ALTERNATING CURRENT
BY Er AMIT BHARDWAJ
1-The instantaneous current and voltage of an a.c. circuit are given by i = 10 sin 314 tA and v =
50 sin (314t+π2)V
2--Define the term ‘wattless current’.
3-Mention the two characteristic properties of the material suitable for making core of a
transformer.
4-When an ac source is connected across an ideal inductor, show on a graph the nature of
variation of the voltage and the current over one complete cycle. (Comptt. Delhi 2012) Answer:
5-A heating element is marked 210 V, 630 W. Find the resistance of the element when
connected to a 210 V dc source.
6-Why is the use of a.c. voltage preferred over d.c. voltage? Give two reasons. (All India
2013)
7-A variable frequency AC source is connected to a capacitor. Will the displacement current
change if the frequency of the AC source is decreased? (Comptt. All India 2015) Answer:
8-Plot a graph showing variation of capacitive reactance with the change in the frequency of
the AC source. (Comptt. All India 2015) Answer:
Graph showing a variation of xc capacitive reactance with the change in frequency of AC source.
9-Define ‘quality factor’ of resonance in series LCR circuit. What is its SI unit? (Delhi 2016)
Answer:
Quality factor (Q) is defined as, Q = ω0LR
It gives the sharpness of the resonance circuit. It has no SI unit.
10-Prove that an ideal capacitor in an a.c. circuit does not dissipate power.
11-The circuit arrangement as shown in the diagram shows that when an a.c. passes
through the coil A, the current starts flowing in the coil B.
(i) State the underlying principle involved.
(ii)Mention two factors on which the current produced in the coil B depends.
12-An electric lamp having coil of negligible inductance connected in series with a capacitor and
an a.c. source is glowing with certain brightness. How does the brightness of the lamp change
on reducing the
(i) capacitance, and
(ii) the frequency? Justify your Answer. (Delhi 2009)
Answer:
13-capacitor ‘C’, a variable resistor ‘R’ and a bulb ‘B’ are connected in series to the ac
mains in a circuit as shown. The bulb glows with some brightness. How will the glow of
the bulb change if
(i) a dielectric slab is introduced between the plates of the capacitor, keeping
resistance R to be the same;
(ii) the resistance R is increased keeping the same capacitance? (Delhi 2012
14- The figure shows a series LCR circuit connected to a variable frequency 200 V
source with L = 50 mH, C = 80 µF and R = 40 Ω.
Determine
(i) the source frequency which derives the circuit in resonance; (ii)
the quality factor (Q) of the circuit. (Comptt. All India 2014)
15-An inductor of unknown value, a capacitor of 100 μF and a resistor of 10 Ω are
connected in series to a 200 V. 50 Hz a.c. source. It is found that the power factor of the
circuit is unity. Calculate the inductance of the inductor and the current amplitude.
