CBSE Physics Sample Question Paper : Set III
Q. 16. Suppose a helical spring is suspended from the roof and a very small weight is attached to its lower end. What will happen to the spring when the current is passed through it ? Give reason to support your answer.
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Q. 17. A charged oil drop falls under gravity with terminal velocity v. The drop is held stationary by applying suitable electric field in a Millikan’s set up and is found to carry 2 excess electrons. Suddenly the drop is observed to move upward with the same speed v. Why does the drop move upward ? Write mathematical relation in support of your answer.
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Q. 18. If emitter and base of a NPN transistor have same doping concentration. Explain how will collector and base current will be effected.
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Q. 19. What is Seebeck effect ? Draw a curve to show the variation of thermo e.m.f.- with the gradual increase in the temperature of hot junction. Mark the neutral temperature and temperature of inversion of the thermocouple on the curve.
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Q. 20. Draw the logic symbol and write the truth table of AND gate. With the help of a circuit diagram describe the realisation of AND gate using semiconductor diodes.
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Q. 21. What is meant by the current sensitivity of a moving coil galvanometer ? On what factors does it depend ? In an exercise to increase current sensitivity of a galvanometer by 25% its resistance is also increased 1-5 times. How will voltage sensitivity of the meter be affected ?
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Q. 22. An electron beam of initial energy 18 keV moving horizontally is subjected to a horizontal magnetic field of 0-4 G normal to the initial direction. Calculate the vertical deflection of the beam over a distance of 30 cm.
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Q. 23. When 100 V D.C. is applied across an inductor, a current of 1A flows through it. If the same inductor is connected to a 100 V A.C. source, the current reduces to 0-5A. Why is the current reduced in the later case ? Calculate the value of reactance of the inductor.
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Q, 24. Monochromatic light of wavelength 589 nm is incident from air on a water surface. What are the values of wavelength, frequency and speed of (a) reflected light ; (b) refracted light ? Refractive index of water is 1-33.
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Q. 25. A radioactive isotope decays in the following sequence :
If the mass number and atomic number of A2 are 176 and 71 respectively, find the mass number and atomic number of A1 and A. Which of the three elements are isobars ?
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Q. 26. With the help of a circuit diagram explain the use of NPN transistor as an amplifier in common emitter configuration. What is the phase relationship between the collector and base voltage ?
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Q. 27. Using data given below, state which two of the given lenses will you prefer to construct a best possible (i) telescope, (ii) microscope. Also indicate which of the selected lenses is to be used as an objective and as an eye-piece in each case.
lenses :Power: Aperture (A)
L1: 6D :1 cm
L2: 3D :8 cm
L3: 10 D :1 cm
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Q. 28. State the postulates of Bohr’s atomic model. Derive an expression for the total energy of an electron in the nth orbit.
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Q. 29. A parallel plate capacitor with air between its plates having plate area of 6 x 10-3 m2 and separation between them 3mm is connected to a 100 V supply. Calculate the charge on each plate of the capacitor. Explain what would happen when a 3 mm thick mica sheet of dielectric constant 6 is inserted between the plates (a) while voltage supply remains constant (b) after the supply is disconnected.
Alternative Question
Derive an expression for the energy stored in a capacitor of capacitance C when it is charged by connecting it to a battery of P.D. V. Now the capacitor is disconnected from the batter}’, what will be the .energy stored in the capacitor when (a) separation between the plates is doubled and (b) an uncharged and identical capacitors is connected across it ?
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Q. 30. A small town with a demand of 800 kW of electric power at 220V is situated 15 km away from an electric plant generating power at 440 V. The resistance of the cable carrying power is 15 ohm. The town gets power through a 4000—220 V down transformer at a sub-station in the down.
(a) Calculate the line losses in form of heat.
(b) How much power must plant supply, assuming there is a negligible power loss due to leakage ?
© Calculate the voltage drop in the line.
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