Why does lightning not flash when the sky is clear? And why does it not flash even when there are clouds, if no storm is blowing? Collect the information you need about these questions from various sources such as a library, the internet or a knowledgeable person, and discuss what you have collected in class.
How a cloud gets charged
Lightning too is a flow of electricity. In the sky, particles of vapour join together and a cloud is formed. In a cold cloud there are very small droplets of water, and these small droplets of water slowly fall downwards. Because of the heat below, hot air is evaporated and goes upwards. Because of this two way movement, friction takes place, there is an exchange of electrons, and electric charge is produced in the cloud.
Read that paragraph again and count how many new ideas it actually contains. None. Two things move past each other, water droplets falling and warm air rising. There is friction between them. Electrons are exchanged, so one part ends up with a surplus and another with a shortage. That is word for word the comb and the hair, only the comb is now a cloud several kilometres tall and the hair is the air rushing through it. This is why the second thinking question has such a simple answer. Lightning needs a storm because a storm is what supplies the rubbing. Still clouds on a quiet day have nothing moving through them, so no friction, no separation of charge, and no lightning. And on a clear day there is not even a cloud to do the rubbing in. It is the same experiment every time, and only the size changes. But size changes everything, because the charge that makes a comb pick up paper is harmless, and the same charge multiplied by an enormous cloud can kill a person.
Generally, positive charge is produced in the upper cloud and negative charge in the lower cloud. Electrons move across from the cloud that has the negative charge to the cloud that has the positive charge. This is called electric discharge. When electric discharge takes place, a very large amount of heat, light and sound energy is produced. Sometimes an electric discharge onto a house, a tree or the ground is called lightning striking. When lightning strikes, a house or a tree is damaged.
Look carefully at the last two panels of figure 8.9. The cloud is marked negative and the ground below it is marked positive, and the roof of the house is marked positive too, although nobody has rubbed anything down there. Where did that charge come from? From exactly the same place as the charge on a piece of paper under a comb. The huge negative charge overhead pushes the electrons in the ground and in the roof downwards and away, leaving the top surface short of electrons and therefore positive. Nothing was given to the house. Its own charges were simply pushed around inside it, which is what you worked out for a small piece of paper in the first topic of this unit. Now you can see why a strike happens. There is an enormous negative charge above and an induced positive charge below, and the air between them is an insulator holding the two apart. When the charge grows large enough, the air finally gives way and the charge jumps through it, which is the flash you see.
Three safety questions with one answer
When lightning strikes, the rumbling sound of thunder is heard only some time after the flash of lightning is seen. What might the reason be? Why should one not sit under a tree while lightning is flashing? And why should electrical goods in the house not be used while lightning is flashing?
Two of those three questions have the same answer, and it is one sentence: an electric discharge takes the easiest and shortest path it can find to the ground. A tall tree standing alone in a field is a much shorter path from the cloud to the ground than the open field beside it, so the discharge is far more likely to pass through the tree, and anybody sheltering underneath is in the way of it. Electrical wiring in a house is a ready made conducting path running all through the building, so a strike that reaches the wiring can travel along it and out through whatever is plugged into it, which is why appliances should be switched off and unplugged during a storm. Hold on to that one sentence, because in a moment it will also explain how a lightning rod protects a whole house.
The flash and the bang are made at the same instant by the same discharge, so neither one starts first. They arrive at different times because they travel at wildly different speeds. Light travels at about three hundred thousand kilometres in a single second, so for anything happening in the sky above you it arrives essentially the moment it happens. Sound, as you learnt in the unit on waves, is a mechanical wave that has to be passed along from one air particle to the next, and it manages only about three hundred and forty metres per second. That means sound needs roughly three seconds to cover one kilometre. So you can turn the gap into a measurement. Count the seconds from the flash until you hear the thunder and divide by three, and that is roughly how many kilometres away the strike was. A gap of nine seconds means about three kilometres. A gap of three seconds means about one kilometre, which is close enough to be dangerous. And if the flash and the bang arrive together, the strike is right on top of you and you should already be indoors.
Lightning rod
A lightning rod is placed on the roof or the upper part of a house in order to save the house from lightning. This rod is joined to a copper wire and the copper wire is carried down under the ground, which is called earthing.
Earthing is done in order to save a house from lightning. For this, a somewhat broad copper rod, of the kind shown in the figure, is placed on the roof or the upper part of the house. This carries the high voltage that arrives when lightning strikes away towards the ground, without letting it enter inside the house. This rod is joined with copper wire and the wire is taken to a pit dug in the ground. In the pit dug in the ground, pieces of coal and salt are placed, in which there is an arrangement for a continuous flow of water so that it stays damp. The copper wire taken into that damp place is joined to the rod buried in the pit. Lightning rods are used in buildings constructed using modern technology, such as earthquake resistant schools, apartments, commercial buildings and cinema halls.
Many people imagine a lightning rod somehow pushes lightning away, and that misunderstanding hides how clever the device really is. It does the opposite. It invites the strike, and then it decides where the strike goes. Remember the sentence from earlier: a discharge takes the easiest and shortest path to the ground. A tall pointed metal rod at the very top of a building is the shortest path, and copper conducts far better than bricks, wood or people, so it is also by far the easiest. The strike therefore chooses the rod almost every time, and the copper wire hands it straight down into the earth without it ever passing through the building. Now look again at the pit at the bottom, with coal, salt and a supply of water. That is there for one reason, which is to make the last part of the road as easy as possible. Dry soil is a poor conductor, and damp salty soil is a much better one, so the charge can spread out into the earth quickly instead of piling up at the bottom of the wire. Every single part of this design is the same sentence: give the charge a road, and make the road easy.
Staying safe in a storm
| What to do | Why, in terms of the path to the ground |
|---|---|
| Go inside a proper building as soon as thunder is heard | A building offers many easier paths than a person does |
| Do not shelter under a tall tree standing on its own | The tree is the shortest path down, and you are standing in it |
| Do not stand in open ground, on a hilltop, or in water | With nothing taller nearby, you become the shortest path yourself |
| Switch off and unplug electrical goods during a storm | House wiring is a ready made conducting path running everywhere |
| Fit a lightning rod on a tall building | It offers a path so easy that the strike takes it instead of the house |
Notice that every row of that table is the same sentence applied to a different situation. Nobody has to memorise five separate rules. There is one rule, which is that the discharge takes the easiest and shortest path to the ground, and every piece of advice above is simply about making sure that path is not you.
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