Charging by induction: rod near → finger on → finger off → rod away.
Depending on the materials of the rollers and belt, a Van de Graaff dome can become positive or negative; in this model it is positive. Only some charges are drawn: a real balloon gains billions of electrons. Red + = positive, blue − = negative (electrons). Never use a Van de Graaff if you have a heart pacemaker; in a real lab the volunteer stands on an insulating stool.
When two different materials are rubbed together, the one lower in this list usually pulls electrons off the other. Pick two and rub. (Real orders vary between sources, surfaces and how humid it is.)
The Van de Graaff generator in the 3D lab builds up hundreds of thousands of volts of static charge. Pick Basic (7 labels) or Advanced (12), then try the blank diagram, the label quiz and match the jobs. On a phone, swipe the diagram sideways.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back.
Each line describes the job of one part. Tap a part name, then tap the box next to the job it does. Tap a filled box to take the word back.
Tap a card, then tap the box it belongs in. Press Check when all 12 are placed.
A full worksheet with an answer key: label the Van de Graaff, conductor or insulator, the balloon, the triboelectric series, charging an electroscope by induction, and explain and calculate questions (high school: Coulomb’s law and sparks).
The force between two charges is F = k q₁q₂ / r², with k = 8.99 × 10⁹ N·m²/C². Like charges repel (F > 0), opposite charges attract. Charges here are in nanocoulombs (1 nC = 10⁻⁹ C), typical for rubbed objects.
The charge on one electron is e = 1.60 × 10⁻¹⁹ C, so 1 nC is about 6 billion electrons. A lightning stroke moves around 5 to 20 coulombs.
Atoms have a nucleus of positive protons (and neutral neutrons) with negative electrons around it. Normally the numbers are equal, so objects are neutral. When you rub a balloon on wool, some electrons move from the wool to the balloon: the balloon now has extra electrons (negative) and the wool is missing some (positive).
Two negative balloons push apart; a negative balloon and the positive sweater pull together. A charged object can even attract a neutral one: it pushes or pulls the electrons in the neutral object a little, leaving the near side with the opposite charge (induced charge or polarization). That is why a rubbed balloon sticks to a wall and picks up small bits of paper.
In insulators (rubber, plastic, glass, wool, dry air) electrons cannot move far, so charge stays where it was rubbed. In conductors (metals, salt water, your body) electrons move freely, so charge spreads out or flows away. Connecting an object to the ground (earthing) lets charge flow to or from the huge Earth. An electroscope uses this to show charge, and can be charged by contact or by induction.
When enough charge builds up, the voltage gets so high that air breaks down and a spark jumps: the shock from a doorknob, the crackle when you take off a sweater, and on a giant scale lightning. Static is useful in photocopiers and laser printers, spray painting and smokestack dust filters, and dangerous near fuel, flour dust and electronics, so people use earthing wires and anti-static straps.
Rub the balloon four times and hold it near the wall, then near the second balloon (rub that one too). In the electroscope, charge it by induction: rod near, finger on, finger off, rod away. On the Van de Graaff, find the widest gap that still sparks.
| Useful | How static helps |
|---|---|
| Photocopiers and laser printers | Charged toner powder sticks only to the charged parts of a drum, then onto the paper |
| Spray painting | Charged paint drops spread out (they repel) and are attracted to the earthed car body, so less paint is wasted |
| Smokestack dust filters | Smoke particles are charged and stick to plates of opposite charge |
| Defibrillators | Store charge in a capacitor and release it through the chest to stop a dangerously irregular heartbeat, so a normal rhythm can return |
| Dangerous | How people stay safe |
|---|---|
| Lightning | Lightning rods give a safe path to the ground; stay out of open fields and away from tall trees in storms |
| Fuel, flour dust and grain | Earthing wires and conducting hoses stop sparks that could set off an explosion |
| Electronics | Anti-static wrist straps and bags stop small sparks damaging computer chips |
| Shocks from doorknobs and cars | Touch metal with a key first; humid air and anti-static sprays help |
Static electricity is electric charge that builds up on the surface of an object and stays there, instead of flowing as a current in a circuit. It happens when electrons are moved from one material to another, usually by rubbing, leaving one object negative (extra electrons) and the other positive (missing electrons).
Key takeaway: charge that builds up and stays put.Different materials hold on to their outer electrons with different strengths. When rubber and wool (or hair) touch and rub, many points come into close contact and electrons move to the rubber, which holds them more strongly. The balloon becomes negative and the wool or hair positive by exactly the same amount.
Key takeaway: electrons move to the material that holds them more strongly.This is a basic property of the electric force. Two objects with the same kind of charge push each other away; opposite charges pull together. The force gets weaker with distance: double the distance and it becomes a quarter as strong (Coulomb’s law).
Key takeaway: like charges repel, opposite charges attract.The wall is neutral, but the balloon’s negative charge repels the electrons in the wall’s molecules slightly, so the side of each molecule nearest the balloon becomes a little positive. This induced charge is close to the balloon, so the attraction beats the repulsion from the farther negative charges, and the balloon sticks until its charge leaks away.
Key takeaway: it induces an opposite charge on the wall’s surface.In conductors such as metals, electrons can move freely through the material, so charge spreads out and can flow away to the ground. In insulators such as rubber, plastic, glass and dry air, electrons stay with their atoms, so charge stays where it was put. Static charge builds up best on insulators.
Key takeaway: conductors let charge move; insulators trap it.An electroscope has a metal cap joined by a metal rod to two thin metal leaves inside a jar. When the leaves get the same charge they repel and spread apart. A charged rod brought near moves electrons inside the electroscope (a negative rod pushes them down into the leaves, a positive rod pulls them up into the cap), so both leaves get the same sign of charge and spread apart (induction); touching the cap shares charge (conduction). Earthing the cap lets charge flow to or from the ground.
Key takeaway: leaves with the same charge spread apart.A motor turns a rubber belt. At the bottom, rubbing and a comb of sharp points put charge onto the belt; the belt carries it up inside an insulating column, where another comb passes it to the metal dome. Charge spreads over the outside of the dome, and the voltage can rise to hundreds of thousands of volts until a spark jumps to a nearby earthed sphere.
Key takeaway: a moving belt carries charge up to a metal dome.Walking on a carpet rubs electrons between your shoes and the carpet, so your body builds up charge. When your hand gets close to a metal doorknob, the charge jumps across the small gap as a spark, which you feel as a shock. It happens most on dry winter days. In humid weather a thin film of water forms on skin, clothes and surfaces and lets charge leak away.
Key takeaway: your charged body sparks to the metal.In a thunderstorm, ice crystals and soft hail collide in the strong updrafts, and charge is separated: the top of the cloud usually becomes positive and the lower part negative. When the voltage is high enough (hundreds of millions of volts), the air breaks down and a giant spark, lightning, flows between the cloud and the ground or inside the cloud.
Key takeaway: lightning is a giant static spark.The rules are real: electrons move when you rub, like charges repel, the wall is polarized, the electroscope follows the true induction and conduction steps, and the Van de Graaff sparks when the voltage reaches about 3 kV per millimeter of air. But only a handful of charges are drawn (a real balloon gains billions of electrons), and the numbers are simplified.
Key takeaway: real rules, simplified numbers.