Important Questions of AC

                                 "Om Sai Ram"

                       Worksheet of Alternating Current

Q.1) An alternating voltage (in volts) = 100 sin 100t is connected to one microfarad capacitor. The reading of the ammeter shall be :

(a) 100 mA
(b) 20 mA
(c) 1 mA
(d) 10 mA

(Q.2) What doesa large value of Q imply in context with resonance?

(Q.3) What is the dimension for ?

(Q.4) What is the phase difference between the voltage drop across L and C in a series LCR circuit connected to an a.c. source?

(Q.5) Peak value of emf of an a.c source is E0. What is its rms value ?

(Q.6) When a.c is fed to moving coil galvanometer, it shows no deflection. Why?

(Q.7) Is it possible to use capacitor instead of choke coil for reducing a.c.?

(Q.8) How is it possible to improve the quality factor of a series resonance circuit?

(Q.9) In a pure LC circuit, what is the energy stored when peak current is IO?

(Q.10) What is the minimum value of power factor and when does it occur?

(Q.11) Using an AC voltmeter, the potential difference in the electrical line in a house is read to be 230 Volt. If the line frequency is known to be 50 cycles per second, the equation of the line voltage will be:

(a) V = 230 sin (100^π t)
(b) V = 325 sin (100^π t)
(c) V = 440 sin (100^π t)
(d) V = 115 sin (100^π t)

(Q.12) Ratio of the mean value over half cycle to the r.m.s value of an AC is:

(Q.13) A capacitor of capacitance 1µF and a resistor of resistance 10⁵ are connected in series with a battery of emf 100 volt and negligible internal resistance. Time constant of the circuit is (a) 10^–6s
(b) 10^–4s
(c) 10^–1s
(d) 1000 s

(Q.14) A coil of resistance 300 and self inductance 1.5 henry is connected to an a-c source of frequency 100/ hertz. The phase difference between voltage and current is :

(a) 0°
(b) 30°
(c) 45°
(d) 60°

(Q.15) The power factor varies between:

(a) 0 to 0.5
(b) 0.5 to 1
(c) 0 to 1
(d) 1 and 2

(Q.16) A 20 µF capacitor is connected across a 200V, 50 Hz ac supply. The peak current through the circuit is,

(a) 0.6A
(b) 6.28 A
(c) 8.856 A
(d) 0.8866 A

(Q.17) A capacitor in an ideal LC circuit is fully charged by a DC source. Then it is disconnected from DC source, the current in the circuit:

(a) becomes zero instantaneously
(b) grows monotonically
(c) decays monotonically
(d) oscillate infinitely

(Q.18) The tuning circuit of a radio receiver has a resistance of 50 , an inductor of 10 mH and a variable capacitor. A 1 MHz radio wave produces a potential difference of 0.1 mV. The value of the capacitance to produce resonance is (Take ):

(a) 2.5 pF
(b) 5.0 pF
(c) 25 pF
(d) 50 pF

(Q.19) Maximum power is dissipated in an LCR circuit at resonance

(a) through L
(b) through R
(c) through C
(d) through LR

(Q.20) An LC circuit has an inductance 2.81 mH and a capacitance of 9pF. The capacitor is initially charged with a 12 V battery. Total energy stored in the circuit is:
(a) 6.48 × 10^-10 J
(b) 12.96 × 10^-10J
(c) 6.70 × 10^-9 J
(d) 12.48 × 10^-10 J
(Q.21) The sum of the instantaneous current values over one complete cycle is zero, and the average current is zero. This means average power consumed is:

(Q.22) A 12 W resistor and a 0.21 henry inductor are connected in series to an a.c. source operating at 20 volt, 50 cycle. The phase angle between the current and source voltage is: (a) 30º
(b) 40º
(c) 80º
(d) 90º

(Q.23) An LCR series circuit is connected to a source of alternating current. At resonance, the applied voltage and current flowing through the circuit will have a phase difference of

(a) π /2
(b) π
(c) π/4
(d) 0

(Q.24) The maximum current through a pure capacitor of capacitance C is I₀. The maximum emf is E  If the frequency is 50 Hz, the average power dissipated through the capacitor is :

(Q.25) The peak value of house hold line voltage is:
(a) 220 V
(b) 311 V
(c) 1.414 V
(d) 440 V
(Q.26) An instrument is rated 200 W, 484 for a 220 V supply. The peak voltage and the rms current through the instrument is:
(a) 220V, 0.45 A
(b) 220 V, 0.86 A
(c) 311 V, 0.45 A.
(d) 311 V, 0.86 A

(Q.27) The average power dissipation in a pure capacitor in a.c circuit is

(a) 1/2 CV²
(b) CV²
(c) 2 CV²
(d) zero

(Q.28) Iron has been used as the core of the transformer to reduce the loss due to:

(a) Flux leakage
(b) Hysteresis
(c) Eddy currents
(d) Resistance

(Q.29) Through which of the ac circuit elements both the emf and current are in the phase ?

(a) Impedance
(b) Inductive reactance
(c) Capacitive
(d) Resistance

(Q.30) The current in an ac circuit is I = I0 sin and the emf is V = V0 cos . The power dissipation in the circuit is :

(a) zero
(b) I₀V₀ / 2
(c) I₀ V₀
(d) I₀² V

(Q.31) In an RCL circuit, capacitance is changed from C to 2C. For the resonant frequency to remain unchanged, the inductance should be changed from L to :

(a) 4 L
(b) 2L
(c) L/2
(d) L/4

(Q.32) In an a.c. circuit V and I are given by V = 100 sin (100 t) volts I = 100 sin (100 t + /3) mA The power dissipated in the circuit is:
(a) 104watt
(b) 10 watt
(c) 2.5 watt
(d) 5.0 watt
(Q.33) In a circuit L, C and R are connected in series with an alternating voltage source of frequency f. The current leads the voltage by 45°. The value of C is:

(Q.34) An instantaneous current from an a.c. Source is given by I=5 sin314t. What is the rms value of the current?

(a) 2.35 A
(b) 3.4 A
(c) 3.54 A
(d) 5.4 A

(Q.35) In an ideal inductor, L = 4 H and = 100 rad/s. The power developed is

(a) V₀ I₀
(b) V₀ I₀ / 2
(c) 2 V₀ I₀
(d) Zero

(Q.36) Which of the following quantities remain constant in a step-down transformer?

(a) Current
(b) Voltage
(c) Power
(d) Induced e.m.f

(Q.37) Justify the statement that “An inductor acts as a conductor for d.c”.

(Q.38) Why is a capacitor always able to bypass a.c.?

(Q.39) Why cannot we use a.c. for electrolysis purpose?

(Q.40) How can we improve power factor of given circuit, by use of appropriate capacitance in the circuit?

(Q.41) In an a.c circuit, there is no power consumption in an ideal inductor. Explain.

(Q.42) State why A.C. voltage preferred over D.C. voltage?

(Q.43) If the voltage in an A.C. circuit is given by

a) Find peak and rms value of voltage.
b) Frequency of A.C.

(Q.44) An A.C. source of voltage is connected to a resistor of resistance 20 ohm. Find the rms value of the current through the resistor.

(Q.45) Obtain the resonant frequency and Q-factor of a series L-C-R circuit with L = 3H, C = 27μF and R = 7.4 ohm(H)

(Q.46) What can be said about power losses in a purely capacitive and inductive circuits?

 (Q.47) A pure inductor of 25 mH is connected to a source of 220 V. Find the inductive reactance and rms current in the circuit if the frequency of the source is 50 Hz ?

(Q.48) A capacitor, resistor and inductor of 40 mH are connected in series to an a.c. source of frequency 60 Hz. Calculate the capacitance of the capacitor, if current is in phase with the voltage.

(Q.49) Show that the resonance of series LCR circuit driven by a.c. source is given by ?           (Q.50) An a.c. source of frequency 50 Hz is connected to a 50 mH inductor and a bulb. The bulb glows with some brightness. Calculate the capacitance of the capacitor to be connected in series with the circuit so that the bulb glows with maximum brightness.

(Q.51) A 100 μF capacitor in series with a 50 §Ù resistance is connected to a 150 V, 30 Hz supply.

(a) What is the maximum current in the circuit?
(b) What is the phase difference between current and voltage?

(Q.52) 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 rms voltage?

(Q.53) Find the power consumed in a circuit having a resistance of 30 ohm in series with an inductance of 40 ohm in series with an a.c. with peak current of 1 ampere and peak voltage of 220 volt.

(Q.54) A power transmission line feeds input power at 2200 Vto a step down transformer with its primary windings having 2000 turns.What should be the number of turns in the secondary in order to get output power 220 volt?

(Q.55) Define power factor.What is the maximum and minimum value of power factor of an a.c. circuit?

(Q.56) The output voltage of an ideal transformer connected to a 240V a.c. mains is 24V.If this transformer is used to light a bulb with rating 24V, 24W,calculate the current in the primary coil of the circuit.

(Q.57) A 10 μF capacitor is charged to a 25 volt of potential.The battery is then disconnected and pure 100 mH coil is connected across the capacitor so that LC oscillations are set up.Calculate the maximum current in the coil.

(Q.58) State the condition under which the phenomenon of resonance occurs in a series L-C-R circuit.Plot a graph showing variation of current with frequency of a.c. source in a series L-C-R circuit.

(Q.59) A capacitor, a resistor and a 40 mH inductor are connected in series to an a.c. source of frequency 60 hz.Calculate the capacitance of the capacitor, if current is in phase with the voltage.

(Q.60) A coil having a resistance of 7 ohm and an inductance of .6H is connected to an AC source of 110 volts having frequency 20 Hz.Find the rms and peak value of the current in the circuit.

(Q.61) State the reasons for energy losses in transformers.

(Q.62) What is meant by impedance? Using a phasor diagram or otherwise derive the expression for the impedance of an a.c. circuit containing L, C and R in series. Find the expression for resonant frequency.

(Q.63) What is choke coil? Why is it preferred to resistance in a.c. circuit? If the frequency of emf is increased, then what will be the effect on the currents flowing through the circuits given below?

(Q.64) An a.c. circuit containing 800 mH and a 60 μF capacitor is in series with 15 ohm resistance.They are connected to 230 volt, 50 hz a.c. supply.Obtain the average power transferred to each element and the total power absorbed.

(Q.65) An a.c. circuit having an inductor and a resistor in series draws a power of 560 Wfrom an a.c. source marked 210 V,60hz.If the power factor of the circuit is 0.8 ,calculate

(a) the impedance of the circuit.
(b) the inductance of the inductor used.

(Q.66) A 100 mH inductor , a 25 μF capacitor and a 15 ohm resistor are connected in series to a 120 volt,50 hz a.c. source. Calculate the

(a) impedance of the circuit at resonance.
(b) current at resonance.
(c) resonant frequency.

(Q.67) When a series combination of inductance and resistance are connected with a 10V ,50 hz a.c. source, a current of 1 A flows in the circuit.The voltage leads the current by a phase angle of .Calculate the value of resistance and inductance.

Physics Classes By Er.Shyam Pachori add: C.L 81 D.D Nagar                           Mob.: 9039803981 Batches MWF Time: 3:30 - 5 Pm