In transformers, the current changes depending on the principle of electromagnetic induction and the relationship between the primary and secondary coils. Current in the primary coil generates a magnetic field in the core, which induces a voltage in the secondary coil. The ratio of the number of turns in the primary coil to the number of turns in the secondary coil, called the turns ratio, determines the relationship between the input current and the output current.

If the transformer increases the voltage, the current in the secondary coil decreases proportionally to save power (assuming ideal conditions). Conversely, if the transformer decreases the voltage, the current in the secondary coil increases.

Transformers operate on alternating current (AC). The alternating nature of alternating current allows the creation of a changing magnetic field, essential to the induction process used by transformers to transfer energy from the primary coil to the secondary coil.

Changing the direction and magnitude of the alternating current generates a corresponding alternating magnetic field in the transformer core, allowing the efficient transfer of electrical energy between circuits at different voltage levels.

Power in transformers is ideally conserved, meaning that the input power to the primary coil is equal to the output power to the secondary coil, minus losses due to inefficiencies. Power in a transformer is the product of voltage and current.

Therefore, when a transformer increases the voltage, the current decreases, and when it decreases the voltage, the current increases, while the product of voltage and current (power) remains constant. Real transformers have losses due to resistance in the coils and other factors, so the output power is slightly less than the input power.

Copper loss in transformers, also called resistive loss or I²R, varies with load current because it is proportional to the square of the current flowing through the windings.

As the load current increases, the resistive losses in the transformer windings increase, resulting in higher copper losses. Indeed, the electrical resistance of the windings causes power to be dissipated in the form of heat. Therefore, copper losses increase significantly with higher load currents, impacting transformer efficiency under varying load conditions.

The operation of a transformer depends on the electromagnetic induction effect of alternating current.

This effect is characterized by the ability of a changing magnetic field, produced by alternating current in the primary winding, to induce an electromotive force (EMF) in the secondary winding. The alternating nature of the current is crucial, as it creates an ever-changing magnetic field necessary for the induction process. This principle allows transformers to efficiently transfer energy between circuits while changing voltage and current levels based on application requirements.