188 Sources of the Magnetic FieldQ30.25A “hard” ferromagnetic material requires much more energy per molecule than a “soft”ferromagnetic material to change the orientation of the magnetic dipole moments. This way, a hardferromagnetic material is more likely to retain its magnetization than a soft ferromagnetic material.Q30.26The medium for any magnetic recording should be a hard ferromagnetic substance, so that thermalvibrations and stray magnetic fields will not rapidly erase the information.Q30.27If a soft ferromagnetic substance were used, then the magnet would not be “permanent.” Anysignificant shock, a heating/cooling cycle, or just rotating the magnet in the Earth’s magnetic fieldwould decrease the overall magnetization by randomly aligning some of the magnetic dipolemoments.Q30.28You can expect a magnetic tape to be weakly attracted to a magnet. Before you erase the informationon the tape, the net magnetization of a macroscopic section of the tape would be nearly zero, as the
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