Transformer fundamentals: A voltage is induced in the secondary winding primarily due to which physical action?

Introduction / Context:
Transformers transfer energy via magnetic coupling. Understanding what actually induces voltage in the secondary clarifies how turns ratio, flux, and core behavior relate and helps avoid common misconceptions about “voltage being transferred” directly through wires.

Given Data / Assumptions:

• Ideal transformer model for explanation.
• Alternating voltage is applied to the primary winding.
• Core provides high mutual coupling so most primary flux links the secondary.

Concept / Approach:
Applying AC to the primary creates a time-varying flux in the core. According to Faraday’s law, any coil linked by changing flux experiences an induced emf proportional to the rate of change of flux and the number of turns. Thus, the secondary voltage is induced by the primary-generated mutual flux that cuts the secondary turns. The turns ratio determines the magnitude relationship V2/V1 ≈ N2/N1 but is not itself the mechanism; the mechanism is changing flux linkage.

Step-by-Step Solution:
Primary AC current → time-varying core flux φ(t).Secondary turns are linked by φ(t) → e2 = N2 * dφ/dt.Therefore, V_secondary arises from mutual flux set by the primary.Magnitude relationship is governed by the turns ratio, but induction is due to dφ/dt.

Verification / Alternative check:
Open-circuit test: with the secondary open, a voltmeter still measures near-rated voltage on the secondary—evidence that induction is caused by primary-driven flux, not by load current.

Why Other Options Are Wrong:
Secondary magnetic field acting alone: without primary excitation, there is no changing flux to induce voltage.Primary turns ratio as a cause: the ratio sets the proportionality but does not cause induction.Secondary counter-emf independent of the primary: secondary emf is induced by the primary-created flux.

Common Pitfalls:
Equating turns ratio to mechanism; always attribute induction to changing flux as per Faraday’s law.

Final Answer:
the alternating magnetic field produced by the primary that links the secondary (mutual flux in the core)