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Assume that terrorists manage to detonate an EMP in the middle of the United States. Its range is long enough for the pulse to hit and effectively render useless all unprotected hardware.

Let's assume that this is a casual buyer and decides just to wrap it in aluminum foil. As for the strength of the pulse, how about lets have be the average strength of the EMP generated by a standard size nuke.

For example, an E1 kind of EMP could create a pulse with strength of up to 1 MeV (million electron volts)

Background information: Depending on the size of the gaps in the cage, a Faraday cage can shield an object inside of it from radiation all over the spectrum. If the object is not touching the cage, the object will be sheilded.*

Could a faraday cage protect electronics from an EMP? Could a stronger EMP still damage the electronics within the cage?

DevilApple227
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  • depends on pulse's waveform as well as the frequency and the intensity of the pulse.also the thickness of the cage is important in this regard. Please make the question more specific by putting in some numbers or estimates. . – Bruce Lee Jan 20 '16 at 19:33
  • And describe how a terrorist group will (1) get a bomb, and (2) get it high enough that EMP would be a real concern (hint - a Cessna won't do). – Jon Custer Jan 20 '16 at 19:55
  • Ignore the HOW. I'm more curious about the physics behind it. – DevilApple227 Jan 20 '16 at 19:56
  • @JonCuster I'm voting to close your comment as off-topic. Have you tried posting it at the terror stackexhange? – Asher Jan 20 '16 at 20:58
  • If you are talking about a "cage" in terms of wrapping it in a conductive material, then this question has a partial answer http://physics.stackexchange.com/questions/160137/faraday-cage-in-real-life – ProfRob Jan 20 '16 at 21:53

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It all depends on your construction of the Faraday cage... for a sufficiently well constructed cage (multilayer, continuous, RF gaskets on all seams) the answer is "yes". It's much easier to add another 3 dB of isolation than to double the power of your EMP generating device.

Imagine you have a cage that provides just 3 dB of shielding. If you put that cage inside another cage, you have 6 dB. And so it continues. This is one case where sufficient isolation is indeed possible (assuming you are far enough from the blast that the shield isn't mechanically compromised).

Floris
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  • How strong would the pulse have to be to run a genuine risk of melting the cage? Or would this never happen? – DevilApple227 Jan 20 '16 at 20:02
  • The electromagnetic pulse is unlikely to be sufficiently energetic to melt the cage - but the heat of the explosion that generated the pulse could... – Floris Jan 20 '16 at 20:03
  • What I mean is something like, "How strong would it have to be so that the current in the Faraday cage would be strong enough to melt it" – DevilApple227 Jan 20 '16 at 20:04
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    The short answer - "ridiculously strong". Look at how induction heaters work: they induce eddy currents which heat the target material. Eddy currents require $\frac{dB}{dt}$ - if it's a single pulse, the way to increase the current is either to increase the B, or decrease the time. But the shorter the time, the less power is dissipated. With a pulse, it could never work. You would need a high power continuous RF signal (even then, it would be insanely hard. Inside MR systems they have really strong and rapidly changing magnetic gradients, and lots of eddy currents - but nothing melts). – Floris Jan 20 '16 at 20:08
  • The "heat of the explosion" won't be a concern. An EMP device has to be exploded out in the exosphere. Unless it's a truly gargantuan bomb, it'll simply be too far away (too high in the sky) for direct radiation to harm objects at ground zero. – Solomon Slow Jan 20 '16 at 20:20
  • @jameslarge - "has to be" exploded in the exosphere? - presumably if you want just the EMP effect yes, but it would still work at close range. But it would cause additional destruction. – Floris Jan 20 '16 at 20:23
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If you wrap your electronics in aluminium kitchen foil then the appropriate equation for the electric field transmission factor, that takes into account reflection from the foil and attenuation in the foil is $$\frac{E_t}{E_i} \simeq 4 \frac{\eta_{\rm Al}}{\eta_0} \exp(-t/\delta) = 0.47 \omega^{-1/2} \exp(-22 \omega^{1/2} t),$$ where $t$ is the foil thickness and $\omega$ is the "frequency" of the EM radiation (see Faraday cage in real life ). The transmitted power fraction would be the square of this.

Typical foil has $t \sim 3\times 10^{-5}$ m and the lowest frequencies have the highest transmission factors. According to this extensive report, an E1 HEMP is less important than lightning strikes for frequencies below 1 MHz. At 1MHz ($\omega \sim 6 \times 10^6$ Hz), the formula above gives a transmission factor of $3\times 10^{-5}$. Given a typical HEMP E-field peak of around 50,000 V/m (same report), then this amount of attenuation is sufficient to reduce the signal to that typical from a strong FM radio station.

So my conclusion is that tinfoil would protect your phone from an EMP. However it is simply not practical or possible to completely enclose all electronic and electronic devices (e.g. they often need cables in or out or some kind of opening, that may leave them vulnerable).

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ProfRob
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