Question

why there are air gap between poles of electromagnet​

Answer 1

Answer:

If the current in the coil is increased, the strength of electromagnet increases the length of air gap between the poles. If we reduce the length of air gap between the poles of an electromagnet, then its strength increases.

The gap reduces effective permeability of a given magnetic circuit and allows storing much greater energy before saturation is reached. Increasing the gap reduces the inductance, so the winding must have more turns to compensate accordingly.