Which device opposes a change in current?

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

Answer: (A)

Inductors resist changes in current because of self-inductance. When current through an inductor changes, it creates a changing magnetic flux that induces an emf described by e = -L di/dt. This opposing emf, per Lenz’s law, acts to keep the current from changing quickly: if you try to raise the current, the inductor produces a counter emf that fights the rise; if you try to drop the current, it releases energy to help keep it flowing. In DC steady state di/dt is zero, so the opposition disappears, but during transients an inductor is what smooths or resists abrupt current changes.

Capacitors, by contrast, resist changes in voltage (i = C dv/dt), not current itself. Resistors simply follow Ohm’s law without generating a back emf to oppose a change. Transformers rely on changing currents to induce voltages, not primarily to oppose current changes.