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CBSE XII Sample / Mock 2015 : PHYSICS with Answers

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Sample Question Paper Class XII Physics (Applicable for March 2015 Examination) Time Allowed: 3 Hours Maximum Marks: 70 General Instructions 1. All questions are compulsory. There are 26 questions in all. 2. This question paper has five sections: Section A, Section B, Section C, Section D and Section E. 3. Section A contains five questions of one mark each, Section B contains five questions of two marks each, Section C contains twelve questions of three marks each, Section D contains one value based question of four marks and Section E contains three questions of five marks each. 4. There is no overall choice. However, an internal choice has been provided in one question of two marks, one question of three marks and all the three questions of five marks weightage. You have to attempt only one of the choices in such questions. 5. You may use the following values of physical constants wherever necessary. c = 3 X 108 m/s h = 6.63 X 10-34 Js e = 1.6 X 10-19 C o = 4 X 10-7 T m A-1 0 = 8.854 X 10-12 C2 N-1 m-2 1 4 0 = 9 X 109 N m2 C-2 me = 9.1 X 10-31 kg mass of neutron = 1.675 X 10-27 kg mass of proton = 1.673 X 10-27 kg Avogadro s number = 6.023 X 1023 per gram mole Boltzmann constant = 1.38 X 10-23 JK-1 Section A 1. What is the value of the angle between the vectors and for which the potential energy of an electric dipole of dipole moment , kept in an external electric field , has maximum value. 1 2. Name the colours corresponding to the digits 4 and 7 in the colour code scheme for carbon resistors. 1 3. State which of the two, the capacitor or an inductor, tends to become a SHORT when the frequency of the applied alternating voltage has a very high value. 1 4. Redraw the diagram given below and mark the position of the centre of curvature of the spherical mirror used in the given set up. 1 5. In the given diagram C(t) stands for the carrier wave and m(t) for the signal to be transmitted. What name do we give to the wave labeled as Cm(t) in the diagram? 1 Section B 6. Calculate the value of the unknown potential V for the given potentiometer circuit. The total length (400 cm) of the potentiometer wire has a resistance of 10 and the balance point is obtained at a length of 240 cm. 2 7. Name the phenomenon which proves transverse wave nature of light. Give two uses of the devices whose functioning is based on this phenomenon. 2 OR Name the phenomenon which is responsible for bending of light around sharp corners of an obstacle. Under what conditions does this phenomenon take place? Give one application of this phenomenon in everyday life. 8. The equivalent wavelength of a moving electron has the same value as that of a photon having an energy of 6 X 10-17 J. 2 Calculate the momentum of the electron. 9. The short wavelength limit for the Lyman series of the hydrogen spectrum is 913.4 . Calculate the short wavelength limit for Balmer series of hydrogen spectrum. 2 10. (a) Arrange the following networks in increasing order of the number of computers that may be present in the network: Internet ; LAN ; WAN (b) What is the minimum number of satellites that enables a Global Positioning System (GPS) receiver to determine one s longitude/latitude position, i.e., to make a 2D position fix. 1+1=2 Section C 11. Eight identical spherical drops, each carrying a charge 1 nC are at a potential of 900 V each. All these drops combine together to form a single large drop. Calculate the potential of this large drop. 3 (Assume no wastage of any kind and take the capacitance of a sphere of radius r as proportional to r). 12 . The current flowing in the galvanometer G when the key k2 is kept open is I. On closing the key k2 , the current in the galvanometer becomes I/n, where n is an integer. Obtain an expression for resistance Rg of the galvanometer in terms of R, S and n. To what form does this expression reduce when the value of R is very large as compared to S? 3 13. The magnitude F of the force between two straight parallel current carrying conductors kept at a distance d apart in air is given by = 3 0 1 2 2 Where I1 and I2 are the currents flowing through the two wires Use this expression, and the sign convention that the: Force of attraction is assigned a negative sign and Force of repulsion is assigned a positive sign . Draw graphs showing dependence of F on (i) 1 2 (ii) d when the product 1 2 is maintained at a constant positive value. (iii) d when the product 1 2 is maintained at a constant negative value. when d is kept constant OR The given graphs show the variation of intensity of magnetization I with strength of applied magnetic field H for two magnetic materials P and Q. (i) Identify the materials P and Q. (ii) For material P, plot the variation of Intensity of Magnetisation with temperature. Justify your answer. 14. Find the value of the phase lag/lead between the current and voltage in the given series LCR circuit. Without making any other change, find the value of the additional capacitor, such that when suitably joined to the capacitor ( C= 2 F) as shown, would make the power factor of this circuit unity. 3 15. Explain how one observes an inconsistency when Ampere s circuital law is applied to the process of charging a capacitor. How this contradiction gets removed by introducing the concept of an additional current , known as the displacement current ? 3 16. A point object O is kept at a distance of 30 cm from a convex lens of power +4D towards its left. It is observed that when a convex mirror is kept on the right side at a distance of 50 cm from the convex lens, the image of the object O formed by the lens-mirror combination coincides with the object itself. Calculate the focal length of the convex mirror. 3 17. The arrangement used by Thomas Young to produce an interference pattern is shown in the given diagram. Justify why there would be no change in the fringe width when the main illuminated slit (S) is shifted to the position as shown. 3 18. A given number of atoms No of a radioactive element with a half life T is uniformly distributed in the blood stream of a 3 (i) normal person A having total volume V of blood in the body (ii) person B in need of blood transfusion having a volume of blood in the body. The number of radioactive atoms per unit volume in the blood streams of the two persons after a time nT are found to be N1 and N2. Prove mathematically that the additional volume of blood that needs to be transfused in the body of person B equals ( 2 1 2 )V 19. A student has to use an appropriate number of (i) NAND gates (only) to get the output Y1 (ii) NOR gates (only) to get the output Y2 From two given inputs A and B as shown in the diagram. 3 Identify the equivalent gate needed in each case. Show how one can connect an appropriate number of (i) NAND (ii) NOR gates respectively in the two cases to get these equivalent gates . 20. The data given below gives the photon energy (in eV) for a number of waves whose wavelength values (in nm) are also given. 3 Wavelength (in nm) 200 400 600 800 1000 1200 6.216 3.108 2.072 1.554 1.243 1.036 Photon Energy (in eV) (Without doing any calculation/taking any reading), explain how one can use this data to draw an appropriate graph to infer (i) photon energy corresponding to a wavelength of 100 nm. (ii) the wavelength value (in nm) corresponding to a photon energy of 1 eV. (iii) velocity of light assuming that the value of Plank s constant is known. 21. A (sinusoidal) carrier wave C(t) = AC sin c t 3 is amplitude modulated by a (sinusoidal) message signal m(t) = Am sin m t Write the equation of the (amplitude) modulated signal. Use this equation to obtain the values of the frequencies of all the sinusoidal waves present in the modulated signal. 22. Give reasons for the following: 3 (i) The Zener diode is fabricated by heavily doping both the p and n sides of the junction (ii) A photodiode, when used as a detector of optical signals is operated under reverse bias. (iii) The band gap of the semiconductor used for fabrication of visible LED s must at least be 1.8 eV. Section D 23. Dimpi s class was shown a video on effects of magnetic field on a current carrying straight conductor. She noticed that the force on the straight current carrying conductor becomes zero when it is oriented parallel to the magnetic field and this force becomes maximum when it is perpendicular to the field. She shared this interesting information with her grandfather in the evening. The grandfather could immediately relate it to something similar in real life situations. He explained it to Dimpi that similar things happen in real life too. When we align and orient our thinking and actions in an adaptive and accommodating way, our lives become more peaceful and happy. However, when we adopt an unaccommodating and stubborn attitude, life becomes troubled and miserable. We should therefore always be careful in our response to different situations in life and avoid unnecessary conflicts. Answer the following question based on above information: a) Express the force acting on a straight current carrying conductor kept in a magnetic field in vector form. State the rule used to find the direction of this force. b) Which one value is displayed and conveyed by grandfather as well as Dimpi? c) Mention one specific situation from your own life which reflects similar values shown by you towards your elders. 2+1+1=4 Section E 24 a) State the theorem which relates total charge enclosed within a closed surface 5 and the electric flux passing through it. Prove it for a single point charge. b) An atom was earlier assumed to be a sphere of radius a having a positively charged point nucleus of charge +Ze at its centre. This nucleus was believed to be surrounded by a uniform density of negative charge that made the atom neutral as a whole. Use this theorem to find the electric field of this atom at a distance r (r<a) from the centre of the atom. OR A Dipole is made up of two charges + q and q separated by a distance 2a. Derive an expression for the electric field due to this dipole at a point distant r from the centre of the dipole on the equatorial plane. Draw the shape of the graph, between | | and r when r>> . If this dipole were to be put in a uniform external electric field , obtain an expression for the torque acting on the dipole. 25. State the law which relates to generation of induced emf in a conductor being moved in a magnetic field. 5 Apply this law to obtain an expression for the induced emf when one rod of a rectangular conductor is free to move in a uniform, time independent and normal magnetic field. Apply the concept of the Lorentz (magnetic) force acting on a moving charge to justify the expression obtained above. OR An a.c. voltage V = Vm sin t is applied across an inductor of inductance L. Apply Kirchoff s loop rule to obtain expressions for (i) the current flowing in the circuit (ii) the inductive reactance L Hence find the instantaneous power Pi supplied to the inductor. Show graphically the variation of Pi with t. 26. (a) Explain, with the help of a diagram, how is the phenomenon of total internal reflection used in (i) an optical fibre (ii) a prism that inverts an image without changing its size (b) A right angled prism made from a material of refractive index is kept in air. A ray PQ is incident normally on the side AB of the prism as shown. Find (in terms of ) the maximum value of upto which this incident ray necessarily undergoes total internal reflection at the face AC of the prism. 2+3=5 OR State Huygen s principle in wave-optics. How did Huygen explain the absence of the backwave? Use this principle to draw the refracted wave front for a plane wave incident from a denser to a rarer medium. Hence obtain Snell s law of refraction. Class XII Physics Sample Question Paper (Marking Scheme) Time Allowed: 3 Hours Maximum Marks: 70 01. P.E. = . = p E Cos P.E. is maximum when Cos = -1, i.e. = (1800) ( ) ( ) 02. 4 yellow 7 Violet ( ) ( ) 03. The capacitor (1) 04. (1) 05. Cm (t) is the frequency modulated wave. 06. The current through the potentiometer wire = (1) 3 (290 + 10) = 10-2 A Potential drop per unit length of the 10 2 10 1 potentiometer wire = = = X 10 3 400 4 Balancing length (l) = 240 cm (given) ( ) (1) V= l = 1 4 (= 60 mV) X 10 3 240 = 6 10 2 ( ) 07. Polarization (1) Two Uses: Polaroids can be used in sunglasses, window panes, photographic cameras, 3D movie cameras (Any Two) ( + ) OR Diffraction; Condition: Size of the obstacle sharpness should be comparable to the wavelength of the light falling. (1+1/2) Any application (1/2) 08. E = Energy of the photon = h = = ( ) Wave length of the moving electron = = Momentum of the electron = p = = = = 6 10 17 3 108 kg 1= 2 X 10 25 kg 1 ( ) ( ) ( ) 09. Rydberg formula for the wavelengths of spectral lines in hydrogen spectrum is 1 =R( 1 2 - 1 2 ) ( ) The short wavelength limit for the Lyman series would be 1 = R( R= 1 1 1 2 2) = R 1 913.4 ( ) The short wavelength limit for the Balmer series, would be 1 1 = R ( 2 - = 4 1 22 )= = ( ) 4 4 X 913.4 = 3653.6 ( ) 10. a) LAN, WAN, Internet (1) b) Three (1) 11. Let the radius of each drop be r. The capacitance C of each drop is kr, where k is a constant. Also q = CV, V = 900 volt ( ) charge on each drop = q = (kr X 900) C ( ) Total charge on all the eight drops = Q = 8 q =7200 kr ( ) Let R be the radius of the large drop. Then 4 3 4 3 R =8X r 3 3 R = (8) r = 2r Capacitance of the large drop = kR = 2kr Potential of the large drop = = 7200 2 = 3600V ( ) ( ) volt ( ) 12. With key K2 open, the current I in the galvanometer is given by I = + When K2 is closed, the equivalent resistance, say R , of the parallel combination of S and is given by = + The total current, say drawn from the battery would now be ( ) = + This current gets subdivided in the inverse ratio of S and ; Hence the current through G, would now be given by = = = + = ( ) + ( + ) . 1 + [ + + ] ( ) + + But = = 1 + ( + ) Or n RS + n S = ( ) + + = RS + R + S or (n-1) RS = R - (n-1) S or (n-1) RS + [ ( 1) ] ( 1) = ( ) ( 1) This is the required expression When R >>S , we have ( 1) = (n-1) S ( ) 13. We know that F is an attractive (-ve) force when the currents I1 and I2 are like currents i.e. when the product I1I2 is positive. Similarly F is a repulsive (+ve) force when the currents I1 and I2 are unlike currents, i.e. when the product I1I2 is negative. (1/2) Now F (I1 I2 ), when d is kept constant and F 1 when I1I2 is kept constant. The required graphs, therefore, have the forms shown below. (1/2) (1+1/2+1/2) OR (i) P- Paramagnetic ( ) Q-Ferromagnetic materials ( ) (ii) (1) 14.The current, I, leads the voltage, V, by an angle where tan = = 1 = 500 and = L = 100 , R = 400 Here tan = 1 = 450 (1/2) ( ) The power factor becomes unity when = 00. Hence we need to adjust C to a new value C where = = 100 (1/2) ( ) Thus, phase angle is 450 with the current LEADING the voltage. To make power factor as unity we need to have Xc also equal to 100 ohms. For this C needs to have a value of 10 F. (1/2) We, therefore need to put an additional capacitor of (10-2), i.e, 8 F in parallel with the given capacitor (1/2) 15. Discussion on the observed inconsistency (1) Discussion on Removing the contradiction through the concept of an additional current, called the displacement current . (1 +1) (Pages 270 and 271 of NCERT Book, Part I) 16. Focal length of the convex lens = 1 m (1/2) Let v be the position of the image, I, of the object formed by the convex lens alone. We then have 1 1 ( ) = 1 v = 150cm (1/2) Hence the distance of the image (formed by the convex lens alone ) from the convex mirror would be (150-50) cm ,i.e.,100 cm. This distance equals the radius of curvature of the convex mirror. (1/2) Hence focal length of the convex mirror equals100/2, i.e., 50 cm. ( ) Radius of curvature of the convex mirror R = LI LM ( ) (1/2) 17. Calculation of fringe width, the normal set up (1 ) Calculation of fringe width, in the changed set up (1) Observing that = ( ) (Reference : NCERT Book, Part II, pages 363, 364) 18. Initial number of radioactive atoms, per unit volume, in the blood streams of persons A and B are ( 0 /V) and ( 0 /V ) respectively. ( ) After a time nT (T = Half life), these numbers would get reduced by a factor 2n. ( ) 1 Hence N1= ( ) . 2 1 And N2 = ( ) . ( ) 2 1 = ( ) 2 or = V 1 2 Additional volume of blood needed by person B is V- = V V 1 2 =( 2 N1 N2 )V ( ) ( ) 19. The equivalent gates in the two cases are the OR gate and the AND gate respectively. (1/2 + ) (i) A combination of three NAND gates, connected in the manner shown, would be equivalent to an OR gate. (Fig 14.46 (b) Page 511) (1) (i) A combination of three NOR gates connected in the manner shown would be equivalent to an AND gate. (Fig 14.48 (b) page 512) 20. One can calculate the values of and 1 1 (1) and plot a graph between E (photon energy in eV) (in nm-1). (1) The resulting straight line graph can be used to (ii) read the value of (iii) We have E = 1 1 1 nm-1 (1/2) ( in nm-1) corresponding to E = 1eV (1/2) (i) read the value of , corresponding to = 100 The slope of the graph (after appropriate adjustment of the units) would equal hc. Since h is known, one can calculate c. (1) 21. The equation of the (amplitude) modulated signal is ( ) = [ ( + sin ) ] sin (1) This can be rewritten as ( ) = [ ( 1 + sin ) ] sin Where = / = modulation index ( ) = sin + = sin + 2 2 2 sin . sin (1/2) (1/2) [cos ( ) cos ( + ) ] (1/2) These are the three sinusoidal waves present in the amplitude modulated signal. The frequencies of these three waves are 1 = 2 = 2 2 and 3 = (1/2) + 2 22. (i) Heavy doping makes the depletion region very thin. This makes the electric field of the junction very high, even for a small reverse bias voltage. This in turn helps the Zener diode to act as a voltage regulator . (1) (ii) When operated under reverse bias, the photodiode can detect changes in current with changes in light intensity more easily. (1) (iii). The photon energy, of visible light photons varies from about 1.8 eV to 3 eV. Hence for visible LED s, the semiconductor must have a band gap of 1.8 eV. (1) 23. (i) Expression for the force in vector form (1) - Statement of Flemings left hand rule (ii) (iii) (1) Adaptation to different situations and flexible and adjustable attitude (1) - Sharing excitement in classroom learning with family members Avoiding unnecessary arguments in conflicting situations in everyday life (1) 24. (a) Statement of Gauss s law Simple Proof (1) (2) (Pages 33 and 34 of NCERT Book I) (b) Set be the uniform density of negative charge. We then have 4 3 3 = = 3 4 3 (1/2) Taking a sphere of radius r (centred at the nucleus) as the Gaussian surface, we have ( ) 4 2 = (1/2) Where Q is the net charge enclosed by the Gaussian surface. Now 4 3 Q = (+Ze) + ( - . 3 3 = Ze Ze ( 3 ) ) = Ze ( 1- 3 3 (1/2) ) Substituting this value of Q, we get E(r) = 4 0 = (12 4 0 ( 1 2 3 3 - ) 3 (1/2) ) OR Derivation of the expression for [ = ) 2 ( 3 ] (page 28 NCERT Book I) 4 0 ( 2 + 2 )2 | = For r >> a, | 2 4 0 3 (3) Thus, the graph has the form shown. (1) Derivation of the expression for torque (Page 31, NCERT Book I) (1) 25. Statement of Faraday s law of e-m induction (1) Derivation of the expression for induced emf (2) Justification on the basis of the concept of Lorentz s force (2) (Page 212 + page 213 NCERT Part I) OR Applying Kirchoff s loop rule to obtain expressions for (i) (ii) Current flowing in the circuit Inductive reactance of L (21 2) (1/2) Finding expression for instantaneous power, Pi, the graph has the form shown. (1) (1) 26. (a) Explaining the use of the phenomenon of total internal reflection in (i) an optical fibre (NCERT Page 322 fig 9.16 + Explanation) (1/2 +1) (ii) a prism that inverts an image without changing its size. (NCERT Page 322 fig 9.15 9(c) + Explanation) (1/2 +1) (b) (1/2) The angle of incidence at the face AC = i But i = A (Angle between two lines is the same as the angle between their perpendiculars) Also = /2 A = /2 i (1/2) The minimum value of I, so that there is total internal reflection at the face AC, equals ic 1 where ic = sin 1 ( ) (1/2) The maximum value of corresponding to the minimum value of i (= ic) is therefore, 1 max = /2 ic = /2 sin 1 ( ) (1/2) OR Statement of Huygen s principle (1) Absence of back wave (Page 354 NCERT book part II) Drawing the refracted wave front (1) (11 2) Obtaining Snell s law of refraction (Fig 10.5 Page 357 NCERT book part II) (11 2)

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