Question

A capacitor, an inductor and an electric bulb are connected in series to an AC supply of variable frequency. As the frequency of the supply is increased gradually, then the electric bulb is found to

• A. increase in brightness
• B. decrease in brightness
• C. increase, reach a maximum and then decrease in brightness
• D. show no change in brightness

Answer 1

Answer: (C)

increase, reach a maximum and then decrease in brightness

$I=\frac{V}{\sqrt{R^2+{(\omega L-\frac{1}{\omega C})}^2}}$

it is clear from te expression that as $\omega$ increases, ${(\omega L-\frac{1}{\omega C})}^2$ decreases first then it starts increasing.

So the brightness increases first and then decreases.