thickness of steel plate required to stop interaction between 3' dia neodymium magnet
I don't think you can shield one magnet from another one... you didn't say what the shielding would be "between".
The shielding effect depends on the grade of steel... you need enough that it won't saturate, so how much you'd need also depends on the strength of the magnet.
Three feet diameter neodymium magnets?
I'm sorry,that was a mistake its 3 inch dia
You can't cancel magnetic field but can redirect it which effects the interaction between magnets or other materials.
Yes, thickness of shielding material depends on various factors which includes magnetic field strength, its Frequency and material permeability.
Even 3" is very big.
You're not going to ever completely stop the interaction. If you put up a piece of iron, the magnet will still feel the force from the iron.
I got my answer.
Thanks for your involvement.
What is the answer?
Well It depends on your question.
I just posted that question to get an insight of some other problem.
If you can elaborate your question then maybe I can help you.
"Steel" is not a well specified material. There are lots and lots of different types, and many are almost completely non-magnetic (this is especially true for stainless steel); i.e. the steel won't affect the magnetic field at all (this is why you can build e.g. NMR and MRI machines out of steel). Iron is of course ferromagnetic, but pure iron is not actually used that often.
Hence, in order to be able to be able to start to answer the question you need to specify the type of steel used.
Plain Carbon Steel
You can not shield them completely.
How thickness affects shielding is shown here
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