Here, we have used the fact that the current in the secondary is due to induction; that is, it only exists if the current in the
primary exists. According to Lenz’s law, the flux produced by this current is always in opposition to the flux due to
the primary. Therefore, the flux in the core is small (it is zero for an ideal transformer and for a nonideal transformer with
zero losses).
To more easily identify the emfs induced in various coils, the so-called dot convention is used. A dot is placed on the
terminal of the coil which, when a current flows into the dot, produces a flux in the direction of the net flux in the core. In the
case of transformers, this means that when the current increases on a dotted terminal, all dotted terminals experience an
increase in emf. A current flowing into a dot produces a positive emf and a current flowing away from a dot produces a
negative emf. In Figure 10.13b, I1 flows into the dot and I2 flows away from the dot. The emfs in Eqs. (10.48) and (10.49)
that are associated with I1 are positive whereas those associated with I2 are negative.
The induced emfs in Eqs. (10.48) and (10.49) may also be understood in terms of impedances. In particular, in the
frequency domain, d/dt is replaced with jω and the emfs in Eqs. (10.48) and (10.49) become