Transformer works on principle of MUTUAL INDUCTION. When voltage is applied to the primary winding of the transformer, current flows through the current called magnetising current results in magnetic flux (phi-1) further results in an emf induced in the primary coil which opposes the magnetising current as per Lenz's law. This primary flux lines pass through the low reluctance path i.e, magnetic core and links with the secondary coil thus inducing an emf at secondary coil of the transformer. When the transformer is loaded, secondary current starts flowing completing the entire Electrical circuit. Thus power flow from primary to secondary circuit is simply based on the mutual induction.