Answer the following questions :
(a) In any a.c. 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 rins 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 d.c. voltage and an a.c. voltage of high frequency. The circuit consists of an inductor and a capacitor in series. Show that the d.c. signal will appear across C and the a.c. signal across L.
(d) A choke coil in series with a lamp is connected to a d.c. 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 a.c. line.
(e) Why is choke coil needed in the use of fluorescent tubes with a.c. mains ? Why can we not use an ordinary resistor instead of the choke coil ?

Explanation

(a) Yes, the applied instantaneous voltage is equal to the algebraic sum of the instantaneous voltages across the series elements of the given a.c. circuit. The same is not true for rms voltage, because voltages across different elements may not be in phase.
(b) Out of syllabus.
(c) For d.c., impedance of L is negligible and of C very high (infinite), so the d.c., signal appears across C. For high frequency a.c., impedance of L is high and that of C is low. So, the a.c., signal appears across L.
(d) For a steady state d.c., inductance of the choke coil has no effect, even if it is increased by inserting an iron-core. For a.c., the lamp will shine dimly because of additional impedance of the choke. It will dim further when the iron core is inserted which increases the choke.s impedance. br> (e) A fluorescent tube needs less current for discharge (glow) to take place. A choke coil joined with fluorescent tube reduces the current flowing because in an a.c. circuit the choke offers an inductive reactance X1. However, power consumed by choke is negligible because current
and voltage in choke coil differ in phase by `pi/2` . Thus, we save in power.
If we use an ordinary resistor instead of a choke coil with the fluorescent tube, no doubt current in the circuit will fall to the desired level. However, it involves loss of power `pi/2` Thus, the fluorescent tube will consume more power.