A light bulb and an open coil inductor are connected to an ac source through a key as shown in Fig. The switch is closed and after sometime, an iron rod is inserted in to the interior of the inductor. The glow of the light bulb (a) increases (b) decreases (c) is unchanged as the iron is inserted. Give your answer with reasons. What will be your answer if ac source is replaced by dc source ?

Explanation

As the iron rod is inserted, the magnetic field inside the coil magnetizes the iron increasing the magnetic field inside it. Hence, the inductance of the coil increases. Consequently, the inductive reactance of the coil increases. As a result, a larger fraction of the applied ac voltage appears across the inductor, leaving less voltage across the bulb. Therefore, the glow of the light bulb decreases.