Faraday's Cage

[Kedas]

no' date=' it wouldnt effect you, due to the fact that it is impossible to create a potential difference.

[/quote']

 

in a ideal situation the conductor has a resistance of zero but not here.

assume a metal path of 1 ohm that you touch then a current of 100A will have a voltage of 100V.

 

Obvious it won't affect you that much but you will still feel it.

 

 

about the frequency/spectrum see link in message on top #61

[5614]

http://www.boltlightningprotection.com/Elemental_Faraday_Cage.htm

They mention a spectrum for lightning 1kHz to 1MHz.

 

but' date='

 

A DC signal has an infinite wavelength and zero frequency.

 

therefore, either lighning is not DC, but AC, or someone is wrong, which is why, kedas, i asked the question!

 

so again, i re-ask, is lightning DC? and what is it's wavelength?

 

i would have thought that lightning was dc because; lightning is a discharge or static electricity. if the clouds are positive, electrons go to the clouds, this is one direction, DC, and this is all works vice versa, if the clouds are negative, but you still get a DC signal...

[swansont]

Your question depends on what you are trying to shield - there's the current of the lightning stroke, and the EM radiation that it gives off.

 

You can obviously be protected from the current by a properly grounded lightning rod. In that sense the lightning is basically a DC signal as far as understanding the dynamics.

 

But the short duration of the stroke and the acceleration of the charges means that EM radiation will be given off. You car isn't going to protect you against that, since radio waves obviously can penetrate inside.

[Cap'n Refsmmat]

Methinks that the skin effect is what protects you from lightning, so it doesn't matter what the wavelength is.

edit: oops, swansont beat me to it.

[5614]

ok, what i was thinking of was having some kind of metal suit, which you could wear in a lightning storm, and be safe, due the to the faraday cage effect;

 

however, i might need a breathing whole, so to work out, what diameter, i could make this, i need to know the wavelength of the attacking wave!

 

so what size hole could be, in this "suit" to allow me to breathe, without letting the electric current inside?

im not that fussed about EM radiation in the form of radio waves, but obviously am worried if it goes to x-ray/gamma radiation, but it doesnt, so i dont think i need to worry about EM radiation... i think.

[Kedas]

EM=electric/magnetic radiation they go hand in hand.

Don't forget that you ARE a conductor in a strong field (a bad one though)

 

If you want a whole size take the the size of the wholes in the metal of your microwave.

[5614]

EM=electric/magnetic radiation they go hand in hand.

yes, i know, whats your point?

 

Don't forget that you ARE a conductor in a strong field (a bad one though)

whats this got to do with anything? we're trying to protect ourselves in a faraday cage, so the fact that we are bad conductors, which i know, is a pointless sentence!

 

If you want a whole size take the the size of the wholes in the metal of your microwave.

 

why? do lighning bolts produce microwaves? or is it just of a similar wavelength?

 

additionally, if you had a hole, which didnt break the shielding effect of a faraday cage, for lightning, make the whole have a diameter of 'X', can i have loads of 'X' size holes? or just one? or as many as i want, just not actualy connecting?

[Kedas]

my point is that it isn't because it was 'just' an EM radiation that it won't be a current when it passes a conductor (You).

 

I already said that the spectrum was between 1Khz and 1Mhz

The microwave frequency is a lot higher but if you want to be on the safe side.

it isn't an ideal filter you know see my link again with diff. dB values.

 

yes you can make more holes just like in the microwave oven, the holes there have the same function. (only they block just one frequency 2.45 GHz, you block a range.)

[Lance]

Are you actually serious about this?? It doesn’t matter what size the hole is. Everybody has already said it’s not the faraday cage it’s the skin effect that protects you. As long as there’s a straight path from above your head to under your toes it doesn’t matter.

 

By the way, I would NOT recommend intentionally attracting lightning with a lightning rod above your head… but that’s just me

 

 

Edit: Oh wait... I was looking at the wrong page... I don’t think this is still relevant. Was looking at post #80.

[5614]

ok, going back to what we KNOW works.

 

a faraday cage.

 

a simple total enclosure, a big metal enclosed room, [a typical faraday cage]

 

now, obviously this wont let any current inside it. however, say if you replaced one of the walls with something else, say rubber. this means that the faraday cage is now not a total insulating case, so would it still work? [remeber that rubber was used]

 

additionally, all websites about faraday cages say that it must be made of a good conducting material.... is any metal good enough? or must it be a very good conducting one? and why!???

[swansont]

ok' date=' going back to what we KNOW works.

 

a faraday cage.

 

a simple total enclosure, a big metal enclosed room, [a typical faraday cage']

 

now, obviously this wont let any current inside it. however, say if you replaced one of the walls with something else, say rubber. this means that the faraday cage is now not a total insulating case, so would it still work? [remeber that rubber was used]

 

additionally, all websites about faraday cages say that it must be made of a good conducting material.... is any metal good enough? or must it be a very good conducting one? and why!???

 

Rubber's a good insulator, but I don't think it will keep out EM radiation, just current. Wood's a good insulator, too, but doesn't do much as far as shielding goes. You no longer have a grounded surface, so no Faraday cage.

 

The reason that the surface will be an equipotential is that charges feel a force when there is a potential difference, and this causes them to move in such a way that they will minimize their energy. If you have a poor conductor, it will take longer to equilibrate. I'm guessing that the equilibration time would be related to the frequency of radiation that would be passed by the material. e.g. if the material took a millisecond to respond, radiation higher than about a kHz would easily be passed.

[5614]

so by replacing a metalic wall with an insulator such as rubber, whilsts you will still keep out an electrical current you would not keep out an EM radiation.

[5614]

in the guiness book of records 2004, the worst lightning spark incident was when a plane was hit by it, and everyone died? pressumably this would have been because the current got to the fuel or the engine, or it broke part of the plane apart, but assuming that the fusealage was whole, would they have been electrocuted?

 

[remeber, metal case, but glass windows]

[Lance]

No you would not. That hapened more than 40 years ago.

 

http://www.usatoday.com/weather/resources/askjack/2003-05-06-lightning-airplanes_x.htm

[5614]

lol, id read that exact same article before, i was doubting my own knowledge.... its just coz the airplanes went down, im guessing maybe a wing broke off, or it [lightning] got to the fuel or something.

[Lance]

How could a wing possibly brake off? Lightning can not damage modern planes anyway...

[Kedas]

if the fuel thank explodes then everything can break off

the only other possible way I see is local heat that reduces the structural strength of the airplane but temperature would recover soon and there is an obvious big safety margin about what force it can take.

[5614]

im not sure, but it did happen.

[RICHARDBATTY]

If you take a strike as being around 300Kv and your cage's resistance to ground as being about 0.4 ohms, then the current flowing would be 750,000 amps. Your cage would need to be able to take this. Ouch If your cage heated under this stress the current would try to find another path of lower resistance and you would be a candidate.

[swansont]

If you take a strike as being around 300Kv and your cage's resistance to ground as being about 0.4 ohms, then the current flowing would be 750,000 amps. Your cage would need to be able to take this. Ouch If your cage heated under this stress the current would try to find another path of lower resistance and you would be a candidate.

 

Except you can't figure it that way. The potential difference is measured from ground to cloud, and won't be equal to what is measured across the cage. The current will be limited by the resistance of the atmosphere, similar to how the current through a resistor doesn't get larger when you get to the leads, which have small R. It's a series circuit.

 

Typical discharges are of order 5,000-20,000 amps, though larger values can certainly occur.

[RICHARDBATTY]

Never thought of that I had a vision of fried human and paniced sorry.

[5614]

about the plane:

 

a total of 81 people aboard a Boeing 707 jet airliner died when their plane was struck by lightning when flying near Elkton, Maryland, USA, on december 8 1963

[Plecostomus]

what exactly is it?

[[Tycho?]]

What exactly is what?

[5614]

i think he means whats a faraday cage... to which i say read the first page, i posted some links there for those who didnt know what it was.... if you still dont understand, then ask again.