Mission impossible : modulating only the magnetic field of an EM wave ?

[Externet]

Hi.
There is no way to divorce the electric and the magnetic fields in a propagating wave; or is there a way ?
Could the magnetic field only be modulated ? As in a wave that propagates in a medium where the electric field is impeded/blocked to pass trough ?

If that magnetic field is stationary -as a magnet- ; varying its field also forces the creation of a varying electric field -or not?

If an electromagnetic wave is emitted in confinement of a box that allows magnetism to pass trough its walls but blocks the electric field from propagating trough those walls, outside the box there will be no traveling wave, right ?

Please educate me on these basics...

 

[zorro]

Hi.
There is no way to divorce the electric and the magnetic fields in a propagating wave; or is there a way ?
Could the magnetic field only be modulated ? As in a wave that propagates in a medium where the electric field is impeded/blocked to pass trough ?

E and H in a propagating plane wave are related by the intrinsic impedance of the medium: Z0=|E|/|H|=sqrt(µ/ε) (377 ohms for free space, less for other dielectrics).

If that magnetic field is stationary -as a magnet- ; varying its field also forces the creation of a varying electric field -or not?

According to Maxwell equations, there would be an electric field in space, but the relationship between electric and magnetic field near the magnet is not the same that in a propagating plane wave.

If an electromagnetic wave is emitted in confinement of a box that allows magnetism to pass trough its walls but blocks the electric field from propagating trough those walls, outside the box there will be no traveling wave, right ?

Yes, like in a waveguide made of aluminium.

Abolish the deciBel !

Why?

 

[SLK001]

A propagating field has both E and H components - they do not exist without each other. The only E and H fields that are by themselves are static.

How do you vary the magnetic field of a magnet? A magnet has a static field in relation to itself - to the world, it may look like a varying field (say a rotating magnet), but it is still static in relation to the magnet.

Boundary conditions in your box would force both fields to zero at the wall. This is why shields that are made of high conductive materials that effectively block E fields, but do little for H fields are able to block the entire EM field.

 

[Externet]

...
How do you vary the magnetic field of a magnet?

Allow to speculate/say that a permanent magnet rod has a winding on it. Its magnetic field is present and detectable outside the aluminium/copper/whatever 'E-shielding' box.
Applying RF to its superimposed winding, the static magnetic field would vary per the waveform, sort of 'modulated'. Outside the box, there will be the same static magnetic field with no magnetic 'modulation'; inside the box, yes.
Is that correct ?

 

[SLK001]

Allow to speculate/say that a permanent magnet rod has a winding on it. Its magnetic field is present and detectable outside the aluminium/copper/whatever 'E-shielding' box.
Applying RF to its superimposed winding, the static magnetic field would vary per the waveform, sort of 'modulated'. Outside the box, there will be the same static magnetic field with no magnetic 'modulation'; inside the box, yes.
Is that correct ?

No, that is not correct. Applying RF to the windings won't alter the magnet's H field. It WILL, however, create its own EM field, so there will be an EM field inside the box, but only the static H field from the magnet on the outside.

 

[vfone]

A propagating field has both E and H components - they do not exist without each other. The only E and H fields that are by themselves are static.

This explain everything. End of story.