For which purpose is a transformer fundamentally used in electrical engineering practice—considering energy transfer between circuits of the same nature?

Introduction / Context:
Transformers are passive electromagnetic devices used pervasively in power grids and electronics. Their core function is to transfer energy between circuits via magnetic coupling, typically changing voltage and current levels while (ideally) conserving apparent power and frequency.

Given Data / Assumptions:

• Alternating current (AC) source to establish a time-varying flux.
• Two windings magnetically coupled on a common core.
• Neglecting losses for fundamental description.

Concept / Approach:
Only a time-varying current can induce a continuous voltage in the secondary; thus, transformers operate with AC (or time-varying waveforms). The device couples energy from one AC circuit to another, enabling isolation, voltage transformation, and impedance matching.

Step-by-Step Explanation:
Primary AC establishes alternating core flux.This changing flux links the secondary and induces an AC voltage proportional to turns ratio.Load on the secondary draws current; primary current adjusts to supply the power magnetically.Therefore, transformers couple AC-to-AC energy; they are not DC energy converters without additional circuitry (e.g., choppers/inverters).

Verification / Alternative check:
If a DC source is applied to an ideal transformer, after transients no changing flux exists, and no sustained secondary voltage is induced—further confirming the AC-only nature for steady transfer.

Why Other Options Are Wrong:

• DC-to-DC, AC-to-DC, DC-to-AC transfers require active conversion circuits; a transformer alone cannot perform these functions.
• “Convert chemical energy” describes batteries/fuel cells, unrelated to transformers.

Common Pitfalls:

• Assuming a transformer “steps DC voltage”—not possible without switching.
• Overlooking isolation and impedance matching roles in addition to voltage transformation.

Final Answer:
to couple electric energy from one ac circuit to another