Which phenomenon describes a magnetic material changing length in response to a changing magnetic field?

• A. Magnetostriction
• B. Refraction
• C. Piezoelectricity
• D. Rarefaction

Answer: (A)

Magnetostriction is the change in dimensions of a magnetic material when the magnetic field changes. When a ferromagnetic material is exposed to a magnetic field, its magnetic domains reorient and the crystal lattice experiences small strains, causing the material to lengthen or shorten. This effect is intrinsic to how magnetic order couples to the lattice, and it’s the phenomenon that explains why the material physically changes size in response to the field.

Refraction involves bending of waves as they pass between media with different speeds, piezoelectricity is deformation caused by an electric field in certain crystals, and rarefaction refers to a region of lower pressure in a traveling wave—none of these describe a length change driven by a magnetic field, so magnetostriction is the correct concept.