Suppose we have applied a voltage to a transformer with an open secondary, we will get a certain flux in the core and a certain voltage at the secondary depending upon the turns ratio.
Now, suppose we connect resistance to the secondary and therefore current will start to flow through it and a reflected current will flow in the transformer's primary winding. But the magnitude of flux remains the same with or without the load.
So my question is: What exactly changes inside the magnetic core when we connect a load to the transformer, which results in power transfer from primary to secondary?
As far I understand, change in flux induces a voltage but does not carry any power, and change in flux remains the same with or without the load.
I asked the exact same question on electronic stack exchange and someone suggested to ask it on this forum for a more complete answer from a physics point of view.