Simple Nature - Light and Matter

a low permeability, while the other one has a very high one. Wewill violate Faraday’s law unless the component of the H field parallelto the boundary is a continuous function, H ‖,1 = H ‖,2 . Thismeans that if µ/µ o is very high, the component of B = µH parallelto the surface will have an abrupt discontinuity, being muchstronger inside the high-permeability material. The result is thatwhen a magnetic field enters a high-permeability material, it tendsto twist abruptly to one side, and the pattern of the field tends tobe channeled through the material like water through a funnel. Ina transformer, a permeable core functions to channel more of themagnetic flux from the input coil to the output coil. Figure j showsanother example, in which the effect is to shield the interior of thesphere from the externally imposed field. Special high-permeabilityalloys, with trade names like Mu-Metal, are sold for this purpose.j / A hollow sphere with µ/µ o = 10, is immersed in a uniform, externally imposed magnetic field. Theinterior of the sphere is shielded from the field. The arrows map the magnetic field B. (See homework problem47, page 727.)The very last magnetic phenomenon we’ll discuss is probably thevery first experience you ever had of magnetism. Ferromagnetism isa phenomenon in which a material tends to organize itself so that ithas a nonvanishing magnetic field. It is exhibited strongly by iron714 Chapter 11 Electromagnetism