Why does a transformer require $AC$ (alternating current) rather than $DC$ (direct current) to operate?

• A. Because $AC$ allows the transformer to work without any energy losses.
• B. Because $AC$ produces a changing magnetic field in the transformer's core, which induces a voltage in the secondary coil according to Faraday's law.
• C. Because $DC$ reverses the direction of current in the coils, which is necessary for voltage transformation.
• D. Because $DC$ creates a stronger magnetic field in the transformer's core, resulting in higher efficiency.

Answer: (B)

Verified step by step guidance

1. Recall that a transformer operates based on the principle of electromagnetic induction, which requires a changing magnetic flux to induce a voltage in the secondary coil.
2. Understand that alternating current (AC) continuously changes its magnitude and direction, producing a time-varying magnetic field in the transformer's core.
3. Apply Faraday's law of electromagnetic induction, which states that a changing magnetic flux through a coil induces an electromotive force (emf) or voltage in that coil: \(\mathcal{E} = -N \frac{d\Phi}{dt}\), where \(N\) is the number of turns and \(\Phi\) is the magnetic flux.
4. Recognize that direct current (DC) provides a constant current, which creates a steady magnetic field with no change over time, so no voltage is induced in the secondary coil.
5. Conclude that because a transformer relies on a changing magnetic field to transfer energy between coils, it requires AC rather than DC to function properly.