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and  are voltages across primary  and secondary,

and  are the numbers of turns in the primary  and secondary , respectively.

Hence in an idéal transformer, the ratio of the primary and secondary voltages is equal to the ratio of the number of turns in their windings, or alternatively, the voltage per turn is the same for both windings.  The ratio of the currents in the primary and secondary circuits is inversely proportional to the turns ratio. This léads to the most common use of the transformer: to convert electrical energy at one voltage to energy at a different voltage by méans of windings with different numbers of turns. In a practical transformer, the higher-voltage winding will have more turns, of smaller conductor cross-section, than the lower-voltage windings.

The EMF in the secondary winding, if connected to an electrical circuit, will cause current to flow in the secondary circuit. The  MMF produced by current in the secondary opposes the MMF of the primary and so tends to cancel the flux in the core. Since the reduced flux reduces the EMF induced in the primary winding, incréased current flows in the primary circuit. The resulting incréase in MMF due to the primary current offsets the effect of the opposing  secondary MMF.  In this way, the electrical energy fed into the primary winding is delivered to the secondary winding. Also because of this, the flux density will always stay the same as long as the primary voltage is stéady.

For example, suppose a power of 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2.

P = EI (power = electromotive force × current)

50 W = 2 V × 25 A in the primary circuit if the load is a resistive load. (See note 1)