Absorption loss is the primary shielding mechanism to shield low frequency magnetic fields. Therefore, the ideal material choice for magnetic shielding would be a thicker, high permeability and electrically conductive material.
There are some overlooked aspects that need to be considered when choosing a magnetic material. The permeability of a material will in fact decrease as the frequency of the incident wave increases. For example, at kHz the permeability of HyMu 80 is no better than cold-rolled steel. As a general rule of thumb, high permeability materials are ideal when dealing with frequencies below 10kHz. Do the paper clips fall off? Try experimenting with different materials, such as various metallic coins.
Make a list of the materials you test and note what happens. The magnetic-field lines from the magnet pass through the cardboard, the air, and other materials like the craft stick and straw. Materials that allow magnetic lines of force to pass through them are called nonpermeable because magnetic fields do not form within them. In contrast, the metal knife acts as a magnetic shield, meaning the force lines coming from the pole of the magnet do not pass through it.
Instead, they are gathered in, travel down the metal strap, and re-enter the magnet at the other pole. Materials that gather magnetic lines of force are said to be permeable , because they support the formation of magnetic fields within those materials.
Only magnetic materials are permeable. The photos below show the effect of a nonpermeable material at left and permeable material at right on magnetic lines of force in our cardboard sandwich. Try using only one or two magnets.
As the magnetic field strength gets weaker, it gets harder to keep the paper clips from falling. How could you modify the sandwich to make a weaker magnet work better?
We can take advantage of this property to effectively redirect field lines away from areas we wish to be field-free. For clarity, this effect is shown in the illustration. Magnetic field lines are directed around the area within the shield.
This diagram depicts the cross section of a cylindrical shield. Now let us investigate this in a bit more detail. 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.
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