Benjamin Franklin performed a famous experiment of FLYING KITE in thunder cloud in the year 1745. Before his discovery the lightning used to be considered to be “ACT OF GOD”. He also proved that the lightning stroke is because of the discharge of electricity. He also invented protection of lightning. Lightning rods for fixation on tall buildings and grounding for their protection from lightning stroke. Thus Benjamin Franklin is a great scientist in this field.
WHAT IS LIGHTNING?
Lighting is a huge spark which is due to electrical discharge taking place between the clouds within the same cloud or between the cloud and earth or ground. Large number of discharge occur between or within clouds than earth. And enough of them terminate on to the earth and result in serious hazards. This discharge occurs between electrically charged regions of a cloud (called intra-cloud lightning or IC), between two clouds (CC lightning), or between a cloud and the ground (CG lightning).
Many theories have been put to explain the phenomenon where by a thunder cloud becomes electrically charged. However for our purpose it is sufficient to assume that as a result of certain atmospheric process that take place during thunder storms, charges are accumulated in clouds and equal and opposite charges are induced in the earth beneath.
ANOTHER THEORY: LIGHTNING
According to another theory the positive charge as well as negative ions in the air and attach themselves o the small dust particles and small drop of water suspended in the air . Because of polarization they got charged to a certain potential under the storm condition. Thus a cloud gets positive charge or negative charged. Whenever such a cloud passes over the earth, it is induces opposite charge in the earth below.
WHY ELECTRICAL DISCHARGE?
Lightning discharge requires the puncture of air between the cloud and earth. we know about Dielectric Strength of air, breakdown of air requires the electric field is 30 KV/cm. But in in a cloudy weather moisture content in the air is large and for that dielectric strength of air reduces to 10 KV/cm. The mechanism of lightning discharge can be well explained with the help of the figure below…
Whenever a local charge concentration near a cloud causes a the potential gradient to exceed the critical value of breakdown. The air surrounding gets ionized and an electric streamer start from cloud to earth. The current in the streamer is 100 A and speed of the streamer is 0.15 m/µs. Known as PILOT STREAMER because it leads to the lightning phenomenon. Depending upon the state of ionization of the air surrounding the streamer it is branches in several paths and it in known as STEPPED LEADER. The leader steps are of the order of 50 m in length and are accomplished at about microseconds (µs). The charge is bought from cloud through the already ionized path to these pauses. The air surrounding these pauses is again ionized and the leader in this manner reaches the ground or earth.
Once the stepped leader has made contact with earth or ground it is believed that a power returns stroke move very fast toward the cloud through via already ionized path by leader. This streamer is very intense where the current Varies from 1 KA to 200 KA (KA- Kilo Ampere) and the speed is about 10 percent of that of light. It is here where the negative charge of the cloud is being neutralized by the positive induced charged of the earth. This instant gives rise of lightning flash which is observed by Us.
There are may be another cell of charges in the clouds near the neutralized charged cell. This charged cell will try to neutralized through this ionized path. This streamer is called DART LEADER. The velocity of the dart leader is about 3% of the light. The effect of the dart leader is much more severe than that of return stroke.
The discharge current in the return streamer is relatively more large but as it lasts for few microseconds the energy contained in it is small. Therefore this streamer is called the COLD LIGHTNING STROKE while the dart leader is call HOT LIGHTNING STROKE because even though current in this leader is smaller but it lasts for some milliseconds and for that energy contained in it is larger.
it is found that each thunder cloud may contain as many as 40 charged cell and heavy lightning stroke may occur. this is knows as MULTIPLE OR REPETITIVE STROKE.
WAVE FORM OF LIGHTING STROKE:
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