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Class 12 CBSE Notes 2010 : Physics

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ELECTROMAGNETIC INDUCTION AND ALTERNATING CURRENT 1. A choke coil and a bulb are connected inseries to a d.c source.The bulb shines brightly .How does the brightness change when an iron core is inserted in the choke coil? 2. A vertical magnetic poles falls down through the plane of the magnetic meridian.Will any e.m.f be produced between its ends? Give reason for your answer. 3. Power factor of an ac circuit is 0.5. What will be the phase difference between voltage and current in the circuit? 4. Which device will you use to step up/ step down a.c voltage? 5. The e.m.f of ac source is given by the expression E = 300 sin 314 t volts. Write the values of an peak voltage and frequency of source. 6. Which is the best method of reducing current in an a.c circuit and why? 7. Write expression for the average value of the a.c voltage V = Vo sin t over the time interval t =0 and t = / . 8. An electrical element X, when connected to an alternating voltage source, has the current through it leading the voltage by /2 rad. Identify X and write an expression for its reactance. 9. State Faraday s laws of electromagnetic induction. 10. State Lenz law .Show that Lenz law follows from principle of conservation of energy. 11. Draw the graphs showing the variation of reactance of (a) a capacitor and (b) an inductor with the frequency of an a.c. circuit. 12. Derive an expression for induced e.m.f produced by changing the area of a rectangular coil placed perpendicular to the magnetic field. 13. Derive an expression for the instantaneous value of induced e.m.f in a coil when it is rotated in a uniform magnetic field at a uniform angular velocity .How does the e.m.f vary when the coil rotates through an angle of 2 ? (OR) A 44 mH inductor is connected to 220 V, 50 Hz A.C supply. Determine the r.m.s value of current in the circuit. 14. How are eddy currents produced? Give two applications of eddy currents. 15. Derive expression for the self inductance of a long solenoid of length having N turns. 16. A radio frequency choke is air cored coil whereas an audio frequency choke is iron cored. Give reasons for this difference. 17. Define Mutual induction. Write its S.I unit. Give two factors on which the coefficient of mutual inductance between a pair of coils depends. 18. The magnetic flux through a coil perpendicular to its plane is varying according to the relation = (5 t3 + 4 t2 + 2t -5) weber. Calculate the induced current through the coil at t = 2 s of the resistance of the coil is 5 . 19. Derive an expression for average power consumed in an ac circuit over a complete cycle. 20. Distinguish between reactance and impedance. Sketch the graph showing the variation of impedance of LCR circuit with the frequency of applied voltage. 21. (i) A lamp is connected in series with a capacitor. Predict your observations for dc and ac connections. What happens in each case if the capacitance of the capacitor is reduced? ii) A 15 .0 F capacitor is connected to a 220V, 50HZ source. Find the capacitive reactance. 22. A circuit containing a 80 mH inductor and a 60 F capacitor in series is connected to a 230 V, 50 Hz supply. The resistance of the circuit is negligible. (a) Obtain the current amplitude and rms values. (b) Obtain the rms values of potential drops across each element.(c) What is the average power transferred to the inductor ? 23. Explain the basic principle, description and working of a.c generator with neat labeled diagram. 24. A rectangular coil of N turns and area of cross section A, is held in time varying magnetic field given by B = B0 sin t , with plane of coil normal to the magnetic field .Deduce an expression for the e.m.f induced in the coil. (OR)

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