In a series circuit containing a lamp and with no capacitor to store charge, what is the initial behavior when the switch is closed?

In a series circuit containing a lamp and with no capacitor to store charge, what is the initial behavior when the switch is closed?

• A. It would be bright at first, then gradually dim and go out
• B. It would immediately go out
• C. It would stay at the same brightness
• D. It would flicker

Answer: (A)

When the switch is closed in a simple DC circuit with a lamp and no energy-storage element, the applied voltage is felt by the lamp essentially instantly. The filament is cold at the moment of switching on, so its resistance is relatively low and a larger inrush current flows. That brief surge makes the lamp appear brighter at the very first instant. As the filament rapidly heats up to its normal operating temperature, its resistance increases, the current drops to its steady value, and the brightness settles. The lamp stays lit as long as the circuit remains closed; it won’t continue to dim gradually to off unless the power is removed. The initial bright flash is real, but the light does not progressively fade away in a steady DC circuit without a capacitor or some other energy storage or power-limiting element.

When the switch is closed in a simple DC circuit with a lamp and no energy-storage element, the applied voltage is felt by the lamp essentially instantly. The filament is cold at the moment of switching on, so its resistance is relatively low and a larger inrush current flows. That brief surge makes the lamp appear brighter at the very first instant. As the filament rapidly heats up to its normal operating temperature, its resistance increases, the current drops to its steady value, and the brightness settles. The lamp stays lit as long as the circuit remains closed; it won’t continue to dim gradually to off unless the power is removed. The initial bright flash is real, but the light does not progressively fade away in a steady DC circuit without a capacitor or some other energy storage or power-limiting element.