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CBSE Class 12 Board Exam 2025 : Physics

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Army Public School New Cantt., Prayagraj PRE BOARD II Class 12 - Physics Time Allowed: 3 hours Maximum Marks: 70 General Instructions: 1. There are 33 questions in all. All questions are compulsory. 2. This question paper has five sections: Section A, Section B, Section C, Section D and Section E. 3. All the sections are compulsory. 4. Section A contains sixteen questions, twelve MCQ and four Assertion Reasoning based of 1 mark each, Section B contains five questions of two marks each, Section C contains seven questions of three marks each, Section D contains two case study based questions of four marks each and Section E contains three long answer questions of five marks each. 5. There is no overall choice. However, an internal choice has been provided in one question in Section B, one question in Section C, one question in each CBQ in Section D and all three questions in Section E. You have to attempt only one of the choices in such questions. 6. Use of calculators is not allowed. Section A 1. 2. 3. 4. 5. An n-type Ge is obtained on doping the Ge-crystal with a) phosphorus b) gold c) aluminum d) boron [1] If the percentage change in current through a resistor is 1%, then the change in power through it would be: a) 0.5% b) 1% c) 2% d) 1.7% Which of the following pairs of media has the least value of critical angle? a) Glass to air b) Glass to water c) Diamond to water d) Diamond to air [1] Which one of the following has relative magnetic permeability between 0 and 1? a) Sodium b) Aluminium c) Alnico d) Water The potential on the hollow sphere of radius 1 m is 1000 V, then the potential at [1] [1] 1 4 m from the centre of the [1] sphere is: a) Zero V b) 500 V c) 1000 V d) 250 V 1/8 6. Two infinitely long wires carry currents in opposite directions. Then the field at a point P lying midway between [1] them is 7. a) twice the field due to each wire alone b) square of the field due to each wire alone c) zero d) half of the field due to each wire alone Two identical circular coaxial coils A and B, arranged in vertical planes parallel to each other, carry currents in [1] the same direction. If the distance between the coils is decreased at a constant rate, the current: 8. a) increases in both A and B. b) increases in A and decreases in B. c) remains same in both A and B. d) decreases in both A and B. Two similar magnets of magnetic moments M1 and M2 are taken and vibrate in a vibration magnetometer with [1] their i. like poles together ii. unlike poles together. If the ratio of the time periods is 9. b) 5 c) 2 d) 1 , then the ratio of M1 and M2 ( i.e., M1 M2 ) is 3 3 In a wave, the path difference corresponding to a phase difference of is: c) 11. 2 a) 0.5 a) 10. 1 2 b) 2 d) [1] Electric charges under the action of electric forces is called: a) electric field lines. b) electrostatic c) electric flux d) electric field [1] In the half wave rectifier circuit shown which one of the following wave forms is true for VCD, the output across [1] C and D? 12. a) b) c) d) A biconvex lens of glass having refractive index 1.47 is immersed in a liquid. It becomes invisible and behaves [1] as a plane glass plate. The refractive index of the liquid is a) 1.33 b) 1.62 c) 1.51 d) 1.47 2/8 13. Assertion (A): The energy of X-ray photon is greater than that of light (visible) photon. [1] Reason (R): X-ray photon in vacuum travels faster than light photon. a) Both A and R are true and R is the correct b) Both A and R are true but R is not the explanation of A. correct explanation of A. c) A is true but R is false. 14. d) A is false but R is true. Assertion (A): No work is done in taking a positive charge from one point to another inside a positively charged [1] metallic sphere while outside the sphere work is done in taking the charge towards the sphere. Reason (R): Inside the sphere electric potential is the same at each point, but outside it is different for different points. a) Both A and R are true and R is the correct b) Both A and R are true but R is not the explanation of A. correct explanation of A. c) A is true but R is false. 15. d) A is false but R is true. Assertion (A): For best contrast between maxima and minima in the interference pattern of Young's double-slit [1] experiment, the intensity of light emerging out of the two slits should be equal. Reason (R): The intensity of the interference pattern is proportional to the square of the amplitude. a) Both A and R are true and R is the correct b) Both A and R are true but R is not the explanation of A. correct explanation of A. c) A is true but R is false. 16. d) A is false but R is true. Assertion (A): If the frequency of the alternating current in an ac circuit consisting of an inductance coil is [1] increased then the current gets decreased. Reason (R): The current is inversely proportional to the frequency of the alternating current. a) Both A and R are true and R is the correct b) Both A and R are true but R is not the explanation of A. correct explanation of A. c) A is true but R is false. d) A is false but R is true. Section B 17. a. The electric field of an electromagnetic wave is represented as Ex = E0 sin ( t + kz). [2] i. In which direction is the wave propagating? ii. In which direction does the magnetic field oscillate? b. Write two characteristics of electromagnetic waves. 18. Compare the magnetic fields of a bar magnet and current carrying solenoid. [2] 19. Explain the terms depletion layer and potential barrier in a p-n junction diode. How are the [2] i. width of depletion layer, and ii. value of potential barrier affected when the p-n junction is forward biased? 20. Name the spectral series for a hydrogen atom that lies in the visible region. Find the ratio of the maximum to the [2] minimum wavelengths of this series. 21. Two identical circular loops, P and Q, ucK of radius r and carrying currents I and 21 respectively are lying in [2] parallel planes such that they have a common axis. The direction of current in both the loops is clockwise as 3/8 seen from O which is equidistant from the both loops. Find the magnitude of the net magnetic field at point O. OR Two long straight parallel wires A and B separated by a distance d, carry equal current I flowing in same direction as shown in the figure. i. Find the magnetic field at a point P situated between them at a distance x from one wire. ii. Show graphically the variation of the magnetic field with distance x for 0 < x < d. Section C 22. Draw a plot showing the variation of resistivity of a (i) conductor and (ii) semiconductor, with the increase in [3] temperature. How does one explain this behaviour in terms of number density of charge carriers and the relaxation time? 23. The black box, shown here, converts the input voltage waveform into the output voltage waveform as is shown [3] in the figure: Draw the circuit diagram of the circuit present in the black box and give a brief description of its working. 24. a. Use Einstein photoelectric equation to depict the variation of the maximum kinetic energy (Ek) of electrons [3] emitted, with the frequency (v) of the incident radiation. b. A photosensitive surface is illuminated with a beam of (i) yellow light, and (ii) red light, both of the same intensity. In which case will I. photoelectrons have more Ek? II. more numbers of electrons be emitted? Justify your answer in each case. 25. Draw a graph showing the variation of binding energy per nucleon with mass number of different nuclei. Write [3] any two salient features of the curve. How does this curve explain the release of energy both in the processes of nuclear fission and fusion? 26. State Bohr's postulate to explain stable orbits in a hydrogen atom. Prove that the speed with which the electron [3] revolves in nth orbit is proportional to (1/n) 27. In a Young's double-slit experiment, the slits are separated by 0.5 mm and the screen is placed 1.0 m away. It is [3] found that the ninth bright fringe is at a distance of 8.835 mm from the second dark fringe. Find the wavelength of light used. 28. A conducting rod XY slides freely on two parallel rails, A and B, with a uniform velocity 'v'. A galvanometer 'G' [3] is connected, as shown in the figure and the closed circuit has a total resistance 'R'. A uniform magnetic field, perpendicular to the plane defined by the rails A and B and the rod XY (which are mutually perpendicular), is 4/8 present over the region, as shown. i. With key K open: a. Find the nature of charges developed at the ends of the rod XY. b. Why do the electrons, in rod XY, (finally) experience no net force even though the magnetic force is acting on them due to the motion of the rod? ii. How much power needs to be delivered, (by an external agency), to keep the rod moving at its uniform speed when key K is (a) closed (b) open? iii. With key K closed, how much power gets dissipated as heat in circuit 1 State the source of this power OR Figure given below shows an arrangement by which current flows through the bulb (X) connected with coil B, when a.c. is passed through coil A. Explain the following observations: i. Bulb lights up. ii. Bulb gets dimmer if the coil 'B' is moved upwards. iii. If a copper sheet is inserted in the gap between the coils how the brightness of the bulb would change? Section D 29. Read the text carefully and answer the questions: [4] Maxwell showed that the speed of an electromagnetic wave depends on the permeability and permittivity of the medium through which it travels. The speed of an electromagnetic wave in free space is given by c = 1 0 0 . The fact led Maxwell to predict that light is an electromagnetic wave. The emergence of the speed of light from purely electromagnetic considerations is the crowning achievement of Maxwell s electromagnetic theory. The speed of an electromagnetic wave in any medium of permeability and permittivity will be c K r where K is the dielectric constant of the medium and is the relative permeability. r (a) (b) The dimensions of 1 2 0 E 2 ( : permittivity of free space; E = electric field) is 0 a) MLT-1 b) ML-1T-2 c) ML2T-2 d) ML2T-1 Let [ 0] denote the dimensional formula of the permittivity of the vacuum. If M = mass, L = length, T = time and A = electric current, then a) [ ] = ML2T-1 b) c) [ ] = M-1L-3T4A2 d) 0 0 (c) [ 0 ] = MLT-2A-2 [ 0 ] = M-1L-3T2A An electromagnetic wave of frequency 3 MHz passes from vacuum into a dielectric medium with 5/8 permittivity = 4. Then a) wavelength is halved and the frequency b) wavelength and frequency both remain remains unchanged. unchanged c) wavelength is doubled and the d) wavelength is doubled and the frequency remains unchanged frequency becomes half OR The electromagnetic waves travel with a) the speed of light c = 3 108 m s-1 in fluid medium. b) the speed of light c = 3 10 m s-1 in solid medium c) the speed of light c = 3 108 m s-1 in d) the same speed in all media free space (d) Which of the following are not electromagnetic waves? cosmic rays, -rays, -rays, X-rays 30. a) -rays b) X-rays c) -rays d) cosmic rays Read the text carefully and answer the questions: [4] When a charged particle is placed in an electric field, it experiences an electrical force. If this is the only force on the particle, it must be the net force. The net force will cause the particle to accelerate according to Newton's second law. So F e = q E = ma If E is uniform, then a is constant and a = qE m . If the particle has a positive charge, its acceleration is in the direction of the field. If the particle has a negative charge, its acceleration is in the direction opposite to the electric field. Since the acceleration is constant, the kinematic equations can be used. (a) A charged particle is free to move in an electric field. It will travel a) along a line of force, if its initial b) always along a line of force velocity is non zero c) along a line of force, if its initial velocity is zero d) along a line of force, if it has some initial velocity in the direction of an acute angle with the line of force (b) An electron of mass m, charge e falls through a distance h metre in a uniform electric field E. Then time of fall, a) t = c) t = (c) 2hm eE 2hm eE b) t = d) t = 2eE hm 2eE hm An electron moving with a constant velocity v along X-axis enters a uniform electric field applied along Y- 6/8 axis. Then the electron moves (d) a) in a trajectory represented as y = ax b) in a trajectory represented as y = ax2 c) without any acceleration along Y-axis d) with uniform acceleration along Y-axis Two equal and opposite charges of masses m1 and m2 are accelerated in an uniform electric field through the same distance. What is the ratio of their accelerations if their ratio of masses is a) a1 c) a1 a2 a2 = 3 b) a1 = 2 d) a1 m1 m2 = 0.5? = 1 a2 = 0.5 a2 OR A particle of mass m carrying charge q is kept at rest in a uniform electric field E and then released. The kinetic energy gained by the particle, when it moves through a distance y is a) qEy b) qE2y c) qEy2 d) 1 2 qEy2 Section E 31. [5] i. Draw a labelled ray diagram to obtain the real image formed by an astronomical telescope in normal adjustment position. Define its magnifying power. ii. You are given three lenses of power 0.5 D, 4D and 10 D to design a telescope. a. Which lenses should be used as objective and eyepiece? Justify your answer. b. Why is the aperture of the objective preferred to be large? OR In Young's double slit experiment, deduce the condition for (a) constructive and (b) destructive interference at a point on the screen. Draw a graph showing a variation of intensity in the interference pattern against position x on the screen. 32. [5] i. Derive the expression for the energy stored in parallel plate capacitor. Hence, obtain the expression for the energy density of the electric field. ii. A fully charged parallel plate capacitor is connected across an uncharged identical capacitor. Show that the energy stored in the combination is less than the energy stored initially in the single capacitor. OR Derive an expression for potential due to a dipole for distances large compared to the size of the dipole. How is the potential due to dipole different from that due to a single charge? 33. i. When an AC source is connected to an ideal capacitor, then show that the average power supplied by the [5] source over a complete cycle is zero. ii. A lamp is connected in series with a capacitor. Predict your observations when the system is connected first across a DC and then an AC source. What happens in each case if the capacitance of the capacitor is 7/8 reduced? OR i. Describe, with the help of a suitable diagram, the working principle of a step-up transformer. Obtain the relation between input and output voltages in terms of the number of turns of primary and secondary windings and the currents in the input and output circuits. ii. Given the input current 15 A and the input voltage of 100 V for a step-up transformer having 90% efficiency, find the output power and the voltage in the secondary if the output current is 3 A. 8/8

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