Expert: Daniel Mazur - 2/18/2008

Question
QUESTION: hello, i want to use a plate of any material in blocking magnetic fields line.Means,is there a plate exist through which no magnetic field line can pass?Please give me detail.

ANSWER: Hi gagan,

there is no way of blocking a static magnetic field the way you imagine it. It is because magnetic field is not like a stream of particles that you could block out. You can look at it the same way as gravity - both are static FIELDS, neither can be blocked.

The only way to partially reduce the magnetic field is to use a superconducting enclosure. Superconductors "expel" magnetic field from their bulk by means of developing a supercurrent (current of electrons in the superconducting state), which produces a magnetic field in the direction *opposite* to the direction of the external field. Then, the sum of the two contradictory fields can be much less that the magnitude of either of them. It is kind-of the same approach as when you have one strong field and put another magnet (ferromagnet) in it: with the correct orientation of the magnet you will achieve cancellation of the field at some points in space around it.

Note that this is not the same as the electromagnetic radiation, which can be blocked by various materials, depending slightly on the radiation frequency. But generally EM waves can be well blocked by conducting (metal) sheets.

Take care.
Daniel

---------- FOLLOW-UP ----------

QUESTION: sir,
   Can we change the way of a magnetic field line in such a way that there will be no magnetism at required place? please give me detailed knowledge..

Answer
Hi gagan,

the only way to do that is to use another magnet with opposite field, so that at the desired place the two magnetic fields cancel each other. But the "place" without field will probably be just one point, so it is not practical for applications. You cannot shield - *completely protect* - a given volume from penetration of static magnetic field.

There are materials that reduce the field penetrating a given volume, but there is always residual field inside the volume. I read just now that 90% reduction is considered a great success. There is also something called "shield saturation field", above which the shield no longer works. Good shielding materials (mu-metal or others, for example, at www.lessemf.com) reach saturation at 1-2 tesla.

I hope this helps.
Cheers,
Daniel