Question

The resistance of an incandescent lamp is

• A. greater when switched off
• B. smaller when switched off
• C. greater when switched on
• D. the same whether it is switched off or switched on.

Answer 1

Answer: (C)

$R\propto \frac{1}{\tau }$, where $\tau =$
Relaxation time. When lamp is switched on, temperature of filament increase.
hence $\tau$ decrease so R increase.