How can a steel nail be magnetized effectively?

A steel nail can be effectively magnetized by rubbing a bar magnet along its length in one direction multiple times. This method involves aligning the magnetic domains within the steel, which are small regions that can have their magnetic orientations altered when influenced by an external magnetic field. As the bar magnet is rubbed along the nail, the consistent directional motion causes these domains to align in the same direction, resulting in the steel nail becoming magnetized.

This process emphasizes the importance of directionality; rubbing the magnet in one direction ensures that the domains are aligned properly. If the motion were to move back and forth, it could lead to a random orientation of the domains, diminishing the magnetization effect.

Other options do not provide effective means of magnetization: heating the nail tends to demagnetize it instead of magnetizing it, as the heat can disrupt the alignment of the magnetic domains. Placing it in a strong magnetic field for one hour could work, but it may not be as efficient or practical as the rubbing technique, especially without the ability to ensure the correct alignment as in option B. Dropping the nail results in physical shock, which can again disrupt the magnetic domains rather than align them, leading to a loss of any existing magnetization.