Note: The emf in each coil is opposite in sign to the applied voltage on the coil, if any, so that Kirchhoff’s voltage law gives
zero in the primary and secondary circuits. This is the same as saying that the total flux in an ideal transformer is zero. In a
real transformer, Kirchhoff’s voltage law in each circuit results in a small voltage difference (V1 > emf1,V2 < emf2). The
difference is due to losses in the transformer and the transformer ratio changes accordingly.
While most transformers are designed either to transform currents or voltages, they also change the impedance of the
circuit. The impedance of the primary circuit is given by the ratio of emf1 and I1: