In this exhibit you used a crackle tube to- learn about how lightning forms.
Lightning is due to static electricity in the air. When enough static electric charge builds up, this causes a spark of electricity, or a lightning strike.
Lightning strikes can go down from the cloud to the ground (cloud-to-ground), up from the ground to the cloud (ground-to-cloud) or across between clouds (cloud-to-cloud).
Before lightning occurs, the top of the cloud becomes positively charged, and the bottom of the cloud becomes negatively charged. When this difference gets big enough, there is a discharge of electricity, this is the lightning bolt.
The lightning heats up the air it moves through, causing it to rapidly expand. This causes a shockwave, which we hear as thunder.
Lightning takes the shortest possible route from the cloud to either the ground or another cloud. Because of this, lightning often strikes tall structures, such as trees and skyscrapers. For this reason, you should never shelter under a tree during a storm. Tall buildings are often fitted with lightning rods: metal rods that safely conduct the electricity down to the ground without harming the building. Lightning rods are taller than the buildings they are connected to, so they are more likely to be hit than the buildings themselves.
The crackle tube has electrostatic charge in its centre, which acts as the cloud. The edges of the tube act as the ground and are connected up to the circuit. In between are glass beads, coated in phosphor, which act like the air. The charge from the centre tries to find its way to the edges through the beads. The phosphor coating lights up in response to electricity, so we can see this happening. The "lightning" is attracted to us, so we can move around the strikes with our fingers.
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Questions
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| 1 |
Lightning is due to ___________ electricity in the air. |
| 2 |
What are the three routes lightning can take? |
| 3 |
Which part of the cloud must be positively charged for a lightning strike to take place? |
| 4 |
What effect does lightning have on the air it passes through, and how does this cause thunder? |
| 5 |
What is more likely to be hit by lightning, a church steeple or a car? Why? |
| 6 |
Why do you think the "lightning" in the crackle tube is attracted to your fingers? |
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Activities
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| 1 |
Make a thunderstorm diary. Next time there is a thunderstorm, take a note of what the weather was like beforehand, particularly how hot, humid and cloudy it was (you may have to guess). How long did the thunderstorm last and how close did it get? (You can work out how far away a storm is by counting from when you see the lightning, until you hear the thunder. Every five seconds you count is a mile. This works because light moves faster than sound.) Can you use your thunderstorm diary to predict storms? |
| 2 |
Make a lightning chamber from a salt grinder and a pair of old headphones. You need a clear plastic salt grinder with salt in it; it should have a metal rod running all the way through. Cut the ear-pieces off the headphones and attach one to each end of the metal rod in the salt grinder. Put the salt grinder upright, plug your headphone socket into a music player and put some music on. |
