Lightning

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 Lightning.



Lightning

P.K.Ghatak, MD

No. 68.



Lightning is a natural atmospheric phenomenon. Hot most air is lighter than cold air. When these two air masses come in close contact, a violent turbulence occurs. The moist hot air rises on the top; and as the air mass moving upwards, the water vapor in the air freezes into ice crystals and soft hails. The heavier hails fall to the bottom. The violent collision between masses of air, strips the ice crystal of its electrons and ice crystals become positively charged settle on the top of the cold air mass.

The cold air mass by gaining electrons becomes negatively charged. These air masses are separated by wind pressure. The potential difference builds up and when the insulating power of the air is exceeded, an electrical discharge takes place which from the ground, we see as lightning. Lightning between air masses lights up the clouds but do not reach the ground.

During rainy season, negatively charged thick air mass descends low and induces the ground to be positively charged. And lightning occurs when potential difference between these air masses exceeds the insulating capacity of the air. Lightnings descend towards the ground and usually hit tall trees or buildings or people out in the open or has taken shelter under a tree and are electrocuted.

The electricity, like fluids, travels along the paths of least resistance. Occasionally the lightnings travel horizontally along the least resistant path to a considerable distance and then descends towards the ground and strike a tall tree or structure. This often quoted as Bolt from the Blue.

The average voltage of lightning is enormous. Here are some numbers:

    Average Voltage: 300 million Volts

    Average Current: 30,000 Amperes (30 kA)

    Temperature: 50,000°F (5 times hotter than the surface of the sun)

    Duration: Lasts a fraction of a second, but contains massive energy   (1 to 10 billion joules)

In the USA, Florida experiences most lightning strikes and on the average 20 people die per years in the US. 400 people in Bangladesh die from lightning per year and that is the highest number in the world. Bangladesh is followed by the Democratic Republic of Congo. Columbia and Pakistan. However, the record is held by Venezuela. Lake Maracaibo of Venezuela records 233 lightning pr square meter per year and experiences storms 160 nights a year and 10 hours a night.

Benjamin Franklin and his kite:

Franklin was a versatile person. He was a printer and witty writer, a statesman and a signatory of Declaration of Independence Document, a slave owner, the first American ambassador to France, the first Postmaster of the US, a scientist and an inventor of lightning conductor. In 1752, he experimented with flying kite in thunderstorms. He tied a metal key to the kite and use of hemp string as electricity conductor line and dresses himself in silk to prevent getting electric shock. He recorded receiving shocks, similar to static electricity shocks. He proved that the electrically was generated from the gathering thunderclouds. From that experiment he designed the lightning conductor.

In modern days of high-rise buildings are made safe from lightening conductors which was Franklin's invention. Industries producing explosive, petrochemicals, pyrotechnic devices, paints, flour mills, electronic machines and aerospace rockets, missiles and satellites are made safe from the discharge of static electricity which could ignite fire and burn down such facilities.

275 years have passed from Franklin's kite experiment but the precise mechanism of transfer of electron from one to the other when two are rubbed together and then separated is still unknown.



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