QUESTION #512

# Is there any material that can block a magnetic force? Specifically does lead block magnetic fields?

Asked by: Dustin

### Answer

Magnetic fields (forces are caused by magnetic fields) cannot be blocked, no. That is, there is no such thing as a magnetic insulator.

A major reason for this has to do with one of Maxwell's Equations:

del dot B = 0

Which implies that there are no magnetic monopoles. That is, where as you can separate electric monopoles (positive and negative charges) such that an E-field never has to terminate on the opposite charge, you cannot do this with magnetic poles. There do not exist any magnetic monopoles. There is no such thing as "magnetic charge." All magnetic field lines MUST TERMINATE on the opposite pole. Because of this, there is no way to stop them -- nature must find a way to return the magnetic field lines back to an opposite pole.

However, magnetic fields can be re-routed around objects. This is a form of magnetic shielding. By surrounding an object with a material which can "conduct" magnetic flux better than the materials around it, the magnetic field will tend to flow along this material and avoid the objects inside. This allows the field lines to terminate on the opposite poles, but just gives them a different route to follow.

You might want to check out:
http://www.lessemf.com/faq-shie.html#Lead-Copper
Which does a good job to answer your question directly.

As seen there, lead has a permeability of 1. This means that it is no better of a magnetic shield than air.

So the short answers:

• No material can block a magnetic field
• Lead most definitely has little to no effect on magnetic fields
• If you want to block out magnetic "force," your best bet is to re-route magnetic field lines (lines of magnetic flux) around the object that is sensitive to those lines. Do this by shielding the object in a material with a much higher magnetic permeability of the surrounding materials.
Answered by: Ted Pavlic, Electrical Engineering Undergrad Student, Ohio St.

It is an interesting observation that, while there are many analogies between electricity and magnetism, there really isn't an equivalent of a magnetic "insulator". However, engineers, being a clever bunch, have figured out how to get round this problem....

... in order to shield, for example, an electronic device from external magnetic fields, engineers will often use a shell or case that comprises of a material with a very high magnetic permeability - that is, a material that will allow a lot of magnetic flux lines within it, effectively concentrating the lines inside the material, and then "channeling" them away from the delicate electronics inside.

Looking at this from another perspective, sometimes it is necessary to ship magnetic components and materials via airplanes to their destination. There are strict Federal regulations regarding magnetic fields and aircraft instrumentation; a magnet on board must not interfere in anyway with the flight controls for obvious reasons. So, the magnets are usually packed "head to tail" to that their North poles are next to other South poles, and, thin sheets of steel, or other iron-based materials are packed around the magnets in order to "shunt" the field and to keep it from penetrating outside of the box.

Lead, not being a ferromagnetic material, cannot shield or shunt magnetic fields in this way.
Answered by: Gareth Hatch, Ph.D., CEO - Magellica Inc., Illinois