The current through an electromagnet is switched off. What happens to its field?
A scrapyard crane swings a car body over a pile. The crane's magnet is an electromagnet. The operator cuts the current, and the car body drops, exactly where the operator wanted it.
An electromagnet's field is on only while its current flows. Cut the current, and the field is gone: so the magnet lets go of its load at that moment.
A permanent magnet cannot do this. A permanent magnet's field is always on: it cannot be switched off. A crane with a permanent magnet could pick the car body up, but it could never set the car body down on cue.
Can a permanent magnet's field be switched off?
A factory drop-gate must let go of steel sheets on cue. Which magnet suits the job?
A steel door should unlock the moment a guard presses a button. Which magnet should hold it shut: an electromagnet or a permanent magnet?
Pressing the button can cut a current.
An electromagnet's field switches off when its current is cut.
A permanent magnet's field cannot be switched off.
An electromagnet. Cutting its current unlocks the door at once.
Why would a permanent magnet be the wrong choice for the door lock?
A permanent magnet's field is always on.
The door would stay locked even after the guard pressed the button.
Because its field cannot be switched off, the door could never unlock on cue.
Felix says a magnet that lets go when its current is cut must be an electromagnet. Is Felix correct?
A recycling line's electromagnet carries steel cans over a bin, where its current is cut. What happens?
A sorting magnet's steel cans drop the instant its current is cut. Why?
Which job needs an electromagnet rather than a permanent magnet?