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CBSE Class 12 Pre Board 2019 : Physics - Prelim 2 (St Xavier's Sr. Sec. School, Delhi)

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Pre Board Examination 2019 Std. 12 14-01-2019 Set 2 PHYSICS Max. Marks :70 Time : 3 hrs. INSTRUCTIONS: (i) (ii) (iii) (iv) (v) (vi) (vii) Q. Nos. 1 to 5 carry 1 mark each. Q. Nos. 6 to 12 carry 2 marks each. Q. Nos. 13 to 24 carry 3 marks each. Q. Nos. 25 to 27 carry 5 marks each. Use pencil for the diagrams and graphs. Answers should be to the point. Use log tables if necessary. SECTION A 1. The stopping potential in an experiment on photoelectric effect is 15 V. What is the maximum kinetic energy of the ejected photoelectron? (OR) A beam of electron if formed by accelerating electrons by potential V. Show graphically how the de-Broglie wavelength will vary with potential difference. Justify the shape of the graph. (1) 2. Identify the electromagnetic wave whose wavelength lie in the given range and write its -11 -14 one use10 m< <10 m. (OR) In which direction do the electric and magnetic field vectors oscillate in an electromagnetic wave propagating along x- direction? (1) 3. Write a relation for polarization vector P of a dielectric material in the presence of an external electric field E. 4. A convex lens of focal length f1 is kept in contact with a concave lens of focal length f2. Write the focal length of the combination. 5. Write two advantages of connecting cells in parallel. (1) (1) SECTION B 6. The inputs in the form train of pulses are given at A and B of the following combination of gates. Write the necessary supporting truth table and hence sketch the out wave form. (2) A A Y Y B B 7. A step down transformer converts voltage of 2200V to 220V in the transmission line. Number of turns in the primary coil is 5000. Efficiency of the transformer is 90% and its output power is 8kW. Determine (i) number of turns of the secondary coil (ii) input power (2) 8. Which mode of propagation of waves is suitable for television broadcasting and satellite communication and why? Draw suitable diagram depicting this mode of propagation. (2) Std. 12 9. PHYSICS (Set 2) Page 2 A point object is placed on the principal axis of a denser convex surface and forming a real image. Draw a neat ray diagram to show the path of the light. Hence derive relation between object distance, image distance, radius of curvature and refractive indices of media. (2) 10. Considering the case of a parallel plate capacitor being charged, show how one is required to generalize Ampere s circuital law to include the term due to displacement current. (2) 11. In the potentiometer circuit shown, the null point is at X. State with reason, where the balance point will be shifted when (a) Resistance R is increased, keeping all other parameters unchanged; (b) Resistance S is increased, keeping R constant (OR) Explain using appropiate circuit diagrams how we get two sided deflection in meter bridge experiment ? 12. (2) A 16 wire is bent to form a square loop. A 9V battery having internal resistance of 1 is connected across one of its side. Calculate the potential difference across its diagonals. (OR) Two cells of emf 1.5 V and 2.0 V having internal resistances 0.2 and 0.3 respectively are connected in parallel. Calculate the emf and internal resistance of the equivalent cell.(2) SECTION C 13. A series LCR circuit is connected to an AC source. Using the phasor diagram, derive the expression for the impedance of the circuit. Plot a graph to show the variation of current with the frequency of the source, explaining the nature of variation. (3) 14. Draw a schematic diagram of a cyclotron. Explain clearly the role of crossed electric and magnetic field.Derive the expression for the kinetic energy acquired by the particles. (OR) An particle and a proton are released from the centre of cyclotron and made to accelerate (i) Can both be accelerated at the same cyclotron frequency? Give reason to justify your answer. (ii) When they are accelerated in turn, which these of the two will have higher velocity at the exit slit of the dees. (3) 15. (i) (ii) 16. (i) What must be the minimum length of dipole antenna for a carrier wave of frequency 8 5 x 10 Hz? A message signal of frequency of 10 kHz and peak voltage 10 V is used to modulate a carrier wave of frequency 1 MHz and peak voltage 20 V. Determine a) the modulation index, b) the side bands produced. (3) Derive an expression for energy stored in an inductor of self inductance L when the current through the inductor changes form 0 to I. (ii) Self inductance of an air core inductor increases from 0.01mH to 10mH on introducing an iron core in to it. What is the relative permeability of the iron core? (OR) An ideal capacitor of capacitance C is charged by a source of emf. The charged capacitor is disconnected and then connected across an ideal inductor of self inductance L. Show that charge on the plates of the capacitor changes simple harmonically. Hence obtain the expression for frequency and time period of oscillation. (3) Std. 12 17. (i) (ii) (i) (ii) 18. (i) (ii) PHYSICS (Set 2) Page 3 Draw the ray diagram showing the refraction of the light through the glass prism and hence obtain the relation between the refractive index n of the prism, angle of prism A and angle of minimum deviation Dm. A ray of light passing from air through an equilateral glass prism undergoes minimum deviation when the angle of incidence is of the angle of prism. Calculate the speed of light in prism. (OR) Using the phenomenon of polarization, show how transverse nature of light can be demonstrated. Two polaroids P1 and P2 are placed with their pass axes perpendicular to each other. An unpolarised light of intensity Io is incident on P1.A third polaroidP3is kept in between P1 and P2 such that its pass axis makes an angle of 30 with that of P1. Determine the intensity of light transmitted through P1, P2 and P3. (3) In what way is the diffraction from each slit related to the interference pattern in a double slit experiment? Two wavelengths of sodium light 590nm and 596nm are used, in turn, to study the diffraction pattern taking place at a single slit of aperture 2x 10-4m. The distance between the slit and the screen is 1.5m.Calculate the separation between the positions of the first maxima of the diffraction pattern obtained in the two cases. (3) 19. Three circuits, each consisting of a switch S and two capacitors, are initially charged, as shown in the figure. After the switch has been closed, in which circuit will the charge on the left-hand capacitor (i) increase, (ii) decrease and (iii) remain same ? Give reasons. (3) 20. State Gauss s theorem of electrostatics. Using the same establish a relation for the electric field due to a infinitely large conducting charged sheet of density . Write one characteristics of this electric field. (3) 21. An electron and an alpha particle are accelerated through the same accelerating potential. Which one of the two has (a) Greater de Broglie wavelength (b) less Kinetic energy. (3) 22. In a meter bridge shown in the figure, the balance point is found to be 40 cm fromend A. If a resistance of 10 is connected in series with R,balance pointis obtained 60 cm from A. Calculate the values of R and S. (3) 23. (a) (b) (c) (a) Complete the following nuclear reactions (i) 208 204 84 Po 82 Po + (ii) 32 32 15 P 16 S + Write the basic process involved in nuclei responsible for (i) and(ii) +decay. Why is it found experimentally difficult to detect neutrinos? (3) (OR) In a typical nuclear reaction given below, even though the number of nucleons is conserved yet energy is released. Explain how? 2 2 3 1 1H + 1H 2He + 0n + 3.27 MeV (b) Std. 12 24. Show that all the nucleus has density which is independent of mass number. (1 +1 ) PHYSICS (Set 2) Page 4 Show that total energy of an electron rotating in an orbit is half of its potential energy. Hence establish a relation to show that higher gap between energy orbits are less than gap between lower energy orbits. (3) SECTION D 25. (i) (ii) (i) (ii) 26. (i) (ii) (iii) (i) (ii) (iii) (iv) 27. (i) (ii) (i) (ii) Derive the expression for the torque on a rectangular current carrying loop suspended in uniform magnetic field. Distinguish between diamagnetic substance and ferromagnetic substance in respect of their a) intensity of magnetization b) behavior in a non uniform magnetic field c) susceptibility (OR) Draw a labeled diagram of a moving coil galvanometer. Describe briefly its principle and working. Answer the following (a) Why it is necessary to introduce a cylindrical soft iron core inside the coil of the galvanometer. (b) Increasing the current sensitivity of a galvanometer may not necessarily increase it voltage sensitivity. Explain it by giving reason. (5) Draw a well labeled diagram of a compound microscope and explain its working. Derive an expression for its magnifying power for the final image formed at near point. Why is objective lens of a microscope of short aperture and short focal length? In a compound microscope,an object is placed at a distance of 1.5cm from the objective of focal length 1.25cm.If the eye piece has a focal length of 5cm and the final image is formed at the near point, estimate the magnifying power of the microscope. (OR) Draw a labeled diagram of a refraction type telescope when image is formed at 25cm away from the eye piece. Explain its working and derive an expression for its magnifying power. Give its two limitations over the reflecting type telescope. Why is eye piece of telescope of short focal length? A small telescope has an objective lens of focal length 150cm and an eyepiece of focal length 5cm.If this telescope is used to view a 100m high tower 3km away. Find the height of the final image when it is formed 25cm away from the eye piece. (5) Why is Zener diode considered as special purpose semiconductor diode? Draw I-V characteristics of Zener diode and explain briefly how reverse current suddenly increase at the breakdown voltage? Describe briefly with the help of circuit diagram, how a Zener diode works to obtain a constant DC voltage from the unregulated DC supply. (OR) Draw the circuit diagram of an npn transistor at common emitter configuration to work an amplifier. Write the expression for the respective current gain and voltage gain. In a common emitter amplifier circuit the collector resistance is 5k and input resistance is 1k . If the current amplification is 50 and input voltage is 0.01V, calculate the output voltage. What will be the phase difference between input and output voltage? (5) -X-X-X-X-X-X-

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