For those who find that the magic of teaching static electricity has seeped away like an unreliable charge on a damp day, help is at hand. Say farewell to balloons on the ceiling and dancing figures fashioned from tissue paper. The answer is big, and bright and very loud: lightning. Although organising a local lightning strike indoors may seem ambitious, even foolhardy, children will already know a great deal about the subject. This is a good place to start.

Key concepts

* Electrical charge exists in two forms, positive and negative

* Like charges repel but opposite charges attract

* Gas mixtures, such as air, are normally electrical insulators.

What does the class already know? Use a few key questions to stimulate discussion.

Q. How do you know when lightning is about?

A. You see both the flash and hear the thunder.

Q. But do both arrive together? Do light and sound travel at the same speed?

A. No, sound is a slowcoach, travelling at only 330 metres per second compared with 300 million ms for light.

Q. What are the effects of a lightning strike?

A. Split trees, forest fires, damage to buildings and occasionally fatalities can all result from electrical storms.

Q. Can you protect yourself from a lightning strike?

A. High buildings use lightning conductors but they do not always work. Standing under a tree in a storm increases your chances of being struck. Sitting inside a car, insulated from the ground by rubber tyres, is a good place to be.

Q. Is lightning rare on Earth and does it happen on other planets?

A. Not rare exactly, say about 100 lightning strikes each second over the whole planet. Spacecraft have observed electrical storms on other planets, including Jupiter.

What causes lightning?

There are two kinds of electrical charge, positive and negative. In a thunder cloud these charges become separated. The top of the cloud is full of ice crystals with positive charges. The base of the cloud contains water droplets with negative charges.

There is a big potential difference between these areas of the thunder cloud.

A white cloud on a sunny day causes a shadow beneath it which moves along the ground. A thunder cloud does something similar with electrical charges. The ground below the cloud is positively charged: this is called an induced charge.

If you are out in a storm and your hair stands on end, watch out. You are probably in the area of positive charge below a thunder cloud just before lightning strikes.

The electrical charge differences in thunder clouds are neutralised when a current flows between them in a massive spark. This is lightning. Air is a good insulator, its electrical resistance is high. Even so, a voltage of about 10 million volts is enough to give a lightning flash in moist air, which is a better conductor.

Different kinds of lightning strike This is more complicated than you might think. Most lightning discharges never get near the ground. There are lightning flashes between clouds, from clouds to the air and even inside the same cloud. The ones that worry us, those that hit the ground, represent only one fifth of all strikes. Most of these strikes link the base of a thunder cloud with the earth below.

Perhaps the most surprising thing is that lightning travels in both directions. Yes, lightning goes upwards too. First a strike of negative charge is attracted by the positive charge on the ground beneath. Immediately after this, a positive strike returns to the cloud from the ground.

This exchange is very fast and can be repeated, giving the characteristic flickering of lightning. It only takes a fraction of a second and stops when all the charge has been neutralised.

Can we build lightning power stations?

Pupils might think that if only we could trap the energy in lightning, our power needs would be solved for ever. There is a problem. Although there is a lot of power in a lightning strike in terms of watts, it does not last long - less than a millisecond.

Since energy is power multiplied by time, the energy in a strike is low. The power of a typical lightning strike might be enough to run all the electrical devices in your home - for a few hours.

Collecting the lightning strikes would present a few problems too. Where will it strike next?

What about all the noise?

Lightning strikes heat the surrounding air very rapidly. It can get as hot as 20,000 degrees Celsius. At this temperature the air itself glows, it becomes incandescent. This is why there is so much light in a storm. Lightning heats up the surrounding air very rapidly and then it cools again very rapidly. This movement causes the sound waves that we know as thunder.

How do lightning conductors help?

In the mid-18th century the American scientist Benjamin Franklin had a good idea. Since electricity is conducted by metals, why not put a metal strip on your house, joining the ground to the highest point? A lightning conductor like this diverts the lightning away from the main part of the building. This lets the lightning reach earth without damaging the building on its way. It often works quite well.

Ray Oliver is a writer and consultant