CBSE Physics E.M.I. and A.C. Expected Questions : Unit VI
(Long Answer Questions-5 Marks Questions)
1. What is meant by root mean square of effective value of alternating current ? Derive a relationship between it and the peak value. The electric mains in a house are marked 220 V, 50 Hz. Write down the equation for instantaneous voltage.
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2. With the help of a labelled diagram, explain the construction and working of an A.C. generator.
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3. Figure shows a series LCR circuit connected to a variable frequency 220 V source ; L = 40 H; C = 100µF,andR = 40Ω
(i) Calculate the resonance frequency of the circuit.
(if) Obtain the impedance of the circuit and the amplitude.
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4. Derive a relation for electric power in an L-C-R. circuit.
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5. Explain the principle and theory of a transformer. What are the energy loses in transformer ?
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6. Derive an expression for the phase angle of an A.C. circuit with inductor L, a capacitor C and a resistor R in series. Draw the phasor diagram if voltage across the capacitor is greater than that across the inductor. Obtain an expression for resonance frequency of the circuit.
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7. Derive expression for impedance and phase angle of an A.C. circuit with an inductor L, a capacitor C and a resistor R in series. Determine the impedance of the circuit if the reactance of C and L are 340 Ω and 300 Ω respectively and R is 30 Ω.
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8. Distinguish between the terms reactance and impedance for an A.C circuit. A variable frequency 230 V alternating voltage source is connected across a series combination of L = 5.0 H ; C = 80 µ F andR - 40 Ω. Calculate
(i) The angular frequency of the source which derives the circuit in resoance.
(it) amplitude of the current in resonating frequency.
(Hi) sum potential drop across the inductor at resonating frequency.
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9. A student converts a long air core of manganin wire to a 100 V d.c. source and records a current of 1.5 A. When the same coil is connected across 100 V, 50 Hz ax. source, the current reduces to 1-0 A.
(i) Give reason for this observation. (ii) Calculate the value of reactance of the coil.
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1 0. Derive an expression for induced e.m.f. between the ends of a metal conductor moving perpen¬dicularly to a uniform magnetic field.
An a.c. of 1.5 m A rms and angular frequency (o -100 rad s-1 flows through a 10 k Ω resistor and a 0.50µF capacitors in series. Calculate the value of rms voltage across the capacitor and impedance hi the circuit.
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11. What is the difference between reactance and resistance ? An electric lamp running on 80 V d.c. consumes 10 A. The lamp is connected to a 100 V, 50 Hz a.c. source. Calculate the inductance of the choke required.
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12. What is Q-value of series resonant circuit ? What are its uses ?
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13. What is the average power of LCR circuit 7 What is the role of power factor v
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14. What is the role of choke coil in tube lights ? Explain by giving suitable illustration.
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