Transfer of electric power from primary to secondary in a transformer takes place

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electro-magnetically

Explanation: A transformer is a static device that transfers electrical energy from one circuit to another through the process of electromagnetic induction. There is no direct electrical connection (conduction) between the primary and secondary windings; rather, energy is transferred via a time-varying magnetic field in the core.

Working Principle: When an alternating current flows through the primary winding, it creates a time-varying magnetic flux $\Phi$ in the core according to Faraday's law. This flux links the secondary winding, inducing an electromotive force (EMF) based on the rate of change of flux linkage. This dual process of electrical-to-magnetic and magnetic-to-electrical conversion is why it is termed electro-magnetic transfer.

Key Formulas: $e = -N \frac{d\Phi}{dt}$ — Faraday's Law of electromagnetic induction representing the induced EMF.; $\frac{V_p}{V_s} = \frac{N_p}{N_s} = \frac{I_s}{I_p}$ — Ideal transformer turns ratio relationship.

Key Points: Transformers operate on the principle of mutual induction.; The core provides a low reluctance path for the magnetic flux.; An AC source is essential; transformers cannot transfer DC power through mutual induction.; The primary and secondary windings are electrically isolated from each other.

Advantages: Allows voltage step-up or step-down to suit transmission/distribution needs.; Provides electrical isolation between input and output circuits.

Disadvantages: Operates only with AC (time-varying signals).; Physical size and weight can be significant at lower frequencies.

Applications: Power grid voltage transformation.; Impedance matching in electronic circuits.

Additional Info: Option D (conductively) is incorrect because an autotransformer is the only type with a conductive path, but standard two-winding transformers rely purely on electromagnetic coupling.. Option A is incorrect because there is no metallic connection between primary and secondary.

Answer: C is correct — The energy transfer in a transformer occurs via a time-varying magnetic field linkage, defined as electro-magnetic transfer.

Exam Tip: Always remember that 'electrically isolated' means there is no path for DC current between windings; this safety feature is a major benefit of the electromagnetic coupling principle.

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