A electric transformer is an electrical device that increases or decreases the AC voltage from a primary circuit including an electric coil to a secondary circuit including another electric coil.
The principle upon which the voltage and current are transformed (increased or decreased) is magnetic mutual induction. If an AC voltage source supply electrical energy to a coil ,called primary coil, a changing (because the voltage is alternating, AC) magnetic field is created inside and around the coil. (see figure below)
If a second coil (secondary coil) is placed close to the primary coil and within the line of flux, the same changing magnetic field created in the primary coil will induce am emf in the secondary coil (Faraday's law) .
Let Vp be the voltage across the primary coil, Ip the current in the primary circuit and Np the number of turns of the primary coil.
Let Vs be the voltage across the secondary coil, Is the current in the secondary circuit and Ns the number of turns the secondary coil.
For low frequencies of the AC voltage in the primary, we have the following definitions and relationships between the voltages and currents in the primary and secondary circuits.
1) Efficiency η of a transformer is defined by
η = power supplied by secondary / power supplied by primary = Is Vs / Ip Vp
If there are no losses (losses negligible), η ≈ 100%.
If there are losses η < 100%.
2) When losses are negligible, the power in the secondary is equal to the power in the primary.