B Does a Faraday Cage trap EMF inside and increase exposure?

[A13579]

TL;DR Summary

Does EMF emitted from inside a Faraday Cage actually make exposure inside that Cage higher because it can’t escape?

Hi All,
I’d like you to weigh in on a debate I’ve been having.
I’ve been thinking about a Faraday Cage (FC from now on) but from the opposite perspective. Sure it stops EMF getting in. But does that also mean, if there are items emitting EMF in there, that gets trapped within the room. Taking that to a logical conclusion does that mean in the case of a sufficiently large FC, with lots of emitting items, exposure to EMF is higher inside the FC than outside it? Or is there no difference between inside and outside?

 

[berkeman]

Welcome to PF.

Can you tell us what you know so far about the inverse square law of the dissipation of EM waves? And also what you know so far about the constructive/destructive interfernce of multiple EM waves? Thanks.

 

[A13579]

Hi Berkeman,
We know nothing about those things. We’re a couple of simpletons coming to a forum with some really clever people with the intention of getting an answer that is dumbed down to the point we can comprehend it…..
Cheers!

 

[berkeman]

Okay, fair enough. The short answer is that the exposure to EM will not be much higher inside the FC compared to just outside. Have a good day.

 

[A13579]

Okay, fair enough. The short answer is that the exposure to EM will not be much higher inside the FC compared to just outside. Have a good day.

Ok cool so we have an answer. Taking into account my previous reply, why is that?

 

[berkeman]

Ok cool so we have an answer. Taking into account my previous reply, why is that?

Sorry, which part of your previous reply?

Are you familiar with "standing waves" that form when two or more traveling waves (like sound or EM waves) combine? You can get "constructive" and "destructive" interference when two waves of similar/same wavelength combine.

https://en.wikipedia.org/wiki/Standing_wave

As the first animation on that Wikipedia page shows, when you have two waves that are moving past each other (like bouncing around inside your Faraday Cage enclosure), the combination of the two waves can add up to a higher amplitude at some locations and cancel each other out at other locations. Those are called the "nodes" and "antinodes" of the standing wave, respectively.

So the little longer answer to your question about exposure to higher EM inside a Faraday Cage as opposed to outside depends on the wavelength of the EM and the size of the Faraday Cage, and where you happen to be standing in the enclosure (near a node or antinode). But even if you manage to stand right at a node inside the enclosure, you are only exposed to a bit more peak energy than the average that you would experience out in an open field at the same distance from a transmitting antenna.

I hope that makes some sense.