Series AC Circuits | ESE, GATE, & SSC

Q. 1. Is the impedance triangle a phasor diagram?

Ans. An impedance triangle is not a phasor diagram. A value of resistance, reactance, or impedance has magnitude but not direction. Therefore, R. X and Z are not phasors but scalars.


Q. 2. Under what conditions a coil might lose power?

Ans. A coil loses power in the following two cases:

  1. If the coil has resistance R.
  2. The coil is coupled to a circuit that has resistance R in it.


Q. 3. The power factor of a circuit is 80% lagging. What does it mean?

Ans. The power factor of a circuit is a measure of its effectiveness in utilizing the apparent power drawn by it. The greater the power factor of a circuit, the greater its ability to utilize the apparent power. Thus 80% p.1. of a circuit means that it will utilize only 80% p. f. of apparent power. Lagging means that circuit current lags behind the applied voltage i.e. the circuit is inductive.


Q. 4. What do you mean by reactive power in an a.c. circuit?


Ans. When L or C is present in an a.c. circuit, energy is required to build up a magnetic field around L or an electrostatic field in C. This energy comes from the source. However, power consumed in L or C is zero because all the power received from the source in a quarter cycle is returned to the source in the next quarter cycle. This power oscillates back and forth and is called reactive power: A reactive power sloes no useful work in the circuit.

Q. 5. What is the origin of the name reactive power?

Ans. This is the power that flows back and forth in the circuit or reacts upon itself. Hence the name reactive power. Reactive power does no useful work in the circuit.


Q. 6. Energy moves back and forth between the source and inductor in an a.c. inductive circuit. How can a passive device like an inductor possibly produce power?

Ans. The answer is that reactive power really represents energy which, like a pendulum, swings back and forth without ever doing any useful work. Thus inductor acts as a temporary energy-storing device, continuously accepting energy for brief periods and releasing it again.

Q. 7. What is the difference between active power and reactive power in an a.c. circuit?

Ans. The main differences between active power and reactive power are :

(i) Active power (= VI cos Φ) is actually used up in the circuit to do useful work e.g. producing torque in motors and supplying heat, light, etc. On the other hand, reactive power (= VI sin Φ) is neither consumed nor does any useful work in the circuit. In fact, reactive power is a liability to the source because the source has to supply the additional current (i.e. I sin Φ) to provide for this power.
(ii) Active power is unidirectional i.e. it flows from source to load only. However, reactive power flows back and forth between the source and reactance (L or C).
(iii) Active and reactive powers cannot be converted into one another. They function independently of each other and consequently, they can be treated as separate, quantities in electric circuits.

Q. 8. Suppose an inductor is connected in series with a lamp across AC mains. What is its effect?

Ans. The lamp will dim. The current in the circuit will be reduced due to the reactance of the coil.

Q. 9. Why are electrical equipment (e.g. generators, transformers) rated in VA or KVA?

Ans. The maximum apparent power to be carried by electrical equipment huff a direct influent on its physical size. Thus as the current increases, we must employ a larger condo as the line voltage V becomes higher, we have to provide greater insulation, thus the size of electrical equipment depends upon product VI, which of course i., the apparent power.


Q. 10. Can you achieve series resonance without changing supply frequency?

Ans. Yes, by changing L and C in such a way that XL = XC.


Q. 11. A variable capacitor, an inductor and a lamps are connected in series across a.c. mains. The variable capacitor is adjusted till the lamp glows normally. Explain.

Ans. When the capacitor is adjusted, XL and Xc, are equal and cancel each other, forming a series resonant circuit. The current is then determined by the resistance at the lamp and full supply voltage appears across the lamp.

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