What distinguishes the primary side of a transformer from the secondary side?

The primary side of a transformer is distinguished from the secondary side primarily by the input and output voltage levels. In a transformer, the primary side is where the input voltage is applied, while the secondary side is where the output voltage is taken. The fundamental operation of a transformer relies on the principle of electromagnetic induction, which enables it to step up or step down voltage levels based on the turns ratio between the primary and secondary windings.

For instance, if the transformer has a higher number of turns in the secondary winding compared to the primary winding, it will produce a higher output voltage than the input. Conversely, if the secondary winding has fewer turns, the output voltage will be lower. This difference in voltage levels is crucial for various applications in electrical systems, enabling the transformation of voltage to suit the requirements of different electrical loads.

The other choices refer to aspects of transformers that may vary but do not fundamentally distinguish the primary side from the secondary side in the most basic and functional sense. While voltage output characteristics, current type, and connections to different circuits are relevant to the operation and design of transformers, they do not specifically define the primary versus secondary sides as clearly as the input and output voltage levels do.