| Previous Year Questions of March Examinations from 2010 to 2020 |
| Chapter Name: ALTERNATING CURRENT |
| Seema constructed a series LCR circuit in the laboratory as shown in the diagram. She found that the voltages across the inductor and capacitor are equal when the circuit is connected to an ac source. (a) State the condition at which the voltages across L and C become equal. (b) Obtain an expression for the frequency at which this situation occurs in a series LCR circuit. (c) Find the voltmeter and ammeter readings in the circuit |
 |
|
A series LCR AC circuit has great practical importance. It is used for tuning radio, T.V. wireless sets etc. a) Obtain an expression for current in a series LCR AC circuit using phasor diagram. b) Under what condition, this circuit is used for tuning? |
| A) If ‘ω ’ is the angular frequency of a.c., then the reactance offered by inductance ‘L’ and capacitance ‘C’ are respectively, XL =........... and XC = .......... B) An electric bulb and a parallel plate capacitor are connected in series. The bulb glows with some brightness. How will the glow of the bulb be affected on introducing a dielectric slab between the plates of the capacitor? Give reasons in support of your answer. C) Given below are two electric circuits A and B. What is the ratio of power factor of the circuit B to that A? |
 |
|
a) An alternating voltage E = E0 Sin ωt is applied across on LCR circuit. Draw the phasor diagram for the circuit b) Prove that an inductor offers easy path to d.c and a resistive path to a.c c) In the above circuit if L=100mH, C=100μF, R=120 Ω and E=30 Sin(100t) find the i) Impedance ii) Reactance iii) Peak current and iv)Resonant frequency of the circuit |
|
A fascinating behaviour of the series RLC is the phenomenon of resonance a) Explain resonance in an LCR circuit? b) Draw a graphical representation of variation of current amplitude im with ω c) What do you mean by sharpness of resonance? Explain it |
| (a) At resonance, in an LCR circuit, the emf and current are (i) in phase (ii) out of phase (iii) having a phase difference of π/2 (iv) having a phase difference of π/6 (b) In the given circuit, find the value of V |
 |
| (a) In a circuit carrying an ideal coil with negligible resistance, the power dissipated is ........... (b) In the given circuit, find the impedance |
 |
| The phasor diagram of the alternating voltage across an unknown device X and current flowing through it are shown. (a) Identify the device X. (b) Draw the graphical variation of current and voltage with ωt through this device. (c) Draw the phasor diagram which shows the relation among VR, VL ,Vc and I in a series LCR circuit. |
 |
| A series LCR circuit connected to an ac source is shown below: (a) Write an expression for impedance offered by this circuit. (b) Under what condition this circuit is used for tuning radio ? |
 |
|
A transformer is used to change the alternating voltage to a high or low value. (a) What is the principle of a transformer? (b) A power transmission line feeds input power of 2300 V to a stepdown transformer with its primary windings having 4000 turns. What should be the number of turns in the secondary in order to get output power at 230 V? |
|
The current through an AC circuit depends on the magnitude of the applied voltage and impedance of the circuit. (a) Write any two factors on which the impedance of a series LCR circuit depends. (b) Draw and impedance diagram of a series LCR circuit and write the expression for the power factor from the diagram. (c) A sinusoidal voltage of peak value 283 V and frequency 50 Hz is applied to a series LCR circuit in which R = 3 Ω, L = 25.48 mH, and C = 796 μF. Find the impedance of the circuit. |