Re: Electro Magnets: could they be used to generate a shield?

First, I should say that the theory of magnetism is quite complex and an 
area of continuing research, so I will try to answer your question using 
what is known.

We do know that moving charges generate magnetic fields – like electrons 
in the coils of an electromagnet or the unpaired electrons in atoms and 
molecules (like metals).  Depending on how these electrons interact with 
each other we can have several types of magnetic materials.  (As a side 
note, the reason that we need unpaired electrons to observe magnetic 
fields 
is rooted in quantum mechanics, and but we won’t discuss that here).

Lets examine a material where all the electrons are paired – things like 
plastics and wood – most anything you can think of that does not contain a 
metal.  These materials are called diamagnetic – they are repelled by a 
magnetic field, but this repulsive force is very, very small and hard to 
measure without special equipment (devices called Faraday Magnetometers or 
S.Q.U.I.D. Magnetometers – where S.Q.U.I.D. stands for super conducting 
quantum interference device).  So if we have a building or some other 
object with a magnetic field around it, a diamagnetic object would be able 
to penetrate the magnetic field.

Materials that have unpaired electrons – things like metals or organic 
molecules called “radicals” exhibit two main types magnetic behavior that
are described as either paramagnetic or ferromagnetic. 

In a paramagnetic material, the electrons that are unpaired do not 
interact with each other.  Each of these unpaired electrons, acting like a 
tiny bar magnet, is randomly oriented when compared to its neighbors so 
that the paramagnetic material does not generate an easily measurable 
magnetic field.  But, if we apply a magnetic field to the paramagnetic 
material (like using an electromagnet), the material will be attracted to 
the magnetic field (because the electrons – behaving as tiny bar magnets – 
are no longer randomly oriented but start to become ordered).  So if we 
have a building surrounded by a magnetic field, a paramagnetic object 
(like a metallic bullet) will be attracted rather than repelled.

A ferromagnetic material results when unpaired electrons interact with 
each other – specifically, they are aligned in the same direction.  
A bar magnet is a good example of a ferromagnetic material, which can be 
attracted to or repelled by a field, depending on the orientation of the 
magnet and the field.  However, if the bar magnet is free to rotate, it 
will eventually change its position so that it will be attracted to the 
field (Think of placing two bar magnets close together with their north 
poles facing each other on a table but holding one in place.  The bar 
magnet that is free to move will quickly rotate so that its south pole now 
faces the magnet we are holding).  If we quickly alternate the orientation 
of the magnetic field around the building we are trying to protect and 
send a ferromagnetic material toward it, we might be able to slow it down 
or even stop it, but this depends on the speed at which the field is 
changed as well as how quickly the magnetic object is moving toward the 
building and how rapidly the magnetic object rotates as it heads toward 
the magnetic field.  

To my knowledge, this idea is not something that anyone has ever conducted 
research on or published data about, so at this point we can only make 
guesses as to what will actually happen.  Hopefully this has helped answer 
your question.