To calculate the energy required to demagnetize an iron plate magnetized by a permanent magnet, one must consider the magnetic field strength, volume, and magnetic permeability of the iron. Heating the plate to the Curie temperature is one method, but using AC current is preferred for gradual demagnetization. The energy can be calculated using the formula E = 0.5 x B^2 x V x μ, while the AC current needed depends on the frequency and inductance, calculated with I = √(2πfL/E). Additionally, the number of AC cycles required can be determined by dividing the total energy by the energy per cycle. Understanding these parameters is crucial for an effective demagnetization process.