Transformer

A transformer and its close relative, the Induction coil, use a B field created by a Primary Coil to induce a current in a Secondary coil through induction. The B field changes as the Primary Current changes.

The link between the Primary and Secondary is strenghtened by the use of a laminated iron alloy core ( laminated to reduce eddy currents ) to increase the B field. Further, both Primary and Secondary use multiple windings which dictate the strength of the flux between them. More coils, more area hence more flux.

The Primary is the Input and its windings create a resistance to reduce the primary current when no current is taken from the Secondary.

The mutal link of B field gives a very simple relationship between the applied Primary voltage and the Secondary voltage

V_s / V_p = n_s / n_p

As power is approximately conserved across the transformer, P_p = P_s , V_p I_p = V_s I_s

so I_s / I_p = n_p / n_s

Things are never as simple as the animation implies. The animation has deliberately left out the effect of Lenz's Law on the Primary ( the "back emf") and the changes of phase between V_p and I_p, V_s and I_s under different types of load. The assumption is that this transformer is pumping current into a simple resistor in the Secondary stage.