Q:- Temporary magnets are used in which of the following?

1. Loud speakers. 

2. Generators. 

3. Motors. 

4. All of these. 

Hints :- 

Permanent magnets lose their power after some days. So, if we use permanent magnet in those equipment, then it does not give efficient output. But, the above mentioned equipment's have to work efficiently at all time. That is why the temporary magnets (or) the electromagnets are used in the above mentioned machines.