Static electricity is a build-up of electric charge on the surface of an insulating material. Unlike the current electricity that flows in circuits, static charge stays in one place — until it suddenly discharges. The charge arises when electrons are transferred between materials by friction, leaving one material negatively charged and the other positively charged.
What is electric charge and where does it come from?
At the particle level, electric charge is carried by protons (positive, +) in the nucleus and electrons (negative, −) surrounding the nucleus. In a neutral object, the number of protons equals the number of electrons, so the charges balance.
Electrons are on the outside of atoms and can be stripped off by friction. Protons are locked in the nucleus and never move in electrostatic processes.
When two insulators are rubbed together:
- One material holds its electrons tightly → it strips electrons from the other material → it gains electrons → it becomes negatively charged.
- The material that lost electrons has more protons than electrons → it becomes positively charged.
Total charge is always conserved — you cannot create or destroy charge, only transfer it from place to place.
Classic examples:
- Rubbing a balloon on hair → balloon becomes negative (gains electrons from hair), hair becomes positive.
- Rubbing an ebonite (hard rubber) rod with fur → rod becomes negative.
- Rubbing a glass rod with silk → rod becomes positive (silk takes electrons from glass).
What are the rules for attraction and repulsion?
The fundamental rule of electrostatics:
Like charges repel; unlike (opposite) charges attract.
| Charges brought together | Result |
|---|---|
| Positive and positive | Repel (push apart) |
| Negative and negative | Repel (push apart) |
| Positive and negative | Attract (pull together) |
| Charged and neutral | Attract (induction) |
The last row is important and often surprises students. A charged object (say, a negatively charged balloon) can attract a neutral object (such as small pieces of paper). This happens because the balloon's negative charge repels electrons in the paper's atoms towards the far side, leaving the near side with a slight positive charge — and opposites attract. This is called electrostatic induction.
What is an electric field?
An electric field is a region of space in which a charged object experiences a force. Every charged object creates an electric field around it.
Electric field lines show the direction a positive charge would move if placed in the field:
- Lines point away from positive charges (a positive test charge would be repelled outwards).
- Lines point towards negative charges (a positive test charge would be attracted inwards).
- The closer together the field lines, the stronger the field.
Between two parallel plates with opposite charges (like inside a capacitor or lightning cloud), the field is uniform — the lines are parallel and equally spaced. This uniform field is what accelerates charged particles in a particle accelerator.
Why do conductors not hold static charge?
Conductors (metals) contain free electrons — electrons not attached to any particular atom that can move freely through the material. If you charge a conductor, the free electrons redistribute immediately: excess negative charge spreads to the surface and edges, or flows away to earth through any conducting path.
This is why you cannot charge a metal rod by rubbing it unless it is held in an insulating handle — without the insulating handle, the charge flows straight through your body to earth.
Insulators (rubber, plastic, glass, dry wood) have no free electrons. Charge deposited on an insulator stays exactly where it was placed, building up static.
| Property | Conductor (e.g. copper) | Insulator (e.g. plastic) |
|---|---|---|
| Free electrons | Many | Very few |
| Charge builds up? | No (spreads and escapes) | Yes (stays put) |
| Examples | Metals, graphite | Rubber, glass, dry wood |
What are the risks and uses of static electricity?
Risks:
- Fuel tanker sparks: When fuel flows through a pipe, friction can charge the fuel, which charges the tanker. If the tanker is not earthed, the charge builds until a spark jumps to earth — potentially igniting fuel vapour. Tankers are earthed (connected to the ground by a conducting wire) before unloading.
- Lightning: Water droplets and ice crystals in storm clouds collide, transferring electrons so the bottom of the cloud becomes very negatively charged. When enough charge builds, it discharges to the positively charged ground in a lightning strike — a massive electrostatic discharge carrying up to a billion volts.
- Electronic component damage: Static discharge can permanently damage sensitive microchips. Engineers working on circuit boards wear anti-static wrist straps to continuously drain any charge to earth.
Uses:
- Inkjet and laser printers: Ink or toner particles are electrostatically charged and directed precisely onto paper.
- Electrostatic precipitators: Industrial chimneys use charged plates to attract and remove soot and dust particles from exhaust gases, reducing pollution.
- Spray painting: Charged paint droplets are attracted to a grounded metal object being painted, giving an even, waste-free coat (used in car manufacturing).
- Photocopiers: A drum is charged, light removes charge where the paper is white, and toner sticks only to the remaining charged areas.
Frequently asked questions
What is static electricity in simple terms?
Static electricity is a build-up of electric charge on an insulating surface. It is caused by friction transferring electrons from one material to another, making one material negatively charged and the other positively charged. The charge stays put (is static) until it finds a path to discharge — such as a spark.
Why do you sometimes get a shock when touching a door handle?
If you walk across a synthetic carpet, friction transfers electrons to your body, building up a negative charge. When you reach for a metal door handle (a conductor connected to earth), the built-up charge suddenly rushes from your body through the metal to earth in a tiny spark — which you feel as a shock. Wearing leather-soled shoes (an insulator) reduces this build-up compared to rubber soles.
What is the difference between static and current electricity?
Static electricity is a build-up of charge that stays on an insulating surface and does not flow continuously. Current electricity is the continuous flow of electrons (charge) through a conductor in a circuit. Both involve the same charged particles (electrons), but in static electricity they stay put, while in current electricity they flow.
Why does a charged balloon stick to a wall?
A charged balloon (made negative by rubbing on hair) is brought near the neutral wall. The balloon's negative charge repels electrons in the wall's surface atoms, leaving the wall's surface slightly positive near the balloon. Since opposite charges attract, the balloon is pulled towards the wall and sticks. This is electrostatic induction acting between a charged object and a neutral one.
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