If we wind 2 coils on a steel core we can cause almost all of the flux to link both coils and we can further hypostasise the case in which 100% of the flux is linked. In this ideal case it is call an“ideal transformer”. Now let's input a sinusoidal AC voltage on the red coil on the left. This is known as the primary winding. This AC voltage input will cause a small current to flow in the primary coil. The amount of current flowing is limited by the reactance of the primary coil. In an ideal transformer this reactance is 100% inductance so the current will lag the voltage by 90 degrees. This current is known as the magnetization current. Remember that any time a current flows, it will produce a magnetic flux proportional to it which means it too is sinusoidal and in phase with the current...and since we are dealing with an ideal transformer all of that flux flows in the iron and the links both coils. Considering the primary coil we will measure a voltage drop V1 across its terminals and it will be equal to the applied input voltage Vac because of the direct connection in coil 1 the flux produced by the generator is related to the voltage V1 by Faraday’s law which involves the changing flux times the number of turns in the primary coil…in coil 2, the secondary coil, the voltage produced by that same flux (mutual inductance ) is also given by Faraday’s law…. which involves the same changing flux times the number of turns in the secondary coil…That voltage is either either larger or smaller than V1 depending on N1 & N2 but is in phase with the applied voltage Vac.