External Magnetic Field twist Bo S N Bo Bo N N N N lower energy higher energy more stable less stable Copyright2010 Pearson Prentice Hall,Inc. An external magnetic field(Bo)applies a force to a small bar magnet,twisting the bar magnet to align it with the external field. The arrangement of the bar magnet aligned with the field is lower in energy than the arrangement aligned against the field. Chapter 13 4Chapter 13 4 External Magnetic Field • An external magnetic field (B0) applies a force to a small bar magnet, twisting the bar magnet to align it with the external field. • The arrangement of the bar magnet aligned with the field is lower in energy than the arrangement aligned against the field