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Pre Council Examination - 2019 Class XII Paper- I Physics (Theory) Time:-3hrs AL-70 M.M.70 Candidate are allowed additional 15 minutesfor only reading the paper They must not start writing during the time. SECTION - A Question 1. (A) Choose the correct al~ernative (a), (b), (c) or (d) for each of the questions given below : (i) The air between the plates of a capacitor is replaced by a medium of permittivity K. The potential difference between the plates : (a) increases K times (b) decreases K times (c) increas~s .JK times (d) decreases .JK times (ii) The time-constant of a circuit having a capacitor C and a resistor R in series is : (a) (c) (iii) CR (b) C R (d) ./CR R C A ligh~ raf passes from vacuum into a medium of refractive index n. The angle of refraction IS found to be half of the angle of incidence. The angle of incidence is : (a) cos- 1 (b) (~) 2cos-1 ( ~ ) 2 (c) ,_M__ 2 - ~ ' ~}~J._, ----- 2 2sin-1 (n) (d) Jt 2Y1-J {J " 1~ l ~ (iv) Dimensions of Planck's constant are same as of : . (a) force x time (c) energy/time (v) As a result of radioactive decay, emitted during lhis decay are : 92 U 238 (b) energy x distance (d) energy x time nucleus is converted into rj~ 91 c/ ~ fx Pa 234 Th particles (a) a proton and a neutron (b) an a-particle and a J3-particle (c) two fl-particles and a neutron (d) two f3-particles and a proton (B) Answer the following questions briefly and to the point : (i) \ (ii) In an a.c: circ1:it, the voltage of supply is 300 V. What is rms voltage ? How does an induct<;>r behave in a d.c. circuit? field B and velocity_ vector c are mutually perpendicular to each other, for a linearly polarized ray of light. Identify (name) three different planes, which are mutually perpendicular to each other. (iv) When thin transparent film is placed just infront of one of the slits in the Young's double slit experiment using white light, what change results in frin ge system? (v) Give two differences between diffraction fringes formed by a single slit and interference fringes formed by Young's double slit. (vi) A lens whose radii of curvature are different is forming the image of an object placed on its~s. If the lens is reversed, will the position of the imagE> change? (vii) What will be the nature of graph between de-Broglie wavelength and momentum of a moving particle ? (iii) At any instant, the electric field E, magnetic a (1) +2 Paper - I Physics (Theory) (1-4) SECTION B Question 2. Deduce an expression for the common potential when two capacitors of capacitance C1 and C2 and at potential V 1 and V 2 respectively are connected by a conducting wire. Question 3. Light corresponding to the transition n = 4 to n = 2 in hydrogen atom falls on cesium metal (work function = 1.9 eV) . Calculate the maximum kinetic energy of the photoelectrons emitted. Question 4. ~at is the phase relationship between current and voltage in (i) lnductor, (ii) Cap acitor m an A .C . circuit. Question 5. (a) What is meant by current gain of a transistor ? What is its approximate value for common base amplifier ? OR {b) A ~transistor is a temperature sensitive device. Explain. Question 6. What are the important conclusions that can be drawn from Einsteins' photoelectric equation, hv - W = !mv 2 2 ? . . cJf "~ I \ ~ ' /(g).estion 7 . (_ ~ -::::;- ~ ... ~ -. Calc':1lat~ the heat generated when a condensor... of ,25~ <:~pacity is charged to 1 ~., a1:d 1s d ischarged through 9.'.1 ohm resistanct!. ~ ______,. _J\ ~ ---- - , _~ u eshoh S. -~~ -1 >('1.--~ _jJJ Wh::..t a re acceptor energy levels ? Explain. ~ fl :::.. - ,. i,.;, ~ , / Question 9 . ~ 0 (a) What is meant by doping ? H ow is it done ? What is the effect of temperaturharia on on the resistiv ity of pure semiconductor~~ ~ . ff t ), _ C Explain briefly why a junction diode acts as a rectifie _ , , ,,.,- ~ f Q u e stion 10. ~ " S&:J The power of a thin convex lens of ~ ~hen 1t 1s immersed in a liquid, then it ~e~ves like _a divergent lens of foca~ le th 1~0 cm. c,icul~tee refractive index of the hqwd_. The refrac ve ~ ex of glass 1 1.5 ; ,,,, /;' -:: - () ~ Question 11. r/ ~ ,\.,,~~ A parallel beam of light of wavelength 6 x 10-5 cm falls normally on a stra1g t slit of width 0.02 mm. Calculate the total angular width of central diffraction maximum and -also linear width as observed on a screen 2 m away. ~ Question 12. v_:--.....__ , Why do transmitters broadcasting programmes use different carrier frequencies? SECTION-C Question 13. (b) c. \ r Obtain the relation RR 1 = ~ 2 3 ,-E -.. for a Wheats.t one's bridge. The symbols have their standard 4 meaning. (2) +2 Paper - I Physics (Theoi:y) (1-4) fl) Question 14. (a) Fin~ t_h e magi:ietic field at the point pin the figure given below. The curved portion is a semicircle and the straight wires are long. 1 ,_ _P---~---1 OR 1 (b) In the figure given below, a long, straight current carrying wire AB is fixed, while the rectangular loop carrying current is free to move. In which direction would the loop . move ? Explain. B A Question 15. A gt'lm ophone disc of brass of diameter 30 cm rotates horizontally at the rate of 100/3 rev on:tions per minute. If the vertical component of earth's magnetic field is 0.01 Wbm-2 , then calculate the e.m.f. induced between the centre and the rim of the disc. ~stion 16. W In Young's double slit experiment the slits are 0.589 mm apart and the interference isth ob_serve~ on a_ screen ~laced at a di~ce of 100 cm from the slits. _It i~ found that the 9 br~ght frmg~ is_ ~t a d1~ce ~;__~ frpro ..the-'!tf:k~ which 1s se~d from the centre of the fringe pattern. Fmd tne wavelength the hght used. '9T = of light is incident on ~f:c"!;of a 60 ~rism. By rotating tl:le prism, the minimum angle of deviation is measured to bel9..0 Find the refractive index o f the prism . material. If the prislft la -placed in water (relractiv~ indexM, then wtiat would l> the new minimum angli!Ohl~iation? . .",VV\( ~ (b) A ;!?allel 7 Question 17. \ /{ 3 ~~ The interference fringes for sodium light (A = 5890 A) a acmble slit experiment have an a~gular width of 200. For what wavelength will this width be 10% greater? in luestion 18. i) Calculate the number of photons that are emitted from the green line of mercury (A.= 4961 x 10-10 m) to do one joule of work. .i) Which is more energetic : a.-partide, X-ray , fl-particle or y-rays ? (3) +2 Paper - 1Physics (Theory) (1-4) Question 19. Obtain a formula for the fr~guency_of_radiation emitted when a hy_drogen atom de-exc~t~s from level n to level (n --=--1.L_ For largen ,- show that this frequency equals the slas9ical frequency ofrevolutio~e electron 1n 'the orbit. SECTION-D Question 20. (a) Obtain an expression for the magnetic flux density fl at the centre of a circular coil of radius R having N turns when a current i flows through it. OR (b) With the help of a neatly labelled diagram, prove that the magnitude of motional e.m.f. is ~ve1: ~ye = Blv, where ' l' is the length of the metallic rod and 'v' is the velocity with which ~t 1s pulled in a transverse magnetic field 'B'. Question 21. (a) Distinguish between intrinsic and extrinsic condition in semiconductors. Explain the term donor impurity and acceptor impurity. OR {b) In fig. rhe circuit S)'D:\bol of a logic gate and two input waveforms f I A and B are shown ,----. v s---- a--- \\ Name the logic gate. A~ (ii) Write its truth table. (iii) Give the output waveform. o a o 0 Question 22. (a) (i) A n arrow slit of wid th 0.3 is illumin te normally by a parallel beam of light oi wavelength 6 x 10-7 m. Fraunhoffer diffraction pattern is obtained on a screen placed in the focal plane of a convex lens of focal length 25 cm. The lens is placed quire close to the slit. Find the width of central maxima on the screen. (ii) Draw a ray diagram to show how the image of a distant object is formed by ; reflecting astronomical telescope when in normal use. State one advantage over i refracting type telescope. (i) '' OR On the basis of Huygen's wave theory, prove Snell' s law of refracti?n of light. Dra..,. neat and labelled diagram. (ii) The absolute refractive indices of water and glycerin are 1.33 and-~1.A6 respective}~ Find the critjcal angle for a ray travelling from glycerin .into water. (b) (i) (4) +2 Paper -1 Physics (Theory) (1-4) i
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