Which principle underpins the operation of relays, solenoids, and transformers?

• A. Friction
• B. Electric resistance
• C. Magnetism
• D. Capacitance

Answer: (C)

Magnetism is the principle at work. When current flows through a coil, it creates a magnetic field. In a solenoid, that magnetic field exerts force on a movable core or plunger, turning electrical energy into motion. In a relay, the same magnetic pull operates to move contacts and switch circuits. For a transformer, the changing magnetic field in one winding links to another winding through a magnetic core, inducing voltage and transferring energy without direct electrical contact. Friction, resistance, and capacitance may influence performance, losses, or storage, but they don’t explain the fundamental way these devices operate. The driving idea is magnetic fields and electromagnetic induction.