Which device increases or decreases the voltage of alternating current?

• A. Resistor
• B. Inductor
• C. Transformer
• D. Diode

Answer: (C)

Transformers change AC voltage using electromagnetic induction between two windings wrapped around a magnetic core. When AC flows in the primary winding, it creates a changing magnetic flux in the core, which induces a voltage in the secondary winding. The voltage change depends on the turns ratio between the windings: more turns on the secondary produce a higher voltage (step-up), while fewer turns produce a lower voltage (step-down). This requires alternating current because the changing current is what continually builds and collapses the magnetic field to induce voltage in the other winding.

This is why the transformer is the best choice. A resistor simply drops voltage proportional to current and wastes power as heat; an inductor stores energy and opposes changes in current but doesn’t set a new voltage level by itself; a diode rectifies AC to DC and doesn’t transform the AC voltage amplitude.