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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 lamps 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?

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Solution

(a)
Yes, 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 because instantaneous voltages are in same phase.
The same is not true for rms voltage, because voltages across different elements may not be in phase.

(b)

When circuit is broken, a high Voltage is induced, which may cause spark. If capacitor is used, the high voltage will be used to charge the capacitor, thus avoiding sparks and ensuring safety.

(c)

For dc, impedance of L is negligible and of C very high (infinite),so the dc signal appears across C. For high frequency ac,impedance of L is high and that of C is low. So, the ac signal appears across L.

(d)

If an iron core is inserted in the choke coil(which is in series with lamp connected with ac line), then lamp will glow dimly. This is because the choke coil and the iron core increases the impedance of the circuit.

(e)

A choke coil reduces voltage across the tube without wasting power, as average power consumed in inductive circuit is zero. A resistor would waste power as heat, which will lead to loss of energy.


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