Why does lightning usually strike tall buildings? Give reason
Answers
Lightning happens when a cloud builds up a static charge of electricity. Initially the charge is confined to the cloud, because air is a pretty good insulator, but it does tend to create a matching static charge (of opposite sign) in the ground below. Eventually, enough charge builds up that it can ionise the air, forming a conducting path to ground (the free electrons and ions in the ionised air conduct electricity). This is the lightning strike: it’s basically a gigantic spark.
Fairly obviously, the shorter the distance between the cloud and the ground, the easier it is for the path of ionised air to form – you don’t have to cross such a large distance. So, other things being equal, the lightning will tend to hit the tallest thing around. Also, the electric field tends to be greater on pointy things, because the electric field is always perpendicular to the surface, and this means that when the direction of the surface is changing quickly you get a stronger field (this text editor isn’t well suited to maths, but there is a nice explanation, with diagrams, at http://www.mwit.ac.th/~physicslab/applet_04/physics_classroom/Class/estatics/u8l4d.html). Therefore, the absolutely most likely thing to be hit is a tall, pointy thing – even in the Middle Ages, people noticed that church steeples got more than their fair share of lightning strikes (it was slightly embarrassing, given that the lightning was seen as an act of God).
Nowadays, tall buildings have a metal rod sticking vertically out of their roof, and running all the way down the building to the ground. This lightning conductor provides a low-resistance path to ground, and makes it much less likely that a lightning strike will damage the building or injure its inhabitants.