The magnitude of net emf of an ideal transformer is given by the formula: E=4.44×f×N×ϕm Where E = RMS value of applied voltage. f = frequency of the transformer. N = number of turns. ϕm = the magnitude of maximum flux in the core. E=111V,f=50‌Hz,N=200 111=4.44×50×200×ϕm ϕm=