Question

Why do the magnetic parts of a mobile
phone become demagnetised when
the cell phone is stored near a powerful
electromagnet?​

Answer 1

Explanation: A magnet can damage the disk (hard disk or floppy disk) in your computer because these are magnetically recorded.  It is unlikely there is very much that is magnetically susceptible in your computer (although a rapid change in magnetism can induce an electric current within the phone that might damage it - but by rapid change, I am talking about switching on a powerful electromagnet within a millisecond or so; I am not talking about moving a bar magnet closer or further away over the period of about half a second).