question_answer 22)

Answer the following questions: (a) In any ac circuit, is the applied instantaneous voltage equal to the algebraic sum of the instantaneous voltages across the series elements of the circuit? Is the same true for rms voltage? (b) A capacitor is used in the primary circuit of an induction coil. (c) An applied voltage signal consists of a superposition of a dc voltage and an ac voltage of high frequency. The circuit consists of an inductor and a capacitor in series. Show that the dc signal will appear across C and the ac signal across L. (d) A choke coil in series with a lamp is connected to a dc line. The lamp is seen to shine brightly. Insertion of an iron core in the choke causes no change in the lamp's brightness. Predict the corresponding observations if the connection is to an ac line. (e) Why is choke coil needed in the use of fluorescent tubes with ac mains? Why can we not use an ordinary resistor instead of the choke coil?  

Answer:

(a) Yes, the first part is true but it is incorrect for r.m.s. voltage because the potential difference across various components may not be in phase. (b) Sparking etc. is avoided by using the broken current to charge some capacitor. (c) dc signal appears across capacitor because a capacitor blocks d.c. whereas a.c. signal appear across inductance. (d) For steady, d.c. inductance acts as a simple conductor. For a.c. a choke acts as voltage dropper. Brightness of a lamp decreases if a choke is put with it. Inserting of iron core will further reduce the brightness of the lamp. (e) Choke is an inductor which drops voltage without any power loss but power is dissipated in the case of resistor.