The transformer coils of the primary and secondary share no electrical connection, but current flows through them due to which phenomenon?

• A. Direct coupling
• B. Inductive coupling
• C. Capacitive coupling
• D. Mutual induction

Answer: (D)

The voltages and currents in the two coils are transferred by magnetic interaction, not by a direct electrical path. When the current in the primary winding changes, it creates a changing magnetic flux that threads the secondary winding. This changing flux induces a voltage in the secondary according to Faraday’s law, and that induced voltage drives current in the secondary. This coupling between the coils—where one coil induces effects in the other without a direct connection—is called mutual induction (mutual inductance) between the windings. Capacitive coupling would involve electric fields across a dielectric and is not how a transformer transfers energy; direct coupling is excluded by the lack of a physical connection.