EMF shielding using a conductor

[KDPhysics]

Homework Statement

How does placing a conductor in the path of a magnetic field of a circular coil shield anything behind the plate?

Relevant Equations

Lenz's Law

So I've been trying to figure out how EMF shielding works. More specifically, I've seen videos where placing a metal conductor in front of a circular coil (with AC running through at radio frequencies) apparently shielded anything behind it.

After searching online, I repeatedly saw Eddy currents popping up.

If I've understood everything correctly, when you place a conductor near a changing magnetic field (such as that of a circular coil), nature abhors any change in magnetic flux. Consequently, a current will be induced, called Eddy current, in the conductor, such that the induced magnetic field opposes the change in magnetic flux.

For example, I have the following set up (a circular coil attached to an AC supply and a conductor beneath it):

Now consider an arbitrary instant in which the coil's magnetic field is decreasing. Then, by Lenz's law an Eddy Current will be induced in the conductor trying to "undo" this decrease in magnetic flux. Consequently, the current will be such that the induced magnetic field points in the same direction as the external magnetic field.
But then there still is residual magnetic field behind the conductor due to both the decreasing external field and the induced field. So, shielding has not occurred?

 

[hutchphd]

The fluctuations will be mitigated by the induced currents and Lenz's Law. A good Faraday Cage can greatly reduce Radio frequency interference.
This is also why metals appear shiny.
Not static fields!

 

[KDPhysics]

Oh, I see. So exactly because the external magnetic field is decreasing, the fact that we have an induced magnetic field keeps the change in magnetic flux approximately constant, thus shielding anything behind.

EDIT: Also, could you elaborate on why this causes metals to be shiny? Sounds very interesting.

 

[hutchphd]

Yes. And of course a conductor will also shield both static and dynamic E fields.

 

[KDPhysics]

One question: if initially (before turning on the AC supply) there is no magnetic field outside, and then after some instants, as we saw in my diagram, there is a net magnetic field outside, this means that there must be some net change in flux right? Perhaps the moment the AC supply is turned on there is a surge in induced current, and then this stabilizes?

 

[hutchphd]

Exactly.
And the light is reflected from a conducting surface because field components are forced to be zero at the surface...this produces a reflected wave like sound off a hard wall...its a little more complicated for light but that is the fundamental issue.

 

[KDPhysics]

Oh ok thanks.