Re: Why do magnets penetrate through plastic

Chelsea,

Magnetic fields will penetrate through any material that is magnetically 
"weak".  The characteristic of a material that governs its response to 
magnetic fields is called magnetic permeability.  It turns out that most 
materials do not affect, and are not affected by magnetic fields in any 
macroscopic way.  Plastics are one example of this.  

There are two general classes of materials that are affected by magnetic 
fields:  ferromagnetic materials and paramagnetic materials.  Ferromagnetic 
materials have their own intrinsic magnetic fields, and are used to make 
permanent magnets.  Paramagnetic materials do not produce their own field, 
but some respond to external fields strongly.  Most materials are either 
weakly paramagnetic or diamagnetic (you can think of this as the opposite 
of paramagnetic, but it is always weak), and the effect of a magnetic field 
on these materials is not easily detectable.

If you want to know more about magnetic fields and the response of 
materials to them, you might want to go to your library for the book 
"Conceptual Physics" by Hewitt.