What causes a voltage to appear across the secondary winding of a transformer when an AC voltage source is connected across its primary winding?

A transformer operates on the principle of electromagnetic induction. When an alternating current (AC) flows through the primary winding, it creates a constantly expanding and collapsing magnetic field around the coil. This changing magnetic field cuts across the turns of the nearby secondary winding. Because the two coils share this fluctuating magnetic field, an alternating voltage is induced in the secondary coil. This specific phenomenon is called "mutual inductance." Distractors like capacitive coupling and mutual capacitance are incorrect because they refer to the interaction of electric fields between conductors separated by an insulator, not the magnetic fields shared by the wire coils of a transformer.