Test B - Alternating Current (AC) Assignment - Class 12

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1. The electric mains in a house are marked 220 V , 50 Hz. Write down the equation for instantaneous voltage.
[1]
2. Sketch a graph showing the variation of reactance of a capacitor with frequency of the applied voltage.
[1]
3. What is the phase difference between the voltage across an inductor and a capacitor in an a.c.
circuit ? [1]
4. In a series LCR circuit. What is the value of power factor at resonance ? [1]
5. An electric lamp connected in series with a variable capacitor and an a.c. source is glowing with some
brightness. How will the brightness change on increasing the value of capacitance and
why ? [2]
6. An inductor L of reactance X
L
 is connected in series with a bulb B to an a.c. source shown in figure. Briefly
explain how does the brightness of the bulb change, when
~
L
B
(a) number of turns of the inductor is reduced and
(b) a capacitor of reactance X
C
 = X
L
 is included in series in the same circuit. [2]
7. Define resonance in LCR series circuit. State the condition for it. Draw resonance curve. [2]
8. Find the time required for a 50 Hz alternating current to change its value from zero to the
r.m.s. value. [2]
9. A 100 ohm iron in connected to a 220 volt. 50 cycles A.C. What is its (i) peak potential difference
(ii) average potential difference and (iii) r.m.s. current ? [2]
10. An alternating current of 1.5 mA and angular frequency 300 rad s
–1
 flows through 10 k ? resistor and a
0.5 µF capacitor in series. Find the r.m.s. voltage across the capacitor and impedence of the
circuit. [2]
11. Derive expression for impedance and phase diff. in LCR series circuit by qualitative method
[Let X
C
 > X
L
]   [3]
12. A coil has a resistance of 10 ? and inductance 0.4 H. It is connected to an AC source of 6.5 V and
30/ ? Hz. Find the average power consumed by the circuit. [3]
13. A variable frequency 230 V alternating voltage source is connected across a series combination
 L = 50 H, C = 80 µF and R = 40 ?. Calculate (a) the angular frequency of the source which drives the
circuit in resonance, (b) the amplitude of the current at resonant frequency and (c) r.m.s. potential drop
across the inductor at resonating frquency. [3]
ALTERNATING CURRENT T E S T - B
X I I
M.M. M.O.
25
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