Re: Magnetism: How do you shield

Hello

Your question is actually about two slightly different things. Magnetism is an effect due to large groups atoms aligning the atomic charge on their nuclei in the same direction. In most materials the atoms are aligned randomly and no gross effect is seen. The EM pulse from a nuclear explosion is an expanding shell of energy that behaves as an Electro-Magnetic Wave with very high energies. The difference is a subtle but important one. Magnetism is static and largely unchanging. When you wave a static magnet near a conductor it induces a changing (alternating) current in the conductor. This is an example of how Electricity and Magnetism are interelated. Electromagnetic (EM) waves are emitted by a number of things but typically they are created by electrons orbiting atoms, as light, or when atoms collide and fuse, again creating light. EM waves by their nature are not dependant on atomic nuclei being aligned in a material. In fact, an EM wave can travel unhindered in a Vacuum whereas a vacuum can not create a magnetic field.

None the less, all substances, even a vacuum, has a property called Relative Permeability, relative to the permeability of free space that is. (There is a similar property called Permittivity for Electricity). Relative permeability determines the strength of a magnetic field or EM in a substance. There is no one substance that acts as magnetic insulator as such, but most insulators are natuarally bad as magnets. The value of the relative permeability just determines how fast the field decays but the field always extends to infinity. It is possible to shield yourself from a magnetic field by surrounding yourself with a sufficiently large thickness of something with a large permeability. Under a large mountain will do the trick, but any sufficiently strong magnetic field could still affect you.

An EM pulse from an atomic bomb is a different thing though. The energy released in the explosion is partly light and partly high energy particles such as Alpha, Beta and Gamma radiation. The real problem is the high energy radiation. Gamma radiation interacts little with matter whereas alpha and beta easily interact with matter. Computers chips and the like are seriously damaged by a large influx of particles. Again, these components can be shielded by burying them under a mountain, but the Gamma radiation could penetrate to some depth. When the gamma radiation hits something it gives it energy which is released as more radiation that causes still more damage your computers. Plus, a sufficiently high energy EM wave is hardly stopped by a few hundred meters of rock. When this passes by a conducting material, such as silicon or wires, it induces an electrical current that could cause the equipment to be damaged. You can shield against this by

1. Burying yourself deep enough.
2. Building a Faraday cage around you.

I hope this goes some way to answering your question. I have had to be less than precise as a full description of EM Waves and Permeability easily runs to a book. If you wish to know more I would reccomend any college level physics text on Magnetism or Nuclear Physics. The main ones I use for magnetism and EM Waves is,

W.J. Duffin Electricity and Magnetism, pub McGraw Hill.

Admin Note - A Faraday cage often consists of a grounded copper wire/mesh "box" which encloses something of interest - equipment sensitive to magnetic fields, e.g. Magnetic fields induce electrical currents in the wire mesh. In this sense the mesh "shields" the equipment within the cage from magnetic fields. Scientists frequently use these cages to protect everything from computers to calorimeters or electrophysiology equipment.

-L. Bry, MadSci Admin