Why is the emf max when the plane of the armatute is parallel to mf?
Answers
it's maximum when mf is perpendicular to coil
Do not forget the magnetic field within the armature has to be NON-STATIONARY.
The Electromotive force force or in other words, the induced voltage is proportional to the rate of change of the magnetic field. Hence it is proportional to the change in the overall magnetic flux that crosses the moving armature. the plane of the coil/armature. Now, when the armature/coil is perpendicular to the magnetic field, small change in its angle leads to only small absolute change in magnetic flux. Similarly to how tiny angle change near 90 degrees does not change the sine of that angle much, when compared to the same change of the angle near 0 degrees, the change in sine value of it is substantial.
Hence the reason is geometric, based on the rotation of the armature/magnetic field.