Which device opens or closes a circuit?

• A. Capacitor
• B. Switch
• C. Transformer
• D. Transistor

Answer: (B)

Switches are used to make or break a circuit. Their main function is to provide a controllable path for current: when the switch is closed, the circuit is complete and current can flow; when it’s opened, the path is broken and the current stops. This clear on/off control is exactly what defines a switch.

Capacitors store electrical energy and influence current by charging and discharging, but they don’t serve as a manual on/off control for a complete path. Transformers transfer energy between circuits by changing voltage and current levels through induction, not by opening or closing a circuit. Transistors can act as switches or amplifiers, but their primary role is as an active control element within a circuit rather than the simple device whose main job is to open or close the circuit.