrons from one object to another: gaining electrons makes an object negative, losing electrons positive. Like charges repel and unlike charges attract; a charged object is surrounded by an electric field. Point discharge underlies lightning conductors, while earthing lets tanker trucks safely drain charge.
2Vocabulary cards Vocabulary
The branch of physics studying stationary charges, their interactions and applications.
A basic property of matter with only two kinds, positive and negative; like charges repel, unlike attract.
Charging by rubbing two objects together, transferring electrons so each gains equal and opposite charge.
Electrons moving from one object to another; the gainer becomes negative, the loser positive.
The redistribution of charge in an object when a charged body comes near, unlike charge gathering on the near side.
The region around a charged object in which another charge experiences an electric force.
Imaginary lines from positive to negative charge; denser lines mean a stronger field, and the direction gives the force on a positive charge.
The tendency of charge to leak away at sharp points, on which lightning conductors rely.
Connecting an object to the ground so excess charge flows away, preventing build-up and sparks.
A device that detects charge by the divergence of two metal leaves carrying like charges.
3Step by step Steps
- Step 1: Two kinds of charge and charging by frictionNature has only two kinds of charge: protons are positive, electrons negative. The essence of charging by friction is electron transfer, not the creation of charge: when two objects are rubbed, the one holding its electrons more loosely loses them and turns positive, while the other gains them and turns negative, the two charges being equal in size. For example, rubbing a polythene rod with a dry cloth transfers electrons from cloth to rod, so the rod turns negative; rubbing an acetate strip, the electrons go the other way and the strip turns positive. Charge is conserved: the total charge of the system is unchanged by the transfer.
- Step 2: Like repels, unlike attractsCharges follow a clear rule: like charges repel, unlike charges attract. The test is simple: hang one rubbed plastic rod from a thread and bring a second rubbed rod near — the hanging one swings away, both being negative; bring a rubbed glass rod instead and the plastic rod is attracted. For example, hair drifts apart after combing because friction gives every strand the same charge, so the strands repel; photocopiers use the opposite rule, pulling charged toner onto the drum to form the image.
- Step 3: Induction and the rod lifting paperWhen a charged body nears a small uncharged object, the charges inside it redistribute: the near side gathers unlike charge while the far side keeps like charge. Being nearer, the unlike side attracts more strongly than the far side repels, and the whole object is pulled over — electrostatic induction. For example, bring a negatively charged rod near scraps of paper: the near end of each scrap turns positive and the far end negative; the nearer attraction wins and the paper jumps to the rod. After touching, some charge transfers, and the scrap may suddenly fly off — the two are now like-charged and repel.
- Step 4: Electric fields and field linesAround every charged object there is an electric field: any charge entering it feels an electric force, so the charged body acts at a distance through its field. Field lines are the visual tool: they start on positive charge and end on negative charge and never cross; the direction at a point gives the force on a positive charge there, and the denser the lines the stronger the field. For example, the field lines of an isolated positive point charge radiate outward in all directions; between two oppositely charged parallel plates the lines are parallel and evenly spaced — a uniform field, in which the deflection of an electron beam can be calculated with simple geometry.
- Step 5: Conductors, insulators and earthingConductors contain electrons free to move, so charge spreads over the whole object; insulators hold their electrons fixed, and charge stays where friction put it. That is why friction charging happens mostly between insulators — with nowhere to flow, charge accumulates. Earthing is the most direct way to deal with charge: connect the charged object to the ground with a conductor and electrons flow to or from the earth until the object is neutral again. For example, touch a wall or metal water pipe with your palm before reaching for a door handle in winter and the shock disappears; electronics factory workers wear earthing wrist straps so static from their bodies cannot puncture chips.
- Step 6: Point discharge and the lightning conductorCharge crowds wherever a conductor's surface curves most sharply — at points — making the field there intense enough to ionize the air and let charge leak away gently: point discharge. A lightning conductor is a sharp metal rod on top of a building, wired to the ground: induction piles unlike charge on the tip, which slowly drains the cloud, and if a strike comes the current runs down the rod and cable into the earth, protecting the building. For example, the rods on skyscrapers, lighthouses and old towers safely channel countless strikes every year; the sharp design makes lightning prefer the shortest path.
- Step 7: The tanker's earth chain and static safetyAs a tanker truck runs, fuel sloshing in the tank and friction of the body with air and road pile up static charge; one spark and petrol vapour may explode. A metal chain dragging from the rear keeps the body connected to the ground, draining charge continuously so the tank stays near zero potential — earthing in action. For example, petrol stations require drivers to touch a metal discharger before filling up themselves; when aircraft are refuelled, the hose and the fuselage are bonded with a cable first — both ensure the same potential, ruling out sparks. Static also has its uses: electrostatic precipitators and electrostatic spray painting both rely on fields to catch particles.
4Diagrams Diagrams
5Worked example Example
Example: after being rubbed with a dry cloth, a plastic rod can lift small pieces of paper from the desk. (a) Explain why the rod becomes charged, and state the kind of charge. (b) The paper is originally uncharged — explain why the rod still attracts it. (c) A metal chain touching the ground hangs from the rear of an oil tanker; explain its purpose.
- (a) Rubbing transfers electrons from the cloth to the rod: the rod gains electrons and becomes negative; the cloth becomes equally positive. Charge is transferred, not created
- (b) The nearby rod induces charge separation in each scrap: the near side becomes positive, the far side negative
- (b) The nearer, unlike charges attract more strongly than the farther like charges repel, so the paper is pulled to the rod
- (c) Friction during the journey charges the tanker; the metal chain connects the body to the ground (earthing)
- (c) Charge drains away continuously, so no high potential builds up and no spark ignites the petrol vapour
- Answer: (a) Rubbing transfers electrons from the cloth to the rod, so the rod becomes negative — charging by friction. (b) The negatively charged rod induces charge separation in the paper: unlike charge on the near side, and attraction beats the far-side repulsion. (c) The chain earths the accumulated static charge, preventing sparks. For explanation questions: state how charge transfers or redistributes, then how forces act, then the outcome — all three layers for full marks.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the microscopic nature of charging by friction?
Q2What happens when two rubbed plastic rods approach each other, and why?
Q3Why does a negatively charged rod attract uncharged paper?
Q4Which two principles does a lightning conductor use?
Q5Why does an oil tanker drag a metal chain on the ground?
8Recap Recap
- Charging by friction transfers electrons — it never creates charge; gainer negative, loser positive, total charge conserved
- Like charges repel, unlike charges attract; an electroscope's leaves spread apart by repulsion
- A charged body attracts small uncharged objects through electrostatic induction (the rod lifting paper)
- A charged body is surrounded by an electric field; field lines start on positive and end on negative charge, denser where the field is stronger
- Point discharge (lightning conductor) and earthing (tanker's drag chain) are the two classic defences against static hazards