List three sources of magnetic fields.

Permanent magnets are those magnets which can retain their magnetism for a long period of time. They have fixed north and south pole. A Bar magnet is the perfect example of a permanent magnet.

Electromagnets are the temporary magnet which works when current is passed through it. It works on the magnetic effect of current. It consists of a long coil of insulated copper wire wrapped around a soft iron core that gets magnetised only when the electric current is passed through the coil.

Its strength can be changed by changing the number of turns or by changing the current.

They haven't fixed north and south pole like permanent magnets, because its pole can be changed by changing the direction of current in the coil.

Current-carrying conductors are also a temporary magnet which works when current is passed through it. When current is passed through a current-carrying conductor AB, it produces magnetic field lines around it, which are circular in nature. It can be tested by sprinkling iron filings on the cardboard sheet. The sprinkled iron filings get magnetised and on tapping the cardboard sheet, the iron filings arrange themselves in circles around the wire. A compass placed on the cardboard indicates the direction of the magnetic field.