# How lightning reaches earth

Discussion in 'Physics' started by aliashar86, Nov 28, 2006.

1. ### aliashar86 Thread Starter Active Member

Nov 23, 2006
71
0
"You can find capacitors as big as soda cans, for example, that hold
enough charge to light a flashlight bulb for a minute or more.
When you see lightning in the sky, what you are seeing is a
huge capacitor where one plate is the cloud and the other plate
is the ground, and the lightning is the charge releasing between
these two "plates." Obviously, in a capacitor that large, you can
hold a huge amount of charge! "

Q) How lightning reaches earth (air is insulating medium, it won't allow)?

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
For every insulating medium there is a breakdown voltage. Once this breakdown voltage is reached there are several mechanisms that can allow a discharge. Consider the presence of rain droplets. The space between the cloud and the ground is no longer dry air. Consider also the spark chamber where a high energy particle traveling between the plates can set off a discharge.

3. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
To be fair Wikipedia does a pretty good article on lightning.

One of my fondest memories of University weas calculating the voltage between a cows legs in the moments immediately after a lightning strike using Gauss's Law. It certainly made my Electromganteics Fields module seem more interesting!

Dave

4. ### aliashar86 Thread Starter Active Member

Nov 23, 2006
71
0
So Mr. Dave u success to calculate the voltage or was kicked by the cow.
(sorry if it hurts u)

5. ### aliashar86 Thread Starter Active Member

Nov 23, 2006
71
0
Mr. Papabravo
"Consider also the spark chamber where a high energy particle traveling between the plates can set off a discharge".
plz explain this process again

6. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Here is an article on Spark Chambers

Basically gas ions are created along the path of an energetic particle. When the gas ions return to the ground state they emit photons which can be measured and detected.

7. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Thankfully the cow was theoretical, but we did the same calculations for the same situation however we calculated the voltage between a mans legs!! If I recall the answer was around 920V for typical values!

Dave

8. ### Ashraf Ali New Member

Nov 2, 2007
6
0
hi,
i don't know much abt the topic but i can try to explain the concept in simple terms. .

have you ever tried to insert a plug into the socket after turning it on before hand . . did u notice a spark that jumps out while you do so . . when the voltage is high there's a spark jumping from one terminal to the other . .its a similar concept between the earth and the cloud. . a lightening bolt is jus a spark that jumps out, air has the maxi resistance but the energy of a single bolt in is the order of giga volts so whats a few kms of air column to it ! ! its well above the breakdown voltage of the air resistance so jus like the spark the bolt jumps from one plate of the capacitor (cloud) to other plate (earth surface). .

guess this clears your query . .

9. ### jvjtech Member

Jan 26, 2008
23
0
For the reasons Dave mentions, the recommended position for anyone caught out in a lightning storm is to crouch down with both feet on the ground and together. Holding ones knees with ones arms keeps the arms and hands off the ground. This reduces ones height and maintains as small a contact area with ground as possible. Regards.

10. ### OwenSmith New Member

Sep 5, 2008
3
0
Papabravo,

Interested in the breakdown voltage.

In Glass it is 2-3 Mv

I was wondering if it is possible to use either Glass or Silica or ceramic as a Lens to direct a discharge in a wave form.

Application: EM pulse as is found in Lightning

11. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Air is an insulator if the voltage between two points in it is less than 3000000 V/m. As the voltage reaches this value the air molecules start to ionize and thus they flow in opposite directions according to their charge (positives to negative and negatives to positives). During this movement they collide with other molecules, they ionize them and the process continues until this ions reach a point where they are too much so a big rapid movement (current) is created between this two points. In the case of the lighting strike the two points are the earth and the cloud, the cloud is charged negative on the bottom and positive at the top and the earth is charged positively with induction (due to the negative charge of the cloud at the bottom) on the surface. If the earth and the cloud are close enough or the voltage per meter is over 3000000 then a lighting strike is created. That's why high points on the earth, like trees, are hit more by lighting strikes. Lighting strikes can occur between two clouds too.