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Union University Department of Physics

The Science Guys

Science Guys > February 2004

February 2004

I have heard lead stops nuclear radiation; can you shield or block magnetic fields?

The short answer is no, there is no shield or substance that will effectively block magnetic fields as such. You can however redirect the magnetic field lines, which is what some people call magnetic shielding. Now let us investigate this in a bit more detail.

Actually there is a law named Gauss’s law that tells us something about magnetic fields (this law is also one of Maxwell’s equations which explains all electromagnetic phenomena). This law basically implies that you cannot separate magnetic poles, that is, you cannot isolate just one pole; there must be two magnetic poles, one called north and one south. This is different from electric charges where you can segregate a single positive or a single negative charge. Magnetic poles always come as a pair. The terminology scientists use is that monopoles (single magnetic poles) do not exist.

The magnetic field lines are closed loops and must be continuous between a north and a south pole. In the case of a bar magnet, think of field lines exiting from the north pole, radiating through space, and re-entering the bar magnet at the south pole, continuing through the magnet back to the north pole. Since these field lines must be continuous, they must find a way back to their origin. They cannot be stopped and have nowhere to go.

The field lines can however be redirected. Therefore, it is possible to design a region of space relatively void of magnetic field lines because they have been redirected around that region. Note that you have not stopped them, just redirected them. The field lines still must be continuous and eventually close back on themselves.

In order to redirect magnetic field lines, you offer them a preferred path. Magnetic field lines prefer to travel in materials that have certain magnetic properties, namely materials with high permeability. By placing a material of high permeability (or at least a permeability higher than the region in question) around the region you wish to shield, you effectively offer the field lines a better path to travel. The magnetic lines take that path and stay out of the region you wanted to shield. The high permeability material will "conduct" the magnetic field lines better than its original path. While it is not the same phenomenon, this reminds us of the fact that electricity takes the path of least resistance. The field lines choose the easiest path to travel. Thus, a shell of high permeability material built around an area will effectively keep most of the magnetic field lines in the actual shell itself and out of the area inside the shell.

While lead blocks or stops radioactive emissions such as beta particles or gamma rays, it does nothing to block magnetic fields. The permeability of lead is low and has basically no affect on magnetic fields.