Hint:The electromagnet is basically a solenoid with soft iron as a core and the core of the solenoid has high permeability and low retentivity. When we apply current to the solenoid, the magnetic field is set up within the solenoid.
We know that the electromagnet is made of soft iron rod wind a number of turns of copper wire over it.When we pass current through the copper wire, a magnetic field is generated in the space within the solenoid. The high permeability of soft iron increases the magnetic field one thousand times. The end of the solenoid at which the current is flowing in the solenoid is in an anti- clockwise direction and behaves as North Pole and the remaining side behaves as a South Pole.
When the current flows in the solenoid is off, the soft iron (core) loses its magnetism due to its low retentivity.If we increase the current in the solenoid and the number of turns of the copper wire over the soft iron, then large amount of magnetic field will be generated within the solenoid. As a result, the strength of the electromagnet will increase.
Note:The choice of magnetic material for making electromagnets depends upon the hysteresis loop of the material. The material requires making the core of the electromagnets must have the following characteristics:
-High initial permeability so that the magnetisation is large even for a small magnetising field.
-Low retentivity so that the magnetisation is lost as the magnetising current is switched off.
Thus, soft iron is more suitable for the core of electromagnets.