GCSE Physics Question Analysis Topic: Physics - Electricity
A capacitor 'C', a variable resistor 'R' and a bulb 'B' are connected in series to the ac 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?
(i) The capacitor's capacitance will increase as the dielectric slab is inserted between its plates. As a result, the capacitor's potential drop will decrease (by V = Q/C). As a result of the circuit being configured in series, the potential drop across the bulb will rise. Its brightness will consequently increase.
(ii) The potential drop across the resistor will increase as the resistance R increases. As a result (since both are connected in series), the potential drop across the bulb will decrease. So, it will become less bright.
End of analysis. Great!