THE FIRST COMPARATIVE EXAMINATION 2019-20 Class XII (ISC) PHYSICS Paper - I (Theory) Time: Three hours Maximum marks: 70 Instructions: * Answers to this paper must be written on the answer sheet provided separately. * All working, including rough work, should be done on the same sheet and adjacent to the rest of the answer. * You are NOT allowed to write during the first 15 minutes, This time is to be spent in reading the question paper. Please do not write anything on the question paper except your name and roll number. * Answers to sub parts of the same question must be given in one place only. A list of useful physical constants is given at the end of this paper. The intended marks for questions or parts of questions are given in brackets [ A simple scientific calculator without a programmable memory may be used for calculations. * All questions are compulsory. This question paper is divided in 4 Sections A, B, C and D as follows: Section A: Question number 1 is of twelve marks. All parts of this question are compulsory. Section B: Question numbers 2 to 12 carry 2 marks each with two questions having internal choice. Section C: Question numbers 13 to 19 carry 3 marks each with two questions. having internal choice. ‘Section D: Question numbers 20 to 22 are long-answer type questions and carry 5 marks each, Each question has an internal choice. Section A Answer all questions. Question 1 "A. Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below: [5x1] () A body has a positive charge of 8 x 10°12. It has: (a) anexcess of 5 electrons a deficiency of 5 electrons (©) anexcess of 8 electrons “(d)_a deficiency of 8 electrons. (i) The colour sequence in a carbon resistor is red, brown, orange and silver. The resistance of the resistor is: 21 x 10° + 10% (b) 23 x 10° 2+ 10% (©). 21x10 225% @ 12x10 245% (iii) A charged particle enters into a magnetic field normally. Which of the following would be its trajectory? (a) Parabolic (6) Straight line oy Ceuta (@_ Helical Y-13 ‘This paper consists of cight printed pages. Turn over 0(iv) Awire of resistance R is cut into n equal parts. These parts are then connected in parallel with each other. Then equivalent resistance of the combination is: (@) Rn (&) Rn (© wR ay Rint (v) The variation of magnetic field B due to a long straight current carrying wire with distance x from the wire is represented by the curves given below. Select the correct curve: @ B ae : 7. : f- oa = B. Answer all questions given below briefly and to the point: [7x1] @ State Ampere’s circuital law. FrK 4G. Gi) How is force between two point charges affected if charges are doubled in ‘magnitude and separation between them is halved? \ 6 Gil) A point charge Q is revolved in a circle of radius R around another point charge q. What will be the work done in half the revolution if q is at the centre of the circle? WO (iv) Define capacitance of a conductor. (¥) “Dielectric constant of water is 81.” What do you understand by this statement? Fora oh Becbric Rd ss Wp ae compacel te me (vi) Write one property of a Gaussian surface. (vii) What is the orientation of electric dipole in electric field for unstable equilibrium? O= \®* ao a _ Yo @)Section B Loncvg- Answer all questions. Question 2 Write the relation between drift velocity and current. Name all physical quantities in the relation. LP) Question 3 (a) Calculate the equivalent capacitance between the points A and B in the combination shown in the figure: xe = Qu 2uF] pF uF are a es ea OR (0) Three capacitors C), C2 and Cs are connected to a battery of 100 V. Find the charge on the capacitors C and Cs. 2) tah 12pF ee ys Q=2 = Zoxino = es nile a c 2 — att Zs eye = Ape O41 = Axi00 sUespe SFA Question 4 (What do you mean by quantization of charge? (What is the net charge on a body which is charged by induction? =o [+1] Question 5 The two charges 5 wC and 3 uC are placed 1 metre apart. At what point the net electric field on the line joining them is zero? 23 2] ee Y - z a Question 6 = XS" ea Draw the graph between temperature and resistance for conductors and +0 ee semiconductors. Ray cenicewlaler, &, ue Pl Ek i Sars SoF+8+6 Question 7 e € nes, A set of identical resistors, each of resistance R ohm, when connected in series, has an effective resistance X ohm and when the resistors are connected in parallel, their are effective resistance is Y ohm. Find the relation between R, X and Y. 2) Ros AY Y-13 @ ‘ay (3) ‘Turn over Ka wR = YeRle xen) re CNS, RQRXL Ke Gee 3 SwQuestion 8 _{@) Slate Biot-Savart’s law. Write the veetor form of it OR _-@y Find magnetic field at the centre ‘O’ of the given semicircle: 2A Semicircle 3 Question 9 (i) State the condition of balanced wheatstone's bridge. P/a,* /S ° ¥ (i) How can the sensitivity of Potentiometer be increased? Aecnmigy “ ‘PI Question 10 Two point charges + 10 uCare placed 5 mm apart, forming an electric dipole. = “go ttd Compute electric field at a point on the axis of the dipole 15 em away from the centre of the dipole. 2 Ss, 2] Es wot 2kSoxwe D a xi Ae Question 11 uct 3 if E=61+3}+4k, calculate the electric flux through a space of area 20 square units ae in Y-Z plane. Ke 20 Question 12 = ER= josnwe Define conductivity. Write the vector form of Ohm’s law. 2 Section C Answer all questions. Question 13 { Deri an expression for Electric Potential at a point on the axial line ofa dipole. Gl Question 14 ¥ i y 7 KeA jerive an expression for the capacitance of a parallel plate capacitor C y . symbols have usual meaning. BI : bis Y-13 «) xc RG BatsQuestion 15 @) Bye poe ® am KS > 6, - we . Seco () The given figure shows a cell of emf E and internal resistance r, connected to a voltmeter V and a variable resistance R. Deduce the relationship between V, E, Questi Find the magnetic field at point P as shown in figure: OR Rand r. How will the value of V vary when R is reduced? rion 16 “ind the equivalent capacitance between the points A and B in the following figure: og 2nF () 2uF Ae {+ 4}+-—+B ‘4uF 4pF ab { F OR Br nap a ES) x, he 4pF i i a ae Rue (Sketch the electric field lines for two charges +q and—q separated by aye MW distance d. Gi) Why do two equipotential surfaces not intersect each other? on lS (5) Turn over. 7 by > aan gh wot oie Ah ise Ampere’s circuital law to derive an expression for magnetic field due to an eo ; i sat infinitely long straight current carrying conductor B= Pol: symbols have usual ” oN Pane he wee? reso er, ey 8] . i Pa eee ar P vs Helium atom completes one round of circle of radius 0.8 m in 2 seconds. Find the Ww magnetit field at the centre of the circle. (3) 4 = 0-00 A Question 19 A voltmeter reads 5 volt at full scale deflection and is graded according to its resistance per volt at full scale deflection as 5000 /V. How will you convert it into, valine +iy that reads 20 volt at full scale deflection? Is the voltmeter graded as 5000 2/ V better L than the one graded as 2000 2./ V? B) a ——— sg = wars AP ¥ PGa+R see lovoos = gscer-Q Section D ve Gr ee X Answer all questions. Question 20 A = 1S000 —— (@) @. Anelectrical technician requires a capacitance of 2 uF in a circuit across a ie & potential difference of 1 kV. A large number of 1 uF capacitors are S available to him, each of which can withstand a potential difference of not Ces = more than 400 V. Suggest a possible arrangement that requires the Qa minimum number of capacitors. We agacirte, A 2 (ii) What will happen to charge, potential difference and capacitance of a oy capacitor when a dielectric is inserted in the capacitor after the removal of oR battery? Q RLWALUD AOmes NV bevy, » ae oR © Peowes Ke ee (b) Four charges are arranged as +2uC shown in figure. What should be the magnitude and sign of the charge to be placed at the centre O so that the system remains in equilibrium? Assume that charges are free and side of the given square is a cm. +2yC + 2uC 6) ale Gane [ra mI Y¥-13 (6) Q anz Litav2d, ai ss @ a-d Coaw > eaten - 8 (a) (Inthe circuit given below, find the currents ly, In, Is. BI 1 ro) 1 we + =3v 25, +7 +L) oe 40 ' S oe 65, +4, 22 29 29 Sr, + ata te . fe fe € = f \QTn FHT AHS © Y Gi) Why is potentiometer preferred over voltméter in measuring the emfofa 67, +4s,¥2 f cell? 2] oY 6, OR NG Ta tenes © Sh) 4) Draw a circuit diagram of potentiometer to determine internal resistance =e Ss es ofa cell. Derive the formula used for internal resistance of a cell. - Gy a a a” » Gi) Inthe circuit given below, the length of wire AB = 400 em, Where should ae et the jockey ‘J be placed so that the galvanometer shows zero deflection? [2] 32 Ra eye eiee ol | 2 a) \ 8 Beek? ata B Aan tered: Gy ea oe Bop —2R + BL es ~~ ' 5 Bog 5 Se Question 22 A= 8 eiegou : in ae e ie @O_ Use Gauss’ theorem to derive E =: Bias electric field due to an TES, infinitely long line of charge having a uniform linear charge density 2 at a . ly Sich distance r from it. Bl (ii) A point charge 8.85 pC is placed at the centre of a hemisphere as shown, below. Find electric flux passing through the surface of hemisphere. Qi gf => * =] co Me g = 88S Ai 2 KEBENO™ = lot = poe ma ks Y-1B a Turnover = SXW@ c -(6) Derive an expression of electric potential due to a point charge. B] (ii) Three charges of 2 41C, 4 wC and ~6 j1C are kept at vertices of an equilateral triangle of side 40 cm. Calculate the potential energy of the system of charges. 2) ae tea] Nite oe Useful Constants and Relatior 7 a Aue @_[ Charge of an electron (¢) =1.6x10"C bcp: Meme i) _ | Permittivity of free space (€,) 8.85 «10°? CN mm? (iii) Permeability of free space (10) 4x x 10” NA~ (iv) | Mass ofa proton (m,) 1.65 x 10°" kg (¥)__| Acceleration due to gravity (g) 9.8 Nkg™ (vi) | Mass of an o-particle 6.88 x 107” kg (vii)_[ Charge ofa proton +1.6x 10°C (witt) | 1 8 Ne? Cr? aaa 9x 10? NaC (ix)_| Speed of light in air (c) 3.0 x 10°ms™ (x) __| Mass ofan electron 9.0 x 107" kg 12. = ARK tee AB =e Tdb re tox” Tee ran! ile e "wie ap <* eee o 7 = ot A xio\e ES eos at Yo x16) 3 L oe we Bs, a RN ok KG oe ir Ue Pe \ Ae he Ax \ a es x0" Xo =~ 638xX0 FT ye \ \ = 635. Ai. Whey, ¥-13 (sy