FigQ2 shows an magnetic circuit Assume the magnetic core has

Fig.Q2 shows an magnetic circuit. Assume the magnetic core has zero reluctance and there is no fringing effect in the flux paths, calculate the flux density in the air gap.

Solution

When a magnetic flux is circulated or follow through a closed area or path, is called the magnetic circuit or when a magnetic field circulates in a closed path represented as lines of magnetic flux in a confined area is called Magnetic Circuit. This magnetic circuit forms with permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron etc.

I know that it prevents rotor from rubbing in on the stator and also have read that in salient pole type motor it plays role by adding reluctance torque component but do explain how if you know.or anything else,especially what about in case of uniform air gap .
I don\'t mind anyone telling only some specific role or importance in any specific type of motor because I m interested in knowing every role, importance and necessity of air gap in producing rotation in any kind electric motor .
To say which type of motor ,actually feel free to tell any of the following.
DC motor
synchronous motor
Induction motor
or else if you know.