In a simple circuit where a switch controls a bulb, what happens when the switch is closed?

• A. Bulb B will light
• B. Both bulbs light
• C. Neither lights
• D. Bulb A will light

Answer: (D)

Closing the switch completes the electrical loop and lets current flow through the part of the circuit that the switch controls. In this setup, Bulb A is in the path that becomes energized when the switch is closed, so current passes through it and it lights. Bulb B isn’t in that energized path, so no current flows through it and it stays off. If the switch is opened, the circuit is broken and the bulb in that path would go dark. The key idea is that a closed switch creates a continuous path for current to flow through the connected bulb.