Solution: The induced emf in the coil is due to the change in flux (transformer action) in the coil itself. That is, because the
coil is fed with an AC source, it produces an AC flux which in return induces an emf in itself. After calculating the flux in the
core of the torus, the emf is calculated from Faraday’s law. In (c), the induced emf is also very large because the inductance
of the coil is directly proportional to permeability: