Lightning Phenomenon
Benjamin Franklin performed his famous experiment of flying kite in thunder cloud in 1745.People used to consider lightning as ‘Act of God’.He also proved that the lightning stroke is because of discharge of electricity.He invented lightning rods for fixation on tall buildings and grounding for the protection from lightning strokes.
Fundamentally,lightning is a manifestation of very large electric discharge and spark.Lightning is a huge spark,which is due to the electrical discharge taking place between the clouds,within the same cloud and between the cloud and earth.Large number of discharges occur between or within clouds than to earth and enough discharge terminate on earth and results in serious hazards.
In an active thunder cloud the larger particles usually passes negative charge and smaller carrier are positive.Thus the base of the thunder cloud generally carries the negative charge and the upper part is positive;with the cloud being electrically neutral. Lightning discharge to the earth is usually initiated at the fringe of negative charge centers.
There are various theory explaining the phenomenon of lightning. As a result of certain atmospheric processes that take place during thunderstorms,charges are accumulated in clouds and equal and opposite charges are induced in the earth beneath.According to other theory,the positive as well as negative ions in the air attach themselves to the small dust particles and small drops of water suspended in the air and because of polarisation by induction,they get charged to a certain potential under the storm conditions.Thus a cloud gets charged either positively or negatively.whenever such a cloud passes over the earth,it induces opposite charges in the earth below.
To the human eye the lightning discharge appears at a single luminous discharge.Although at time branches of variable intensity may be absorbed which terminate in mid-air.while luminous main channel continuous Zig-Zag path to the earth.Lightning discharges requires the puncture of air between the cloud and the earth.The stroke is initiated in the region of negative charge sector where local field intensity[(30 Kv peak/cm)]atmospheric air or [(10 Kv/cm)]in the persence of water droplets.
1.
charge centers in cloud.pilot streamer and stepped leader propagate earthward.outward branching of streamers to earth.Lowering of charge into space beneath cloud.
The current in the streamer is of order of 100A and the speed of the streamer is 0.15m/µs.
2.
process a is almost completed.pilot streamer about to strike earth.The leader steps are of the order of 50 m in length and are accomplished in about a microsecond.
3.
Heavy return streamer discharge of negatively charged space beneath earth .This is the most dangerous condition.
once the stepped leader has made contact with the ground it is believed that a power return stroke moves very fast up towards the cloud through the already ionized path by the leader.This streamer is very intense where the current varies from 1 KA to 200 KA and the speed is about 10 % of that of light.About 55kwh of energy is dissipated in one or more strokes that makes discharge.
4.
First charge center completely discharged,development of streamers between charge centers within clouds.
5.
This instant gives rise to lightning flash which is observed by us.There may be another cell of charges in the cloud near the neutralized charged cell.This charged cell will try to neutralize through this ionized path.This streamer is called the dart leader.Discharge between two charge centers. Dart leader propagates to ground along original channel,dart leader about to strike.The velocity of the dart leader is about 3% of that of light.The effect of the dart leader is much more severe than that of the return stroke.
6.
The discharge current in the return streamer is relatively very large but as it lasts only for a few microseconds the energy contained in it is small and therefore this streamer is called the cold lightning stroke while the dart leader is called hot lightning stroke because even though current in this leader is relatively smaller but it lasts for some milliseconds and so the energy contained in it is relatively larger.Thundercloud contain as many as 40 charged cells and a heavy lightning stroke.
fig: Generalized wave shape of Lightning stroke
surge caused due to lightning discharge are generally of non oscillatory nature with a very steep wave front and have gradually decreasing tail.The wave is designated by
- maximum or crest value
- the front time t1 which is the time to rise to the peak value and
- the duration t2 which is the time from 0 to point where the voltage or current has decayed to one half its peak or crest magnitude.
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