How is an electromagnet formed? How does winding the wire around a hard core make the electromagnet stronger?

We know that when  current passes through a  conductor magnetic field is produced around it.We also know that if the  conductor  is in circular form the strength of the magnetic field is increased in the centre of the loop. The strength of the magnetic field is, therefore,

(i)directly proportional to the current passing through the circular loop(ii) and inversely proportional to the radius of the loop.

If we increase the the number of loops (by introducing the insulated  conductor) the magnetic field strength gets stronger by that many number of times. This principle applies to a solenoid, where the insulated condutor is wound in a number of windings/ loops. Now if we introduce a core  of soft iron rod  through the solenoid the magnetic effect is many fold increased and the entire core acts as a magnet. The strength of this electromagnet is proportional to (i) the current passing through the insulated windings (ii) the number of turns in the windings.

The use of the soft iron core has one advantage that it completely looses its magnetism, when the current in the loop is switched off. If we use  steel instead of soft iron , it does not loose all its magnetism when current is stopped.

To make the electromagnet stronger, as already told we have to (1) increase the current passing in the conductor , (2)increase the  the number of loops of the conductor. One more factor is that the more the air gap between the poles , the less the magnetic field strength. For this reason, to reduce the length of air gap between the poles the eletromagnet is bent like a  U-shape. A U-shaped (or horse shoe type) electromagnet  is very much stronger than an electromagnet of bar shape with other identical conditions.