Pull a jumper off over your head in the dark and you may hear a faint crackle — or even see tiny blue sparks. Run a plastic comb through dry hair, hold it over some torn-up scraps of paper, and the scraps leap up to it. Rub a balloon on your sleeve and it clings to the wall, or lifts the fine hairs on your arm. Slide out of a car seat, touch the metal door, and — snap — a stinging little shock.

All of these are the same thing: static electricity. Not electricity flowing round a circuit as a current, but electric charge that has built up and is sitting still on something — "static" just means not moving. Where does that charge come from, why do some things attract and others repel, and why does it sometimes let go in a spark? That is this whole page.

Charging by friction: rubbing moves electrons

Everything is built from atoms, and every atom has a heavy positive nucleus with light electrons — each carrying a scrap of negative charge — buzzing around the outside. Normally an object has exactly as many electrons (negatives) as protons (positives), so the pluses and minuses cancel and it is neutral: no overall charge.

Now rub two insulators together — a polythene rod on a cloth, a balloon on your hair. The rubbing drags some of the loosely-held electrons off one material and onto the other. Watch it happen:

The object that gains electrons ends up with more negatives than positives, so it becomes negatively charged. The object that loses those same electrons is left with more positives than negatives, so it becomes positively charged. The two charges are always equal and opposite, because the very same electrons simply moved from one object to the other.

The single most important idea on this whole page: only electrons ever move. The heavy protons are locked in the nuclei and stay put. So a positive object is not a thing that "collected extra plus charge" — it is an object that has lost some of its electrons.

Charging by friction — the rule

• Rubbing two insulators transfers electrons from one to the other.
• The material that gains electrons becomes negatively charged.
• The material that loses electrons becomes positively charged.
• Only electrons move — protons never do — so the two charges are equal and opposite.

Like repels, unlike attracts

Once things are charged, they push and pull on each other without touching. The rule is short and never varies:

• Like charges repel — two negatives push apart, two positives push apart.
• Unlike charges attract — a positive and a negative pull together.

It is the same "opposites attract" you met with the poles of a magnet, but here it is charge, not magnetism. Flip the sign of each object below and watch the force arrows swap between pushing apart and pulling together.

That balloon stuck to the wall, and the comb lifting scraps of paper, are this same rule at work — even though the wall and the paper started out neutral. The next card explains that twist.

Why a charged rod grabs neutral paper

Here is a puzzle. The scraps of paper are neutral — no overall charge. So why does a charged comb pull them up? If they carry no charge, what is there to attract?

The answer is induction. Bring a negatively charged comb close to a scrap of paper. The comb's negative charge pushes the paper's own electrons to the far side of the scrap and pulls its positive nuclei slightly nearer. The paper is still neutral overall, but now its near side is a little bit positive and its far side a little bit negative.

And unlike attracts: the comb's minus and the paper's newly-near plus pull together more strongly than the comb's minus and the paper's slightly-further-away minus push apart. The attraction wins, and the scrap jumps to the comb. The same trick makes a charged balloon cling to a neutral wall.

Why it builds up on insulators but not conductors

You can charge a plastic rod by rubbing it, but try the same with a metal rod held in your bare hand and nothing sticks. The difference is whether the charge can flow away.

In an insulator — plastic, rubber, glass, dry hair — electrons are held tightly and cannot move through the material. So when rubbing dumps extra electrons on one spot, they are stuck there. The charge builds up and stays: that is why static electricity lives on insulators.

In a conductor — any metal — electrons move freely. Charge a metal rod that you are holding, and the extra electrons simply flow through the metal, through your body, and away into the ground. We say the charge is conducted to earth. It never gets the chance to pile up, so a held metal object stays uncharged. (Stand the same metal on an insulating handle, cutting off the path to earth, and now it can hold a charge.)

This idea — giving unwanted charge a conducting path to earth so it drains away harmlessly — is called earthing, and it turns out to be exactly how we tame the dangerous side of static.

Sparks: when static lets go

If charge keeps building on an insulated object, the "electrical pressure" between it and its surroundings — the potential difference, or voltage — grows and grows. Air is normally an insulator, so nothing happens. But push the voltage high enough and something dramatic gives way.

When the potential difference gets big enough, it rips electrons off the atoms in the air itself — we say the air is ionised — turning a thin channel of air into a temporary conductor. Electrons suddenly leap across that channel all at once: a spark. That rush of charge heats the air so fast it glows and cracks — the tiny snap off a doorknob, the blue flash when you pull off a jumper. This sudden escape of built-up charge is called a discharge.

The shock off a car is exactly this. Sliding across the seat rubs you against the upholstery and charges you up; the moment your finger nears the metal door, the charge discharges through the small gap as a spark, and you feel it. Touch the metal before you fully stand up — keeping contact so the charge drains gently instead of jumping — and there is no snap.

Static at work — and where it's a menace

Static electricity is not just a party trick. Because charged things attract, and because "unlike attracts", engineers put it to real use:

• Spray painting — the paint droplets are given a charge as they leave the gun, so they repel each other into a fine even mist, and are all attracted to the oppositely-charged metal object. The paint even curls round to coat the back, and almost none is wasted.
• Crop spraying — the same trick charges pesticide droplets so they cling evenly to leaves (and wrap round to the undersides) instead of drifting away.
• Dust precipitators in chimneys and power stations — smoke particles are charged as they rise, then attracted onto oppositely-charged plates, cleaning the soot out of the exhaust before it reaches the sky.
• Photocopiers and laser printers — a charged drum attracts fine toner powder only where the image should be, then presses it onto the paper.

But the same sparks that are so useful can be deadly near anything flammable. Fuel flowing through a pipe into an aircraft or a tanker rubs against the pipe and builds up static; a single spark could ignite the fuel vapour and cause an explosion. The cure is exactly the earthing idea from earlier: the tanker, the aircraft and the fuel pipe are earthed and bonded together with metal straps, so any charge drains harmlessly away to the ground before it can ever build up enough to spark.

A first look at the electric field

How does one charge push or pull on another across empty space, with nothing touching? We picture every charge as surrounded by an invisible electric field — a region where any other charge will feel a force. The field is strongest close to the charge and fades away with distance, which is why the paper must be brought near the comb before it jumps.

A positive charge's field points away from it; a negative charge's points towards it. Drop another charge into that field and it is pushed or pulled accordingly — like repelling like, unlike attracting unlike. You will meet electric fields properly later; for now, just picture that invisible reach around every charged object, the thing that lets it act at a distance.