A coil of self-inductance L is connected in series with a bulb and an a.c. source. Brightness of the bulb decreases when

Step-by-step solution

The brightness of the bulb depends on the power dissipated, given by P = I^2 R , so it decreases when the RMS current I decreases. In an AC series circuit with the bulb (resistance R ) and coil (inductive reactance X_L ), the impedance is Z = R^2 + X_L^2 and the current is I = V/Z . A decrease in current requires an increase in impedance, which occurs when X_L increases since X_L = 2 f L . Inserting an iron rod into the coil increases the magnetic permeability, raising the inductance L . This increases X_L , thus i