試題詳解

19. An unmagnetized steel bar is placed inside a solenoid. As the current in the solenoid is slowly increased from zero to some large value, the magnetization of the bar:
(A) increases proportionally with the current
(B) remains zero for a while and then increases linearly with any further increase in current
(C) is unaffected by the current
(D) increases with increasing current at first but later is much less affected by it
(E) increases quadratically with the current.