# Lightning Lab

A high\-school physics lesson on static electricity and lightning: a hand\-cranked Van de Graaff generator on a stormy city rooftop, and the lightning rod beside it\.

By Numa

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/dftxxpUGmHdV)

## Transcript

### 1. Lightning Lab

A storm is rolling in, and this rooftop bench makes lightning small enough to study\. Watch the dome charge, the puppet's hair rise and a spark jump to the brass wand\. A bench spark and a lightning bolt are the same physics\.

Drag the crank, the wand or the lightning rod, and click the cage, to use them\. Drag anywhere else to turn the view\.

### 2. 1 · Rubbing Separates Charge

Turn the crank\. The belt rubs over a roller, picks up charge and carries it to the metal dome\. The dome's voltage, the energy each bit of charge carries, climbs as more arrives\. Like charges repel, so the puppet's hair stands up\.

Charge is only moved, never made\. The ground loses what the dome gains\.

### 3. 2 · Charge Lives on the Outside

Charges on the dome push each other as far apart as they can, so they all end up on its outer surface\. Inside the metal shell, the electric field is zero\. Outside, field lines point straight out from the surface\.

An electric field is the push a charge would feel\. Field lines crowd together where it is strongest\.

### 4. 3 · Points Concentrate Charge

Clip on the steel needle and watch the voltage fall\. Charge crowds onto sharp points, so the field at the tip is very strong\. It tears electrons from air molecules, and charge leaks away as a faint violet glow called corona\.

The tip of a lightning rod does the same under a storm\.

### 5. 4 · Air Breaks Down

Air normally blocks charge\. But when the electric field gets strong enough, about 3 kV per millimetre, charge rushes across as a spark\. A wider gap needs a higher voltage\. Predict: will the 2 cm gap spark more or less often than the 8 cm gap?

A spark heats the air so fast that it bursts outward with a crack: a tiny clap of thunder\.

### 6. 5 · The Storm Strikes

Inside a thundercloud, colliding ice and hail leave the base strongly negative\. Raise the lightning rod by its red grip\. A jagged leader reaches down, the rod sends a streamer up to meet it, and a huge current flashes through and runs down the cable to the ground\.

A rod doesn't attract lightning from far away\. It gives a nearby strike a safe path to ground\.

### 7. 6 · Safe Inside the Shell

Click the cage to set it over the electroscope\. The dome's field spreads the gold leaves apart\. Inside a closed metal shell the field is zero, so the leaves fall together\. A car protects you from lightning the same way, with the current flowing around the outside\.

It's the metal body that protects you, not the rubber tires\.