Which device allows current to flow in one direction only?

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

Answer: (A)

A diode is a one-way current device. It uses a PN junction that creates a barrier to flow when reverse-biased, so current is blocked in that direction. When the anode is more positive than the cathode, the barrier narrows and carriers cross the junction, allowing current to flow with only a small forward voltage drop (around 0.7 V for silicon). This unidirectional behavior is why diodes are used to rectifiy AC into DC and to protect circuits from reverse polarity.

Capacitors store charge and respond to changes in voltage; they can pass AC but block steady DC once charged, so they don’t enforce a single direction of current. Inductors resist changes in current, not direction, and transformers rely on alternating current to transfer energy between windings. None of these enforce current flow in one direction like a diode does.