# Why can't I Measure the Capacity Of Lightning

Discussion in 'General Science & Technology' started by hsaithan93, Oct 30, 2010.

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1. ### hsaithan93Registered Member

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6
Hi

I have a question ..

Why every time i try to Measure the Capacity of lightning i get unlogical numbers

Lightning Volt = 1 x 10^9 v
Joules = 500 x 10^6 j

U= 1/2 x C x V^2

u = Joules
C = Capacity
V= Volt

if any one have any idea

3. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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23,198
Welcome to sciforums.
That should work fine for calculating the order of magnitude of C, which is the capacitance of the Earth cloud capacitor* when the bolt discharged that capacitor, but of course your numbers will only be reasonable estimates.
BTW, with Joules and volts the units of your computed C are Farads which is much to big to be convenient. Thus micro-Farads are more commonly used.

* not the lightning bolt.

Last edited by a moderator: Oct 31, 2010

5. ### kevinalmRegistered Senior Member

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993
Using those numbers, I get a picofarad. (1x10^-12 farad) A bit low but not unreasonable given a 'plate' separation of hundreds (or thousands) of feet.

Last edited: Oct 31, 2010

7. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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23,198
That sounds reasonable to me and may even be an over estimate for the cloud /Earth capacitance if other clouds are nearby. I.e. Cloud 1 to cloud 2 is making part of the capacitor also. Often these different clouds are at quite different potential and lightning bolts go between them too.

8. ### hsaithan93Registered Member

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6
Hey , thx guys ,,,

But my teacher asked us to explain why we cant measure the Capacity of lightning ( cause all the nummbers looks unreasonable )

if any one have an explanation ?

9. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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23,198
Ask teacher to be more clear about what "Capacity of lightning" means. Is that, for example the distance it can arc? Or the ability to do useful work, if its energy were captured? Would that "capture" be during the bolt? or draining the charge off the cloud prior to the bolt? Also if captured from the bolt, how is the relatively large energy dissipated in heating the air and making sound waves (thunder) to be counted - was that useful work or not?

What numbers look "unreasonable" and to whom? They look reasonable to me.

10. ### hsaithan93Registered Member

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6
Hello Billy

First of all , the asignment is about to measure the capacity of Lightning in Farad , Using the Energy of Lightning and The Volt [ U = 1/2 x C x V^2 ].... The teacher wants us to measure the Capacity of lightning so we can know how many Capacitor we need to store the lightning

* When i measure the capacity of lightning i got
C = 1x 10 ^-6 F

The problem that i dont think its reasonable to me because how can the capacity of lightning be less then [ 1 ] which is [ 1x 10 ^-6] ?!

Thats why i dont think its reasonable ..... Can u explain to me Why do u think its reasonable numbers ? shouldn't the Lightning capacity be in Millions or Billions ?? or am wrong

Sorry for wasting ur time

11. ### hsaithan93Registered Member

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6
If you can explain it simply that will be nice

Cause am only in 12 grade not University

thanks

12. ### kevinalmRegistered Senior Member

Messages:
993
If the answer is to be in farads, then it is capacitance you are supposed to be measuring. What is somewhat confusing is that capacitance is not a property of lightning, but of the geometry of the clouds, ground, rain, etc. involved in the lightning strike.

Let's say you have 2 conductors near each other but insulated from each other. Now apply a fixed voltage between the two. For a while charge will flow, diminishing to zero as a charge difference builds up between the conductors. The "capacity" is a measure of how much charge will build up for a given voltage. Specifically, for a conductor arrangement with a capacity of 1 farad 1volt will deposit 1 coulomb of charge.

A farad is a very large capacity. For example, the large filter caps in the output of a computer power supply typically have a value of around 2000 microfarad. That's 2x10^-3 farad. That's about as large a capacitance as you will commonly find in electronic equipment.

13. ### hsaithan93Registered Member

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6
so do you mean am getting weird numbers of Capacity of lightning cause the rain,cloud efffect the operation ?

14. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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23,198
With some slight algebra, your formula becomes:

C = {2U} / (V^2) or {2 x500 x10^6 =1000 x10^6 = 10^9} / (1x10^9 x 1x10^9) = 1 / (10^9) = -1x 10 ^(-9) Farad
Or C= 10^-9 = 0.001 micro Farads.

That is the capacitance of the earth cloud capacitor which was able to store 500 x 10^6 joules, but that small capacitor was able to do so only because the voltage was very large, equal to 1,000,000,000 volts.

No man-made capacitor can have that much voltage applied to it without immediately having an internal arc form, shorting it out. They are rare, but a man-made capacitor rated for 100,000 volts do exists. So if you are to store the same energy in a man-made capacitor you must reduce the voltage on your capacitor by a factor of 10,000. However because the energy stored is proportional directly to C and to the square of the voltage, to keep it the same energy when you reduce the voltage by 10,000 you must increase C by (10,000)^2 or 100,000,000 times larger C is needed. That is 1x10^8 larger than your 1x10^(-9) Farads.

SUMMARY:
I.e. you use a capacitor rated for 100,000 V and 0.1Farad of capacity. I am just guessing but think that capacitor would be about the size of a small walk-in closet. Actually no one would ever make such a capacitor as a single unit - much to dangerous if it failed - shorted out internally. Instead to get 0.1 Farad with that voltage rating, perhaps 100 capacitor of 0.001 F could be used. Even so they should be inside a strong box as even only one failing dumps a lot of energy. 1000 of 100 micro Farad capacitors is more reasonable. I will leave to you to compute how many joules are dumped when one of them fails. Hint there is an easy way to do this in your head.

Note instead of writing these powers of ten as you do, a more convent and common way uses E to represent the exponation. E.g. 100,000 V = 1E5 in this more compact notation. So to check, lets recalculate the energy stored in a 0.1 Farad charged to !e5 volts:

U = 0.5 x C xV^2 = 0.5 x 0.1 x 1E5 x1E5 =5x 1E8 =500 E6 or in your old notation 500x 10^6 joules

Last edited by a moderator: Nov 1, 2010
15. ### hsaithan93Registered Member

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6
Thanks all for your time ans specially Billy T for your effort .. And sorry for wasting your time with me

Thanks all again <3

16. ### dhcrackerRegistered Senior Member

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196
Why not lookup the actual tests and see if its really unreasonable data collected. We launch rockets the spool off wire to make lightning strike and be measured, I forget the name of the experiment but I'm sure google will help you there. It may be slightly different because we are using a conductor and not measuring lighting just passing through the air.. however I see they are very similar strikes in magnitude.