Transformers can be applied in __________ circuits only.

• A. Direct Current
• B. Alternating Current
• C. Direct Current and Alternating Current
• D. AC Lines

Answer: (B)

Transformers work by mutual induction, which requires a changing magnetic field produced by the primary winding. That changing field comes from current that varies over time. In a direct current circuit, the current settles to a steady value, so the magnetic flux becomes constant and no new voltage is induced in the secondary winding. Energy transfer essentially stops after the transient, and the transformer is not doing its job.

In alternating current circuits, the current continually changes direction and magnitude, so the magnetic flux in the core keeps changing. This ongoing variation induces a voltage in the secondary winding proportional to the turns ratio, allowing the transformer to transfer energy and transform voltage.

AC lines are a specific context where this works, but the fundamental requirement is the presence of alternating current. That's why alternating current is the appropriate description for when transformers can operate.