The primary and secondary of a transformer are ________ coupled but _______ connected.

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Examoogle · Accepted Answer

Option A: magnetically, not electrically

Explanation & Concepts:
A transformer operates on the principle of mutual induction where energy is transferred between two windings through a common magnetic flux. Since there is no physical conducting path between the primary and secondary coils, they are magnetically coupled but electrically isolated.

Working Principle: When an alternating voltage is applied to the primary winding, it produces a time-varying magnetic flux in the core. According to Faraday's Law, this flux links the secondary winding and induces an electromotive force (EMF) via mutual induction. Because the core material (laminated steel) provides a low-reluctance path for the flux while being an electrical insulator, no charge carriers flow directly from the primary circuit to the secondary circuit.

Key Formulas: $\frac{V_1}{V_2} = \frac{N_1}{N_2} = \frac{I_2}{I_1}$ — Ideal transformer transformation ratio; $e = -N \frac{d\Phi}{dt}$ — Faraday's Law of Induction

Key Points: Electrical isolation prevents DC current transmission between windings.; Magnetic coupling allows for impedance matching and voltage level transformation.; The primary winding creates a time-varying magnetic field (B-field).; The secondary winding intercepts the flux to produce an induced EMF.

Advantages: Electrical safety (Galvanic isolation); Efficient voltage transformation levels; Reduction of ground loop currents

Disadvantages: Transformer size and weight (Core requirements); Presence of leakage flux

Applications: Power distribution systems; Impedance matching in audio circuits; Galvanic isolation for sensitive electronics

Additional Info: The core is typically constructed from high-permeability, laminated silicon steel to minimize eddy current losses.. Option B is incorrect as it describes autotransformers, where a single winding acts as both primary and secondary, leading to electrical continuity.

Answer: A is correct — In a two-winding transformer, energy transfer occurs solely via a common magnetic field, ensuring the primary and secondary circuits remain galvanically isolated.

Exam Tip: Always remember that in an ideal transformer, the coupling coefficient $k=1$, meaning flux linkage is perfect; real transformers operate with $k

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Option B: electrically, not magnetically

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Option C: magnetically, also magnetically

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Option D: electrically, also electrically

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