Electronic – Does Shielding “Electrically” shield “Magnetically” too


I know it sounds like a newbie question but I can't wrap my mind around it. An electromagnetic field is electric + magnetic field.

So this means that when shielding an equipment offensively, for example from avoiding causing interference with other electronics, we need to shield the electromagnetic waves, that means both electric and magnetic shielding.

So if we put say a radio inside an aluminium box, aluminium is pretty much the most cost effective material you can find. Some may use copper but aluminum is more cost effective.

Now an aluminum box will shield the electrical field very efficiently given if the box has no holes or seams, or if the cables coming out of holes are properly shielded and grounded.

But what about the magnetic field?

Aluminum has very low permeability. So how can the aluminum box shield nearby equipment from the magnetic field of the radio inside it? It does shield the electrical field, but not the magnetic?

Can somebody explain to me how shielding works with electrical/magnetic waves? Because I can't wrap my head around it how can it shield the electrical part but not the magnetic?

Does magnetic field leakage pose any noise danger to the nearby equipment from this theory perspective?

Best Answer

You would not be alone in this one. This is an often misunderstood phenomenon.

Static magnetic fields can not be shielded. They can be re-directed using ferrous materials but even those will not block them.

Electric fields on the other hand can be. Since an electric field is basically a voltage in space, they can not pass through a conductive plate that is held at a fixed potential. Space is shorted out as it were.

Alternating magnetic fields of sufficient frequency however, will not pass through a metal plate. The alternating field generates an eddy current in the plate which generates a cancelling magnetic field.

This is all explained in much better detail here.. Wikipedia

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