Question

How placing an iron core in a solenoid can increase the strength of an electromagnet?

Answer 1

If you want a strong magnet, wrap them around an iron core. Iron is a magnetic material, and it becomes magnetized when you switch on the current. This gives you, in effect, two magnets for the price of one. Steel contains iron, so it will behave in the same way, although not as strongly. Two other ferro-magnetic metals you might come across are nickel and cobalt.
As the soft iron core is a highly ferromagnetic material, It allows (and attracts) the magnetic field lines to pass through it.When such material is used in the electromagnet, the magnetic field lines passing through it increases, thereby, the strength of the electromagnet increases.