Answer:
The concept of zero net current in toroid occurs when we try to determine magnetic field induction outside toroid.
to determine magnetic field outside toroid, we consider a circular loop outside toroid as shown in figure.
to get the net enclosed current inside the loop, let us consider the crosssection AA' in toroid as shown in figure
as we see the crosssection AA' part of figure, we see that currents in the winding are in opposition directions at near to the loop
and far away from loop. Hence the concept of net current zero occurs in toroid when we try to determine
magnetic field induction outside toroid.
Have a Good day...
