# Electromagnetic pulses

Last week in my high school history class, we watched the made-for-TV movie “The Day After,” detailing the effects of nuclear war. In that movie, I heard the term “electromagnetic pulse” to describe a nuke’s ability to cut off all power at and around Ground Zero. My history teacher told us that this would cause cars to stop, planes to fall from the sky, etcetera, FOREVER. My question is, the pulse isn’t a tangible thing, so where does it go that makes all electronics stop functioning for all of time? Is it in the air or does something happen to the ground or WHAT?

I think you misunderstood. An EMP will affect all electronics present, within its range, permanently. Once it’s done (a matter of milliseconds), it’s done. Any new, working electronic devices brought into the area after the blast will function normally. An EMP is nothing more than the name suggests: a particularly strong electromagnetic wave, similar to radio waves and light in its general behavior.

And welcome to the SDMB, by the way. Enjoy your stay

The pulse travels in the air, in the ground (though in material it is attenuated), and basically through anything in its path. The way it shuts down electronics is that the propogating electric and magnetic fields create currents in any conductors it comes across; sometimes very large currents. At the very least, this will blow any circuit breakers or fuses in the line, though it could also destroy electronic components if the field is great enough. The only way to protect against an EMP (AFAIK) is to construct a Gaussian cage around the electronics you are interested in saving. A Gaussian cage is a hollow conducting shell. Any Electromagnetic disturbance will not penetrate the shell as all charge will flow to the outer surface.

EMP pulse

Slightly off topic - would it be possible to build a Gaussian cage with tin foil?

I just have this image of kilometers of tin foil sent to Iraq to shield them from supposed US EMP weapons.

Ooooh, okay. That makes much more sense. So, in essence, a nuclear attack in an solitary area will not make that place uninhabitable forever? That makes me feel much better.

No, the material used to create the shield must be ferromagnetic, like iron, nickel, or better yet, an alloy called mu metal.

Well, if you’re just talking about electronics then you’re right. The explosion will damage nearby electronics, but it won’t linger around ready to disable electronics that are brought in later.

Of course there are other effects of the explosion that linger much longer, i.e. fallout (radioactive material). Until the fallout has been washed away by rain and wind, the place will be uninhabitable. How long this takes depends on many factors, but probably a matter of years or decades rather than millenia.

By the way, Rabid_Squirrel, a metallic cage that blocks electric fields is called a Faraday cage, not Gaussian cage.

It is also called a Gaussian cage, scr4.

[quote]
…possible imperfections in Gaussian cages of reinforced concrete magazines, and reliable ways of testing if a Gaussian cage exists in a reinforced concrete structure with one or more frangible surface.[/quote

Have you seen the term used anywhere else? I did a google search for “Gaussian cage” and only got two hits, compared to 9440 hits for “Faraday cage” so I assumed it was some other obscure term. Maybe I was wrong. (Won’t be the first time, as you all know…)

Well, to be honest that was the only reference I found to it via Google. It is a government document, so of course it must be true and accurate. :toungue-in-cheek:

I used the term Gaussian cage as that was what I believe I was taught in my intermediate E&M courses in my undergrad years…

That was of course many years ago, so the term Faraday cage could have been used with Gauss’s law being used to derive its properties. In short, it is all a blur to me now.

I pulled out the book for the course (Intro to Electrodynamics, Griffiths, Second Edition) and it indeed calls it a Faraday cage. My bad.

The time for the area to become habitable again is years- Hiroshima and Nagasaki are both large cities now.

Dread, what we used at Hiroshima and Nagasaki are firecrackers compared to what we’d use now. And with a good cobalt jacket coupled with some heavy hydrogen, we can make a place a radioactive hell for longer than our pitiful little civilization will survive.

I’d understood that a Faraday cage could be made of any good conductor such as copper. Could a Gaussian cage be a Faraday cage made from ferromagnetic materials?

Strictly speaking, a Faraday cage is just one with good conductivity. A Faraday cage is a Gaussian surface.

A Faraday cage only shields against electric fields.

If you want to shield against magnetic fields as well, then you want to use something with both a high permeability and good conductivity. Such as the mu metal referred to above.

Im suprised the resident gay community hasn’t decoded the OP username yet. Normally only takes 10-15 seconds for them to converge at ground zero.

Actually, any conductive material will shield an electromagnetic pulse, and as aluminum is a better conductor than iron, it’d work better. What makes a ferromagnetic material different from other metals is that it will retain a magnetization for a long time, but in shielding a pulse, we’re not concerned with the long-term effect.

And as for fallout, we can make weapons with very high fallout now, but unless you deliberately do so, a fusion (hydrogen) bomb will generally leave less fallout than a fission (uranium or plutonium, like we dropped on Japan) bomb. So if we’re talking about the United States or Russia bombing a target, fallout recovery time would probably be even less than for Hiroshima or Nagasaki. But a terrorist nuke, on the extremely unlikely chance that they could get one at all, would probably be pretty dirty.

Minor nitpick, Chronos. Magnetically “soft” ferromagnetic materials like the silicon steel used in transformer cores, and the class of ceramics called soft ferrites, hold almost no residual magnetization. This is an important property of transformer cores, otherwise they would saturate almost immediately and become useless. The tendency of materials to hold magnetization is called remanance and is an important design consideration in high-frequency transformers.

I don’t know where you get this idea from Chronos. The effectiveness of a given material at shielding electromagnetic radiation is given by its skin depth:

delta = 1 / sqrt (pi . f . mu . sigma), where

delta = skin depth
f = frequency
mu = permeability
sigma = conductivity