Question

An electric lamp having coil of negligible inductance connected in series with a capacitor and an AC source is glowing with certain brightness. How does the brightness of the lamp change on reducing the (i) capacitance, and (ii) the frequency? Justify your answer.

Answer 1

(i) When capacitance is reduced, capacitive reactance X_c = 1/ωC increases, hence impedance of circuit Z = √(R² + X_c²) increases and so current  I = V/Z decreases. As a result the brightness of the bulb is reduced.

(ii) When frequency in decreases; capacitive reactance Xc = 1/2πnC increases and hence impedance of circuit increases, so current decreases. As a result brightness of bulb is reduced.