Question: Q. 3. A capacitor ’ $C$ ‘, a variable resistor ’ $R$ ’ and a bulb ’ $B$ ’ are connected in series to the $a c$ mains in circuit as shown. The bulb glows with some brightness. How will the glow of the bulb change if (i) a dielectric slab is introduced between the plates of the capacitor, keeping resistance $R$ to be the same; (ii) the resistance $R$ is increased keeping the same capacitance?
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Solution:
Ans. (i) When a dielectric slab is introduced in between the plates of capacitor, its capacitance will increase which tends to decrease the potential drop across the capacitor, i.e., $V=\frac{Q}{C}$. As a result of this, potential drop across bulb will increase as they are connected in series, so the brightness of the bulb will increase.
(ii) As the resistance $R$ increases, potential drop across the resistor will also increase which lead to decrease in potential drop across the bulb, since it is connected in series. Hence, the brightness of the bulb will decrease.
