Faraday disk/ how is the magnetic flux changing?

[John K]

i can not seem to figure out how is the magnetic flux is changing and thus the current is induced , "A "is constant (same disk is spinning ) and "B" is constant and does not change , so how is the current induced ?
and more specifically how is the change of magnetic flux is happening ?
thanks for help .

 

[Simon Bridge]

Welcome to PF;
Who is claiming that the magnetic field must change?
What happens to a charge moving through a magnetic field?