The sheet iron core of a transformer forms what type of circuit?

The sheet iron core of a transformer forms a closed magnetic circuit. This design is crucial for the efficient operation of the transformer, as it minimizes magnetic leakage and enhances the coupling between the primary and secondary windings. A closed magnetic circuit allows magnetic flux generated by the primary winding to return through the core material, effectively linking the energy transferred to the secondary winding.

In contrast, an open magnetic circuit would allow magnetic lines of force to diverge into the air rather than return through the core, which would significantly decrease efficiency and performance. A closed electrical circuit refers to the path through which electric current flows, but it does not pertain to the magnetic properties of the transformer core. Hence, understanding the significance of a closed magnetic circuit in a transformer emphasizes its importance in achieving optimal performance and energy transfer.