Question

A capacitor and an inductance coil are connected in separate AC circuits with a bulb glowing in both the circuits. The bulb glows more brightly when

• A. separation between the plates of the capacitor is increased.
• B. a dielectric is introduced into the gap between the plates of the capacitor.
• C. an iron rod is introduced into the inductance coil.
• D. the number of turns in the inductance coil is increased.

Answer 1

The correct option is B a dielectric is introduced into the gap between the plates of the capacitor.

Let resistance of bulb is = $r$

Effective resistance of circuit is -

With capacitor $R_{eff}=\sqrt{r^2+(\frac{1}{wC}{)}^2}$

With inductor $R_{eff}=\sqrt{r^2+{\left(wL\right)}^2}$

Now if we increase separation between the plates of the capacitor, capacitance will decrease so $R_{eff}$ will increase which will decrease the current. Hence the brightness of bulb will decrease as the brightness of bulb is directly proportional to the current passing through it.

A dielectric will increase capacitance, so $R_{eff}$ will increase, so current will increase and thus brightness of bulb will increase.

Both options C and D will increase inductance of coil making $R_{eff}$ higher, that will reduce the current and makes the bulb less bright.