Download ac-circuits-test-16

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Name:
Duration: 45 min
CLASS 12 CBSE PHYSICS – TEST – AC circuits
School:
Batch:
Max. Score: 25
1. Show that the average power loss per cycle in a pure capacitor connected to an ac
source is zero. Explain with suitable graphs why the power loss will be zero. (2 + 1)
2. Consider an LCR series circuit connected to an ac source of variable frequency but with
a constant rms voltage. Draw the frequency response curve for the circuit. Indicate in
the graph frequency ranges where the circuit is (a) inductive and (b) capacitive. (2)
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3. Power factor can often be improved by the use of a capacitor of appropriate capacitance
in the circuit.(1)
4. 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? Do rms values obey kirchoff’s laws? (2)
5. 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 lamp’s
brightness. Predict the corresponding observations if the connection is to an ac line. (2)
6. An inductor in a high frequency ac circuit nearly amounts to an open circuit. A capacitor
in a high frequency ac circuit nearly amounts to a ‘short circuit’. Explain. (2)
7. An electric lamp in series with a variable capacitor and an a.c. source is glowing with some
brightness. How will the brightness change on increasing the capacitance? Why? (2)
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8. An air core coil and an electric lamp are in series across an a.c. source. How will the
brightness charge if an iron bar is introduced into the coil? (2)
9. When an alternating voltage of 200 V - 50 Hz is applied across a device X, a current of 2 A
flows through the circuit and is in phase with the applied voltage. When the same voltage is
applied across another device Y, the same current flows through the circuit but it leads the
applied voltage by /2 radian. Identify the elements X and Y. If X and Y art put in series
(a) Determine the maximum current in the circuit.
(b) Determine is the time lag between current maximum and voltage maximum
(c) Determine the power consumed per cycle.
(3 points)
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10. An LCR series circuit with 200  resistance is connected to an a.c. source of 100 V and
angular frequency 200 rads-1. When only the capacitance is removed, the current lags
behind the voltage by 600. If the inductor alone is removed the emf leads current by a phase
60o. Calculate the current and the power dissipated in the LCR circuit.
(3 points)
11. Obtain the resonant frequency and Q-factor of a series LCR circuit with L = 3 H,
C = 27 μF, and R = 7.4 Ω. It is desired to improve the sharpness of the resonance of the
circuit by reducing its ‘full width at half maximum’ by a factor of 2. Suggest a suitable
way. Define Q-factor of an ac circuit. (3 points)