Describe a machine or situation where it is important to have the ability to turn magnetic power on and off.

A DC motor requires a magnetic force to create its rotational power

hope that helps

Yes it would be a physical change.

☆ionisation energy increases across a period from na -> ar☆ionisation energy decreases down a group from li -> fr☆ionic radius decreases across a period from na -> cl☆ionic radius increases down a groupfrom f -> ii hope it clarifies your doubts