Which statement describes a transformer?

• A. A device that stores electrical energy in a magnetic field
• B. A device that converts DC to AC
• C. A device that measures resistance
• D. A device that increases or decreases the voltage of alternating current

Answer: (D)

Transformers rely on electromagnetic induction to move energy between two coils. When alternating current flows in the primary winding, it creates a changing magnetic field in the core. This changing field links with the secondary winding and induces a voltage there. The ratio of voltages depends on how many turns each winding has: more turns on the secondary give a higher voltage, fewer turns give a lower voltage. In an ideal transformer, voltage and turns are directly related, and power is roughly conserved, so the current changes inversely with the turns.

Crucially, this only works with alternating current because a changing current is needed to keep the magnetic flux varying; direct current would produce a constant flux after the initial moment and produce little to no secondary voltage. So the device’s defining role is to increase or decrease the voltage of alternating current, transferring energy without storing it like a battery or capacitor.